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USING THE IJG JPEG LIBRARY
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Copyright (C) 1994-2009, Thomas G. Lane, Guido Vollbeding.
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This file is part of the Independent JPEG Group's software.
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For conditions of distribution and use, see the accompanying README file.
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This file describes how to use the IJG JPEG library within an application
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program. Read it if you want to write a program that uses the library.
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The file example.c provides heavily commented skeleton code for calling the
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JPEG library. Also see jpeglib.h (the include file to be used by application
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programs) for full details about data structures and function parameter lists.
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The library source code, of course, is the ultimate reference.
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Note that there have been *major* changes from the application interface
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presented by IJG version 4 and earlier versions. The old design had several
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inherent limitations, and it had accumulated a lot of cruft as we added
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features while trying to minimize application-interface changes. We have
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sacrificed backward compatibility in the version 5 rewrite, but we think the
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improvements justify this.
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Functions provided by the library
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Outline of typical usage
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Mechanics of usage: include files, linking, etc
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Compression parameter selection
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Decompression parameter selection
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Compressed data handling (source and destination managers)
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Progressive JPEG support
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Abbreviated datastreams and multiple images
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Raw (downsampled) image data
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Really raw data: DCT coefficients
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Library compile-time options
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Portability considerations
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Notes for MS-DOS implementors
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You should read at least the overview and basic usage sections before trying
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to program with the library. The sections on advanced features can be read
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if and when you need them.
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Functions provided by the library
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---------------------------------
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The IJG JPEG library provides C code to read and write JPEG-compressed image
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files. The surrounding application program receives or supplies image data a
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scanline at a time, using a straightforward uncompressed image format. All
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details of color conversion and other preprocessing/postprocessing can be
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handled by the library.
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The library includes a substantial amount of code that is not covered by the
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JPEG standard but is necessary for typical applications of JPEG. These
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functions preprocess the image before JPEG compression or postprocess it after
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decompression. They include colorspace conversion, downsampling/upsampling,
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and color quantization. The application indirectly selects use of this code
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by specifying the format in which it wishes to supply or receive image data.
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For example, if colormapped output is requested, then the decompression
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library automatically invokes color quantization.
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A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
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and even more so in decompression postprocessing. The decompression library
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provides multiple implementations that cover most of the useful tradeoffs,
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ranging from very-high-quality down to fast-preview operation. On the
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compression side we have generally not provided low-quality choices, since
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compression is normally less time-critical. It should be understood that the
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low-quality modes may not meet the JPEG standard's accuracy requirements;
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nonetheless, they are useful for viewers.
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A word about functions *not* provided by the library. We handle a subset of
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the ISO JPEG standard; most baseline, extended-sequential, and progressive
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JPEG processes are supported. (Our subset includes all features now in common
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use.) Unsupported ISO options include:
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* Hierarchical storage
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* Nonintegral subsampling ratios
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We support both 8- and 12-bit data precision, but this is a compile-time
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choice rather than a run-time choice; hence it is difficult to use both
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precisions in a single application.
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By itself, the library handles only interchange JPEG datastreams --- in
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particular the widely used JFIF file format. The library can be used by
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surrounding code to process interchange or abbreviated JPEG datastreams that
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are embedded in more complex file formats. (For example, this library is
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used by the free LIBTIFF library to support JPEG compression in TIFF.)
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Outline of typical usage
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------------------------
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The rough outline of a JPEG compression operation is:
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Allocate and initialize a JPEG compression object
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Specify the destination for the compressed data (eg, a file)
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Set parameters for compression, including image size & colorspace
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jpeg_start_compress(...);
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while (scan lines remain to be written)
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jpeg_write_scanlines(...);
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jpeg_finish_compress(...);
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Release the JPEG compression object
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A JPEG compression object holds parameters and working state for the JPEG
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library. We make creation/destruction of the object separate from starting
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or finishing compression of an image; the same object can be re-used for a
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series of image compression operations. This makes it easy to re-use the
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same parameter settings for a sequence of images. Re-use of a JPEG object
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also has important implications for processing abbreviated JPEG datastreams,
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The image data to be compressed is supplied to jpeg_write_scanlines() from
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in-memory buffers. If the application is doing file-to-file compression,
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reading image data from the source file is the application's responsibility.
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The library emits compressed data by calling a "data destination manager",
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which typically will write the data into a file; but the application can
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provide its own destination manager to do something else.
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Similarly, the rough outline of a JPEG decompression operation is:
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Allocate and initialize a JPEG decompression object
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Specify the source of the compressed data (eg, a file)
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Call jpeg_read_header() to obtain image info
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Set parameters for decompression
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jpeg_start_decompress(...);
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while (scan lines remain to be read)
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jpeg_read_scanlines(...);
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jpeg_finish_decompress(...);
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Release the JPEG decompression object
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This is comparable to the compression outline except that reading the
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datastream header is a separate step. This is helpful because information
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about the image's size, colorspace, etc is available when the application
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selects decompression parameters. For example, the application can choose an
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output scaling ratio that will fit the image into the available screen size.
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The decompression library obtains compressed data by calling a data source
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manager, which typically will read the data from a file; but other behaviors
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can be obtained with a custom source manager. Decompressed data is delivered
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into in-memory buffers passed to jpeg_read_scanlines().
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It is possible to abort an incomplete compression or decompression operation
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by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
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simply release it by calling jpeg_destroy().
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JPEG compression and decompression objects are two separate struct types.
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However, they share some common fields, and certain routines such as
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jpeg_destroy() can work on either type of object.
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The JPEG library has no static variables: all state is in the compression
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or decompression object. Therefore it is possible to process multiple
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compression and decompression operations concurrently, using multiple JPEG
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Both compression and decompression can be done in an incremental memory-to-
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memory fashion, if suitable source/destination managers are used. See the
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section on "I/O suspension" for more details.
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Before diving into procedural details, it is helpful to understand the
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image data format that the JPEG library expects or returns.
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The standard input image format is a rectangular array of pixels, with each
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pixel having the same number of "component" or "sample" values (color
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channels). You must specify how many components there are and the colorspace
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interpretation of the components. Most applications will use RGB data
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(three components per pixel) or grayscale data (one component per pixel).
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PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
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A remarkable number of people manage to miss this, only to find that their
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programs don't work with grayscale JPEG files.
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There is no provision for colormapped input. JPEG files are always full-color
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or full grayscale (or sometimes another colorspace such as CMYK). You can
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feed in a colormapped image by expanding it to full-color format. However
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JPEG often doesn't work very well with source data that has been colormapped,
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because of dithering noise. This is discussed in more detail in the JPEG FAQ
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and the other references mentioned in the README file.
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Pixels are stored by scanlines, with each scanline running from left to
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right. The component values for each pixel are adjacent in the row; for
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example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an
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array of data type JSAMPLE --- which is typically "unsigned char", unless
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you've changed jmorecfg.h. (You can also change the RGB pixel layout, say
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to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in
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that file before doing so.)
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A 2-D array of pixels is formed by making a list of pointers to the starts of
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scanlines; so the scanlines need not be physically adjacent in memory. Even
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if you process just one scanline at a time, you must make a one-element
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pointer array to conform to this structure. Pointers to JSAMPLE rows are of
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type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.
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The library accepts or supplies one or more complete scanlines per call.
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It is not possible to process part of a row at a time. Scanlines are always
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processed top-to-bottom. You can process an entire image in one call if you
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have it all in memory, but usually it's simplest to process one scanline at
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For best results, source data values should have the precision specified by
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BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress
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data that's only 6 bits/channel, you should left-justify each value in a
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byte before passing it to the compressor. If you need to compress data
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that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.
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(See "Library compile-time options", later.)
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The data format returned by the decompressor is the same in all details,
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except that colormapped output is supported. (Again, a JPEG file is never
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colormapped. But you can ask the decompressor to perform on-the-fly color
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quantization to deliver colormapped output.) If you request colormapped
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output then the returned data array contains a single JSAMPLE per pixel;
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its value is an index into a color map. The color map is represented as
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a 2-D JSAMPARRAY in which each row holds the values of one color component,
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that is, colormap[i][j] is the value of the i'th color component for pixel
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value (map index) j. Note that since the colormap indexes are stored in
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JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
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(ie, at most 256 colors for an 8-bit JPEG library).
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Here we revisit the JPEG compression outline given in the overview.
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1. Allocate and initialize a JPEG compression object.
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A JPEG compression object is a "struct jpeg_compress_struct". (It also has
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a bunch of subsidiary structures which are allocated via malloc(), but the
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application doesn't control those directly.) This struct can be just a local
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variable in the calling routine, if a single routine is going to execute the
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whole JPEG compression sequence. Otherwise it can be static or allocated
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You will also need a structure representing a JPEG error handler. The part
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of this that the library cares about is a "struct jpeg_error_mgr". If you
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are providing your own error handler, you'll typically want to embed the
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jpeg_error_mgr struct in a larger structure; this is discussed later under
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"Error handling". For now we'll assume you are just using the default error
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handler. The default error handler will print JPEG error/warning messages
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on stderr, and it will call exit() if a fatal error occurs.
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You must initialize the error handler structure, store a pointer to it into
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the JPEG object's "err" field, and then call jpeg_create_compress() to
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initialize the rest of the JPEG object.
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Typical code for this step, if you are using the default error handler, is
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struct jpeg_compress_struct cinfo;
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struct jpeg_error_mgr jerr;
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cinfo.err = jpeg_std_error(&jerr);
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jpeg_create_compress(&cinfo);
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jpeg_create_compress allocates a small amount of memory, so it could fail
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if you are out of memory. In that case it will exit via the error handler;
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that's why the error handler must be initialized first.
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2. Specify the destination for the compressed data (eg, a file).
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As previously mentioned, the JPEG library delivers compressed data to a
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"data destination" module. The library includes one data destination
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module which knows how to write to a stdio stream. You can use your own
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destination module if you want to do something else, as discussed later.
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If you use the standard destination module, you must open the target stdio
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stream beforehand. Typical code for this step looks like:
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if ((outfile = fopen(filename, "wb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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jpeg_stdio_dest(&cinfo, outfile);
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where the last line invokes the standard destination module.
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WARNING: it is critical that the binary compressed data be delivered to the
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output file unchanged. On non-Unix systems the stdio library may perform
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newline translation or otherwise corrupt binary data. To suppress this
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behavior, you may need to use a "b" option to fopen (as shown above), or use
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setmode() or another routine to put the stdio stream in binary mode. See
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cjpeg.c and djpeg.c for code that has been found to work on many systems.
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You can select the data destination after setting other parameters (step 3),
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if that's more convenient. You may not change the destination between
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calling jpeg_start_compress() and jpeg_finish_compress().
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3. Set parameters for compression, including image size & colorspace.
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You must supply information about the source image by setting the following
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fields in the JPEG object (cinfo structure):
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image_width Width of image, in pixels
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image_height Height of image, in pixels
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input_components Number of color channels (samples per pixel)
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in_color_space Color space of source image
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The image dimensions are, hopefully, obvious. JPEG supports image dimensions
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of 1 to 64K pixels in either direction. The input color space is typically
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RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special
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color spaces", later, for more info.) The in_color_space field must be
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assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or
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JPEG has a large number of compression parameters that determine how the
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image is encoded. Most applications don't need or want to know about all
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these parameters. You can set all the parameters to reasonable defaults by
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calling jpeg_set_defaults(); then, if there are particular values you want
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to change, you can do so after that. The "Compression parameter selection"
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section tells about all the parameters.
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You must set in_color_space correctly before calling jpeg_set_defaults(),
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because the defaults depend on the source image colorspace. However the
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other three source image parameters need not be valid until you call
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jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more
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than once, if that happens to be convenient.
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Typical code for a 24-bit RGB source image is
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cinfo.image_width = Width; /* image width and height, in pixels */
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cinfo.image_height = Height;
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cinfo.input_components = 3; /* # of color components per pixel */
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cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
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jpeg_set_defaults(&cinfo);
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/* Make optional parameter settings here */
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4. jpeg_start_compress(...);
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After you have established the data destination and set all the necessary
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source image info and other parameters, call jpeg_start_compress() to begin
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a compression cycle. This will initialize internal state, allocate working
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storage, and emit the first few bytes of the JPEG datastream header.
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jpeg_start_compress(&cinfo, TRUE);
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The "TRUE" parameter ensures that a complete JPEG interchange datastream
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will be written. This is appropriate in most cases. If you think you might
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want to use an abbreviated datastream, read the section on abbreviated
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Once you have called jpeg_start_compress(), you may not alter any JPEG
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parameters or other fields of the JPEG object until you have completed
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the compression cycle.
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5. while (scan lines remain to be written)
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jpeg_write_scanlines(...);
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Now write all the required image data by calling jpeg_write_scanlines()
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one or more times. You can pass one or more scanlines in each call, up
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to the total image height. In most applications it is convenient to pass
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just one or a few scanlines at a time. The expected format for the passed
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data is discussed under "Data formats", above.
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Image data should be written in top-to-bottom scanline order. The JPEG spec
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contains some weasel wording about how top and bottom are application-defined
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terms (a curious interpretation of the English language...) but if you want
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your files to be compatible with everyone else's, you WILL use top-to-bottom
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order. If the source data must be read in bottom-to-top order, you can use
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the JPEG library's virtual array mechanism to invert the data efficiently.
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Examples of this can be found in the sample application cjpeg.
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The library maintains a count of the number of scanlines written so far
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in the next_scanline field of the JPEG object. Usually you can just use
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this variable as the loop counter, so that the loop test looks like
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"while (cinfo.next_scanline < cinfo.image_height)".
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Code for this step depends heavily on the way that you store the source data.
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example.c shows the following code for the case of a full-size 2-D source
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array containing 3-byte RGB pixels:
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JSAMPROW row_pointer[1]; /* pointer to a single row */
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int row_stride; /* physical row width in buffer */
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row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */
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while (cinfo.next_scanline < cinfo.image_height) {
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row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
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jpeg_write_scanlines(&cinfo, row_pointer, 1);
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jpeg_write_scanlines() returns the number of scanlines actually written.
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This will normally be equal to the number passed in, so you can usually
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ignore the return value. It is different in just two cases:
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* If you try to write more scanlines than the declared image height,
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the additional scanlines are ignored.
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* If you use a suspending data destination manager, output buffer overrun
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will cause the compressor to return before accepting all the passed lines.
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This feature is discussed under "I/O suspension", below. The normal
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stdio destination manager will NOT cause this to happen.
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In any case, the return value is the same as the change in the value of
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6. jpeg_finish_compress(...);
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After all the image data has been written, call jpeg_finish_compress() to
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complete the compression cycle. This step is ESSENTIAL to ensure that the
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last bufferload of data is written to the data destination.
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jpeg_finish_compress() also releases working memory associated with the JPEG
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jpeg_finish_compress(&cinfo);
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If using the stdio destination manager, don't forget to close the output
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stdio stream (if necessary) afterwards.
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If you have requested a multi-pass operating mode, such as Huffman code
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optimization, jpeg_finish_compress() will perform the additional passes using
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data buffered by the first pass. In this case jpeg_finish_compress() may take
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quite a while to complete. With the default compression parameters, this will
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It is an error to call jpeg_finish_compress() before writing the necessary
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total number of scanlines. If you wish to abort compression, call
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jpeg_abort() as discussed below.
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After completing a compression cycle, you may dispose of the JPEG object
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as discussed next, or you may use it to compress another image. In that case
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return to step 2, 3, or 4 as appropriate. If you do not change the
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destination manager, the new datastream will be written to the same target.
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If you do not change any JPEG parameters, the new datastream will be written
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with the same parameters as before. Note that you can change the input image
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dimensions freely between cycles, but if you change the input colorspace, you
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should call jpeg_set_defaults() to adjust for the new colorspace; and then
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you'll need to repeat all of step 3.
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7. Release the JPEG compression object.
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When you are done with a JPEG compression object, destroy it by calling
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jpeg_destroy_compress(). This will free all subsidiary memory (regardless of
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the previous state of the object). Or you can call jpeg_destroy(), which
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works for either compression or decompression objects --- this may be more
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convenient if you are sharing code between compression and decompression
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cases. (Actually, these routines are equivalent except for the declared type
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of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy()
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should be passed a j_common_ptr.)
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If you allocated the jpeg_compress_struct structure from malloc(), freeing
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it is your responsibility --- jpeg_destroy() won't. Ditto for the error
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jpeg_destroy_compress(&cinfo);
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If you decide to abort a compression cycle before finishing, you can clean up
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in either of two ways:
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* If you don't need the JPEG object any more, just call
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jpeg_destroy_compress() or jpeg_destroy() to release memory. This is
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legitimate at any point after calling jpeg_create_compress() --- in fact,
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it's safe even if jpeg_create_compress() fails.
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* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call
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jpeg_abort() which works on both compression and decompression objects.
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This will return the object to an idle state, releasing any working memory.
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jpeg_abort() is allowed at any time after successful object creation.
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Note that cleaning up the data destination, if required, is your
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responsibility; neither of these routines will call term_destination().
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(See "Compressed data handling", below, for more about that.)
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jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG
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object that has reported an error by calling error_exit (see "Error handling"
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for more info). The internal state of such an object is likely to be out of
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whack. Either of these two routines will return the object to a known state.
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Decompression details
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---------------------
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Here we revisit the JPEG decompression outline given in the overview.
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1. Allocate and initialize a JPEG decompression object.
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This is just like initialization for compression, as discussed above,
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except that the object is a "struct jpeg_decompress_struct" and you
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call jpeg_create_decompress(). Error handling is exactly the same.
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struct jpeg_decompress_struct cinfo;
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struct jpeg_error_mgr jerr;
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cinfo.err = jpeg_std_error(&jerr);
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jpeg_create_decompress(&cinfo);
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(Both here and in the IJG code, we usually use variable name "cinfo" for
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both compression and decompression objects.)
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2. Specify the source of the compressed data (eg, a file).
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As previously mentioned, the JPEG library reads compressed data from a "data
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source" module. The library includes one data source module which knows how
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to read from a stdio stream. You can use your own source module if you want
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to do something else, as discussed later.
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If you use the standard source module, you must open the source stdio stream
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beforehand. Typical code for this step looks like:
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if ((infile = fopen(filename, "rb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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jpeg_stdio_src(&cinfo, infile);
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where the last line invokes the standard source module.
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WARNING: it is critical that the binary compressed data be read unchanged.
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On non-Unix systems the stdio library may perform newline translation or
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otherwise corrupt binary data. To suppress this behavior, you may need to use
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a "b" option to fopen (as shown above), or use setmode() or another routine to
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put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that
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has been found to work on many systems.
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You may not change the data source between calling jpeg_read_header() and
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jpeg_finish_decompress(). If you wish to read a series of JPEG images from
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a single source file, you should repeat the jpeg_read_header() to
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jpeg_finish_decompress() sequence without reinitializing either the JPEG
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object or the data source module; this prevents buffered input data from
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3. Call jpeg_read_header() to obtain image info.
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Typical code for this step is just
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jpeg_read_header(&cinfo, TRUE);
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This will read the source datastream header markers, up to the beginning
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of the compressed data proper. On return, the image dimensions and other
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info have been stored in the JPEG object. The application may wish to
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consult this information before selecting decompression parameters.
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More complex code is necessary if
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* A suspending data source is used --- in that case jpeg_read_header()
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may return before it has read all the header data. See "I/O suspension",
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below. The normal stdio source manager will NOT cause this to happen.
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* Abbreviated JPEG files are to be processed --- see the section on
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abbreviated datastreams. Standard applications that deal only in
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interchange JPEG files need not be concerned with this case either.
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It is permissible to stop at this point if you just wanted to find out the
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image dimensions and other header info for a JPEG file. In that case,
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call jpeg_destroy() when you are done with the JPEG object, or call
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jpeg_abort() to return it to an idle state before selecting a new data
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source and reading another header.
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4. Set parameters for decompression.
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jpeg_read_header() sets appropriate default decompression parameters based on
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the properties of the image (in particular, its colorspace). However, you
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may well want to alter these defaults before beginning the decompression.
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For example, the default is to produce full color output from a color file.
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If you want colormapped output you must ask for it. Other options allow the
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returned image to be scaled and allow various speed/quality tradeoffs to be
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selected. "Decompression parameter selection", below, gives details.
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If the defaults are appropriate, nothing need be done at this step.
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Note that all default values are set by each call to jpeg_read_header().
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If you reuse a decompression object, you cannot expect your parameter
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settings to be preserved across cycles, as you can for compression.
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You must set desired parameter values each time.
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5. jpeg_start_decompress(...);
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Once the parameter values are satisfactory, call jpeg_start_decompress() to
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begin decompression. This will initialize internal state, allocate working
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memory, and prepare for returning data.
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jpeg_start_decompress(&cinfo);
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If you have requested a multi-pass operating mode, such as 2-pass color
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quantization, jpeg_start_decompress() will do everything needed before data
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output can begin. In this case jpeg_start_decompress() may take quite a while
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to complete. With a single-scan (non progressive) JPEG file and default
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decompression parameters, this will not happen; jpeg_start_decompress() will
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After this call, the final output image dimensions, including any requested
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scaling, are available in the JPEG object; so is the selected colormap, if
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colormapped output has been requested. Useful fields include
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output_width image width and height, as scaled
633
out_color_components # of color components in out_color_space
634
output_components # of color components returned per pixel
635
colormap the selected colormap, if any
636
actual_number_of_colors number of entries in colormap
638
output_components is 1 (a colormap index) when quantizing colors; otherwise it
639
equals out_color_components. It is the number of JSAMPLE values that will be
640
emitted per pixel in the output arrays.
642
Typically you will need to allocate data buffers to hold the incoming image.
643
You will need output_width * output_components JSAMPLEs per scanline in your
644
output buffer, and a total of output_height scanlines will be returned.
646
Note: if you are using the JPEG library's internal memory manager to allocate
647
data buffers (as djpeg does), then the manager's protocol requires that you
648
request large buffers *before* calling jpeg_start_decompress(). This is a
649
little tricky since the output_XXX fields are not normally valid then. You
650
can make them valid by calling jpeg_calc_output_dimensions() after setting the
651
relevant parameters (scaling, output color space, and quantization flag).
654
6. while (scan lines remain to be read)
655
jpeg_read_scanlines(...);
657
Now you can read the decompressed image data by calling jpeg_read_scanlines()
658
one or more times. At each call, you pass in the maximum number of scanlines
659
to be read (ie, the height of your working buffer); jpeg_read_scanlines()
660
will return up to that many lines. The return value is the number of lines
661
actually read. The format of the returned data is discussed under "Data
662
formats", above. Don't forget that grayscale and color JPEGs will return
663
different data formats!
665
Image data is returned in top-to-bottom scanline order. If you must write
666
out the image in bottom-to-top order, you can use the JPEG library's virtual
667
array mechanism to invert the data efficiently. Examples of this can be
668
found in the sample application djpeg.
670
The library maintains a count of the number of scanlines returned so far
671
in the output_scanline field of the JPEG object. Usually you can just use
672
this variable as the loop counter, so that the loop test looks like
673
"while (cinfo.output_scanline < cinfo.output_height)". (Note that the test
674
should NOT be against image_height, unless you never use scaling. The
675
image_height field is the height of the original unscaled image.)
676
The return value always equals the change in the value of output_scanline.
678
If you don't use a suspending data source, it is safe to assume that
679
jpeg_read_scanlines() reads at least one scanline per call, until the
680
bottom of the image has been reached.
682
If you use a buffer larger than one scanline, it is NOT safe to assume that
683
jpeg_read_scanlines() fills it. (The current implementation returns only a
684
few scanlines per call, no matter how large a buffer you pass.) So you must
685
always provide a loop that calls jpeg_read_scanlines() repeatedly until the
686
whole image has been read.
689
7. jpeg_finish_decompress(...);
691
After all the image data has been read, call jpeg_finish_decompress() to
692
complete the decompression cycle. This causes working memory associated
693
with the JPEG object to be released.
697
jpeg_finish_decompress(&cinfo);
699
If using the stdio source manager, don't forget to close the source stdio
702
It is an error to call jpeg_finish_decompress() before reading the correct
703
total number of scanlines. If you wish to abort decompression, call
704
jpeg_abort() as discussed below.
706
After completing a decompression cycle, you may dispose of the JPEG object as
707
discussed next, or you may use it to decompress another image. In that case
708
return to step 2 or 3 as appropriate. If you do not change the source
709
manager, the next image will be read from the same source.
712
8. Release the JPEG decompression object.
714
When you are done with a JPEG decompression object, destroy it by calling
715
jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of
716
destroying compression objects applies here too.
720
jpeg_destroy_decompress(&cinfo);
725
You can abort a decompression cycle by calling jpeg_destroy_decompress() or
726
jpeg_destroy() if you don't need the JPEG object any more, or
727
jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
728
The previous discussion of aborting compression cycles applies here too.
731
Mechanics of usage: include files, linking, etc
732
-----------------------------------------------
734
Applications using the JPEG library should include the header file jpeglib.h
735
to obtain declarations of data types and routines. Before including
736
jpeglib.h, include system headers that define at least the typedefs FILE and
737
size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on
738
older Unix systems, you may need <sys/types.h> to define size_t.
740
If the application needs to refer to individual JPEG library error codes, also
741
include jerror.h to define those symbols.
743
jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are
744
installing the JPEG header files in a system directory, you will want to
745
install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h.
747
The most convenient way to include the JPEG code into your executable program
748
is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
749
machines) and reference it at your link step. If you use only half of the
750
library (only compression or only decompression), only that much code will be
751
included from the library, unless your linker is hopelessly brain-damaged.
752
The supplied makefiles build libjpeg.a automatically (see install.txt).
754
While you can build the JPEG library as a shared library if the whim strikes
755
you, we don't really recommend it. The trouble with shared libraries is that
756
at some point you'll probably try to substitute a new version of the library
757
without recompiling the calling applications. That generally doesn't work
758
because the parameter struct declarations usually change with each new
759
version. In other words, the library's API is *not* guaranteed binary
760
compatible across versions; we only try to ensure source-code compatibility.
761
(In hindsight, it might have been smarter to hide the parameter structs from
762
applications and introduce a ton of access functions instead. Too late now,
765
On some systems your application may need to set up a signal handler to ensure
766
that temporary files are deleted if the program is interrupted. This is most
767
critical if you are on MS-DOS and use the jmemdos.c memory manager back end;
768
it will try to grab extended memory for temp files, and that space will NOT be
769
freed automatically. See cjpeg.c or djpeg.c for an example signal handler.
771
It may be worth pointing out that the core JPEG library does not actually
772
require the stdio library: only the default source/destination managers and
773
error handler need it. You can use the library in a stdio-less environment
774
if you replace those modules and use jmemnobs.c (or another memory manager of
775
your own devising). More info about the minimum system library requirements
776
may be found in jinclude.h.
782
Compression parameter selection
783
-------------------------------
785
This section describes all the optional parameters you can set for JPEG
786
compression, as well as the "helper" routines provided to assist in this
787
task. Proper setting of some parameters requires detailed understanding
788
of the JPEG standard; if you don't know what a parameter is for, it's best
789
not to mess with it! See REFERENCES in the README file for pointers to
790
more info about JPEG.
792
It's a good idea to call jpeg_set_defaults() first, even if you plan to set
793
all the parameters; that way your code is more likely to work with future JPEG
794
libraries that have additional parameters. For the same reason, we recommend
795
you use a helper routine where one is provided, in preference to twiddling
796
cinfo fields directly.
798
The helper routines are:
800
jpeg_set_defaults (j_compress_ptr cinfo)
801
This routine sets all JPEG parameters to reasonable defaults, using
802
only the input image's color space (field in_color_space, which must
803
already be set in cinfo). Many applications will only need to use
804
this routine and perhaps jpeg_set_quality().
806
jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace)
807
Sets the JPEG file's colorspace (field jpeg_color_space) as specified,
808
and sets other color-space-dependent parameters appropriately. See
809
"Special color spaces", below, before using this. A large number of
810
parameters, including all per-component parameters, are set by this
811
routine; if you want to twiddle individual parameters you should call
812
jpeg_set_colorspace() before rather than after.
814
jpeg_default_colorspace (j_compress_ptr cinfo)
815
Selects an appropriate JPEG colorspace based on cinfo->in_color_space,
816
and calls jpeg_set_colorspace(). This is actually a subroutine of
817
jpeg_set_defaults(). It's broken out in case you want to change
818
just the colorspace-dependent JPEG parameters.
820
jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline)
821
Constructs JPEG quantization tables appropriate for the indicated
822
quality setting. The quality value is expressed on the 0..100 scale
823
recommended by IJG (cjpeg's "-quality" switch uses this routine).
824
Note that the exact mapping from quality values to tables may change
825
in future IJG releases as more is learned about DCT quantization.
826
If the force_baseline parameter is TRUE, then the quantization table
827
entries are constrained to the range 1..255 for full JPEG baseline
828
compatibility. In the current implementation, this only makes a
829
difference for quality settings below 25, and it effectively prevents
830
very small/low quality files from being generated. The IJG decoder
831
is capable of reading the non-baseline files generated at low quality
832
settings when force_baseline is FALSE, but other decoders may not be.
834
jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor,
835
boolean force_baseline)
836
Same as jpeg_set_quality() except that the generated tables are the
837
sample tables given in the JPEC spec section K.1, multiplied by the
838
specified scale factor (which is expressed as a percentage; thus
839
scale_factor = 100 reproduces the spec's tables). Note that larger
840
scale factors give lower quality. This entry point is useful for
841
conforming to the Adobe PostScript DCT conventions, but we do not
842
recommend linear scaling as a user-visible quality scale otherwise.
843
force_baseline again constrains the computed table entries to 1..255.
845
int jpeg_quality_scaling (int quality)
846
Converts a value on the IJG-recommended quality scale to a linear
847
scaling percentage. Note that this routine may change or go away
848
in future releases --- IJG may choose to adopt a scaling method that
849
can't be expressed as a simple scalar multiplier, in which case the
850
premise of this routine collapses. Caveat user.
852
jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline)
853
Set default quantization tables with linear q_scale_factor[] values
856
jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl,
857
const unsigned int *basic_table,
858
int scale_factor, boolean force_baseline)
859
Allows an arbitrary quantization table to be created. which_tbl
860
indicates which table slot to fill. basic_table points to an array
861
of 64 unsigned ints given in normal array order. These values are
862
multiplied by scale_factor/100 and then clamped to the range 1..65535
863
(or to 1..255 if force_baseline is TRUE).
864
CAUTION: prior to library version 6a, jpeg_add_quant_table expected
865
the basic table to be given in JPEG zigzag order. If you need to
866
write code that works with either older or newer versions of this
867
routine, you must check the library version number. Something like
868
"#if JPEG_LIB_VERSION >= 61" is the right test.
870
jpeg_simple_progression (j_compress_ptr cinfo)
871
Generates a default scan script for writing a progressive-JPEG file.
872
This is the recommended method of creating a progressive file,
873
unless you want to make a custom scan sequence. You must ensure that
874
the JPEG color space is set correctly before calling this routine.
877
Compression parameters (cinfo fields) include:
879
J_DCT_METHOD dct_method
880
Selects the algorithm used for the DCT step. Choices are:
881
JDCT_ISLOW: slow but accurate integer algorithm
882
JDCT_IFAST: faster, less accurate integer method
883
JDCT_FLOAT: floating-point method
884
JDCT_DEFAULT: default method (normally JDCT_ISLOW)
885
JDCT_FASTEST: fastest method (normally JDCT_IFAST)
886
The FLOAT method is very slightly more accurate than the ISLOW method,
887
but may give different results on different machines due to varying
888
roundoff behavior. The integer methods should give the same results
889
on all machines. On machines with sufficiently fast FP hardware, the
890
floating-point method may also be the fastest. The IFAST method is
891
considerably less accurate than the other two; its use is not
892
recommended if high quality is a concern. JDCT_DEFAULT and
893
JDCT_FASTEST are macros configurable by each installation.
895
unsigned int scale_num, scale_denom
896
Scale the image by the fraction scale_num/scale_denom. Default is
897
1/1, or no scaling. Currently, the supported scaling ratios are
898
8/N with all N from 1 to 16. (The library design allows for arbitrary
899
scaling ratios but this is not likely to be implemented any time soon.)
901
J_COLOR_SPACE jpeg_color_space
903
The JPEG color space and corresponding number of components; see
904
"Special color spaces", below, for more info. We recommend using
905
jpeg_set_color_space() if you want to change these.
907
boolean optimize_coding
908
TRUE causes the compressor to compute optimal Huffman coding tables
909
for the image. This requires an extra pass over the data and
910
therefore costs a good deal of space and time. The default is
911
FALSE, which tells the compressor to use the supplied or default
912
Huffman tables. In most cases optimal tables save only a few percent
913
of file size compared to the default tables. Note that when this is
914
TRUE, you need not supply Huffman tables at all, and any you do
915
supply will be overwritten.
917
unsigned int restart_interval
919
To emit restart markers in the JPEG file, set one of these nonzero.
920
Set restart_interval to specify the exact interval in MCU blocks.
921
Set restart_in_rows to specify the interval in MCU rows. (If
922
restart_in_rows is not 0, then restart_interval is set after the
923
image width in MCUs is computed.) Defaults are zero (no restarts).
924
One restart marker per MCU row is often a good choice.
925
NOTE: the overhead of restart markers is higher in grayscale JPEG
926
files than in color files, and MUCH higher in progressive JPEGs.
927
If you use restarts, you may want to use larger intervals in those
930
const jpeg_scan_info * scan_info
932
By default, scan_info is NULL; this causes the compressor to write a
933
single-scan sequential JPEG file. If not NULL, scan_info points to
934
an array of scan definition records of length num_scans. The
935
compressor will then write a JPEG file having one scan for each scan
936
definition record. This is used to generate noninterleaved or
937
progressive JPEG files. The library checks that the scan array
938
defines a valid JPEG scan sequence. (jpeg_simple_progression creates
939
a suitable scan definition array for progressive JPEG.) This is
940
discussed further under "Progressive JPEG support".
942
boolean do_fancy_downsampling
943
If TRUE, use direct DCT scaling with DCT size > 8 for downsampling
944
of chroma components.
945
If FALSE, use only DCT size <= 8 and simple separate downsampling.
947
For better image stability in multiple generation compression cycles
948
it is preferable that this value matches the corresponding
949
do_fancy_upsampling value in decompression.
952
If non-zero, the input image is smoothed; the value should be 1 for
953
minimal smoothing to 100 for maximum smoothing. Consult jcsample.c
954
for details of the smoothing algorithm. The default is zero.
956
boolean write_JFIF_header
957
If TRUE, a JFIF APP0 marker is emitted. jpeg_set_defaults() and
958
jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space
959
(ie, YCbCr or grayscale) is selected, otherwise FALSE.
961
UINT8 JFIF_major_version
962
UINT8 JFIF_minor_version
963
The version number to be written into the JFIF marker.
964
jpeg_set_defaults() initializes the version to 1.01 (major=minor=1).
965
You should set it to 1.02 (major=1, minor=2) if you plan to write
966
any JFIF 1.02 extension markers.
971
The resolution information to be written into the JFIF marker;
972
not used otherwise. density_unit may be 0 for unknown,
973
1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1
974
indicating square pixels of unknown size.
976
boolean write_Adobe_marker
977
If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and
978
jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK,
979
or YCCK is selected, otherwise FALSE. It is generally a bad idea
980
to set both write_JFIF_header and write_Adobe_marker. In fact,
981
you probably shouldn't change the default settings at all --- the
982
default behavior ensures that the JPEG file's color space can be
983
recognized by the decoder.
985
JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS]
986
Pointers to coefficient quantization tables, one per table slot,
987
or NULL if no table is defined for a slot. Usually these should
988
be set via one of the above helper routines; jpeg_add_quant_table()
989
is general enough to define any quantization table. The other
990
routines will set up table slot 0 for luminance quality and table
991
slot 1 for chrominance.
993
int q_scale_factor[NUM_QUANT_TBLS]
994
Linear quantization scaling factors (percentage, initialized 100)
995
for use with jpeg_default_qtables().
996
See rdswitch.c and cjpeg.c for an example of usage.
997
Note that the q_scale_factor[] fields are the "linear" scales, so you
998
have to convert from user-defined ratings via jpeg_quality_scaling().
999
Here is an example code which corresponds to cjpeg -quality 90,70:
1001
jpeg_set_defaults(cinfo);
1003
/* Set luminance quality 90. */
1004
cinfo->q_scale_factor[0] = jpeg_quality_scaling(90);
1005
/* Set chrominance quality 70. */
1006
cinfo->q_scale_factor[1] = jpeg_quality_scaling(70);
1008
jpeg_default_qtables(cinfo, force_baseline);
1010
CAUTION: You must also set 1x1 subsampling for efficient separate
1011
color quality selection, since the default value used by library
1014
cinfo->comp_info[0].v_samp_factor = 1;
1015
cinfo->comp_info[0].h_samp_factor = 1;
1017
JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS]
1018
JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS]
1019
Pointers to Huffman coding tables, one per table slot, or NULL if
1020
no table is defined for a slot. Slots 0 and 1 are filled with the
1021
JPEG sample tables by jpeg_set_defaults(). If you need to allocate
1022
more table structures, jpeg_alloc_huff_table() may be used.
1023
Note that optimal Huffman tables can be computed for an image
1024
by setting optimize_coding, as discussed above; there's seldom
1025
any need to mess with providing your own Huffman tables.
1028
The actual dimensions of the JPEG image that will be written to the file are
1029
given by the following fields. These are computed from the input image
1030
dimensions and the compression parameters by jpeg_start_compress(). You can
1031
also call jpeg_calc_jpeg_dimensions() to obtain the values that will result
1032
from the current parameter settings. This can be useful if you are trying
1033
to pick a scaling ratio that will get close to a desired target size.
1035
JDIMENSION jpeg_width Actual dimensions of output image.
1036
JDIMENSION jpeg_height
1039
Per-component parameters are stored in the struct cinfo.comp_info[i] for
1040
component number i. Note that components here refer to components of the
1041
JPEG color space, *not* the source image color space. A suitably large
1042
comp_info[] array is allocated by jpeg_set_defaults(); if you choose not
1043
to use that routine, it's up to you to allocate the array.
1046
The one-byte identifier code to be recorded in the JPEG file for
1047
this component. For the standard color spaces, we recommend you
1048
leave the default values alone.
1052
Horizontal and vertical sampling factors for the component; must
1053
be 1..4 according to the JPEG standard. Note that larger sampling
1054
factors indicate a higher-resolution component; many people find
1055
this behavior quite unintuitive. The default values are 2,2 for
1056
luminance components and 1,1 for chrominance components, except
1057
for grayscale where 1,1 is used.
1060
Quantization table number for component. The default value is
1061
0 for luminance components and 1 for chrominance components.
1065
DC and AC entropy coding table numbers. The default values are
1066
0 for luminance components and 1 for chrominance components.
1069
Must equal the component's index in comp_info[]. (Beginning in
1070
release v6, the compressor library will fill this in automatically;
1074
Decompression parameter selection
1075
---------------------------------
1077
Decompression parameter selection is somewhat simpler than compression
1078
parameter selection, since all of the JPEG internal parameters are
1079
recorded in the source file and need not be supplied by the application.
1080
(Unless you are working with abbreviated files, in which case see
1081
"Abbreviated datastreams", below.) Decompression parameters control
1082
the postprocessing done on the image to deliver it in a format suitable
1083
for the application's use. Many of the parameters control speed/quality
1084
tradeoffs, in which faster decompression may be obtained at the price of
1085
a poorer-quality image. The defaults select the highest quality (slowest)
1088
The following fields in the JPEG object are set by jpeg_read_header() and
1089
may be useful to the application in choosing decompression parameters:
1091
JDIMENSION image_width Width and height of image
1092
JDIMENSION image_height
1093
int num_components Number of color components
1094
J_COLOR_SPACE jpeg_color_space Colorspace of image
1095
boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen
1096
UINT8 JFIF_major_version Version information from JFIF marker
1097
UINT8 JFIF_minor_version
1098
UINT8 density_unit Resolution data from JFIF marker
1101
boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen
1102
UINT8 Adobe_transform Color transform code from Adobe marker
1104
The JPEG color space, unfortunately, is something of a guess since the JPEG
1105
standard proper does not provide a way to record it. In practice most files
1106
adhere to the JFIF or Adobe conventions, and the decoder will recognize these
1107
correctly. See "Special color spaces", below, for more info.
1110
The decompression parameters that determine the basic properties of the
1113
J_COLOR_SPACE out_color_space
1114
Output color space. jpeg_read_header() sets an appropriate default
1115
based on jpeg_color_space; typically it will be RGB or grayscale.
1116
The application can change this field to request output in a different
1117
colorspace. For example, set it to JCS_GRAYSCALE to get grayscale
1118
output from a color file. (This is useful for previewing: grayscale
1119
output is faster than full color since the color components need not
1120
be processed.) Note that not all possible color space transforms are
1121
currently implemented; you may need to extend jdcolor.c if you want an
1124
unsigned int scale_num, scale_denom
1125
Scale the image by the fraction scale_num/scale_denom. Currently,
1126
the supported scaling ratios are M/N with all M from 1 to 16, where
1127
N is the source DCT size, which is 8 for baseline JPEG. (The library
1128
design allows for arbitrary scaling ratios but this is not likely
1129
to be implemented any time soon.) The values are initialized by
1130
jpeg_read_header() with the source DCT size. For baseline JPEG
1131
this is 8/8. If you change only the scale_num value while leaving
1132
the other unchanged, then this specifies the DCT scaled size to be
1133
applied on the given input. For baseline JPEG this is equivalent
1134
to M/8 scaling, since the source DCT size for baseline JPEG is 8.
1135
Smaller scaling ratios permit significantly faster decoding since
1136
fewer pixels need be processed and a simpler IDCT method can be used.
1138
boolean quantize_colors
1139
If set TRUE, colormapped output will be delivered. Default is FALSE,
1140
meaning that full-color output will be delivered.
1142
The next three parameters are relevant only if quantize_colors is TRUE.
1144
int desired_number_of_colors
1145
Maximum number of colors to use in generating a library-supplied color
1146
map (the actual number of colors is returned in a different field).
1147
Default 256. Ignored when the application supplies its own color map.
1149
boolean two_pass_quantize
1150
If TRUE, an extra pass over the image is made to select a custom color
1151
map for the image. This usually looks a lot better than the one-size-
1152
fits-all colormap that is used otherwise. Default is TRUE. Ignored
1153
when the application supplies its own color map.
1155
J_DITHER_MODE dither_mode
1156
Selects color dithering method. Supported values are:
1157
JDITHER_NONE no dithering: fast, very low quality
1158
JDITHER_ORDERED ordered dither: moderate speed and quality
1159
JDITHER_FS Floyd-Steinberg dither: slow, high quality
1160
Default is JDITHER_FS. (At present, ordered dither is implemented
1161
only in the single-pass, standard-colormap case. If you ask for
1162
ordered dither when two_pass_quantize is TRUE or when you supply
1163
an external color map, you'll get F-S dithering.)
1165
When quantize_colors is TRUE, the target color map is described by the next
1166
two fields. colormap is set to NULL by jpeg_read_header(). The application
1167
can supply a color map by setting colormap non-NULL and setting
1168
actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress()
1169
selects a suitable color map and sets these two fields itself.
1170
[Implementation restriction: at present, an externally supplied colormap is
1171
only accepted for 3-component output color spaces.]
1174
The color map, represented as a 2-D pixel array of out_color_components
1175
rows and actual_number_of_colors columns. Ignored if not quantizing.
1176
CAUTION: if the JPEG library creates its own colormap, the storage
1177
pointed to by this field is released by jpeg_finish_decompress().
1178
Copy the colormap somewhere else first, if you want to save it.
1180
int actual_number_of_colors
1181
The number of colors in the color map.
1183
Additional decompression parameters that the application may set include:
1185
J_DCT_METHOD dct_method
1186
Selects the algorithm used for the DCT step. Choices are the same
1187
as described above for compression.
1189
boolean do_fancy_upsampling
1190
If TRUE, use direct DCT scaling with DCT size > 8 for upsampling
1191
of chroma components.
1192
If FALSE, use only DCT size <= 8 and simple separate upsampling.
1194
For better image stability in multiple generation compression cycles
1195
it is preferable that this value matches the corresponding
1196
do_fancy_downsampling value in compression.
1198
boolean do_block_smoothing
1199
If TRUE, interblock smoothing is applied in early stages of decoding
1200
progressive JPEG files; if FALSE, not. Default is TRUE. Early
1201
progression stages look "fuzzy" with smoothing, "blocky" without.
1202
In any case, block smoothing ceases to be applied after the first few
1203
AC coefficients are known to full accuracy, so it is relevant only
1204
when using buffered-image mode for progressive images.
1206
boolean enable_1pass_quant
1207
boolean enable_external_quant
1208
boolean enable_2pass_quant
1209
These are significant only in buffered-image mode, which is
1210
described in its own section below.
1213
The output image dimensions are given by the following fields. These are
1214
computed from the source image dimensions and the decompression parameters
1215
by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions()
1216
to obtain the values that will result from the current parameter settings.
1217
This can be useful if you are trying to pick a scaling ratio that will get
1218
close to a desired target size. It's also important if you are using the
1219
JPEG library's memory manager to allocate output buffer space, because you
1220
are supposed to request such buffers *before* jpeg_start_decompress().
1222
JDIMENSION output_width Actual dimensions of output image.
1223
JDIMENSION output_height
1224
int out_color_components Number of color components in out_color_space.
1225
int output_components Number of color components returned.
1226
int rec_outbuf_height Recommended height of scanline buffer.
1228
When quantizing colors, output_components is 1, indicating a single color map
1229
index per pixel. Otherwise it equals out_color_components. The output arrays
1230
are required to be output_width * output_components JSAMPLEs wide.
1232
rec_outbuf_height is the recommended minimum height (in scanlines) of the
1233
buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the
1234
library will still work, but time will be wasted due to unnecessary data
1235
copying. In high-quality modes, rec_outbuf_height is always 1, but some
1236
faster, lower-quality modes set it to larger values (typically 2 to 4).
1237
If you are going to ask for a high-speed processing mode, you may as well
1238
go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
1239
(An output buffer larger than rec_outbuf_height lines is OK, but won't
1240
provide any material speed improvement over that height.)
1243
Special color spaces
1244
--------------------
1246
The JPEG standard itself is "color blind" and doesn't specify any particular
1247
color space. It is customary to convert color data to a luminance/chrominance
1248
color space before compressing, since this permits greater compression. The
1249
existing de-facto JPEG file format standards specify YCbCr or grayscale data
1250
(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special
1251
applications such as multispectral images, other color spaces can be used,
1252
but it must be understood that such files will be unportable.
1254
The JPEG library can handle the most common colorspace conversions (namely
1255
RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown
1256
color space, passing it through without conversion. If you deal extensively
1257
with an unusual color space, you can easily extend the library to understand
1258
additional color spaces and perform appropriate conversions.
1260
For compression, the source data's color space is specified by field
1261
in_color_space. This is transformed to the JPEG file's color space given
1262
by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color
1263
space depending on in_color_space, but you can override this by calling
1264
jpeg_set_colorspace(). Of course you must select a supported transformation.
1265
jccolor.c currently supports the following transformations:
1270
plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
1271
YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.
1273
The de-facto file format standards (JFIF and Adobe) specify APPn markers that
1274
indicate the color space of the JPEG file. It is important to ensure that
1275
these are written correctly, or omitted if the JPEG file's color space is not
1276
one of the ones supported by the de-facto standards. jpeg_set_colorspace()
1277
will set the compression parameters to include or omit the APPn markers
1278
properly, so long as it is told the truth about the JPEG color space.
1279
For example, if you are writing some random 3-component color space without
1280
conversion, don't try to fake out the library by setting in_color_space and
1281
jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an
1282
APPn marker of your own devising to identify the colorspace --- see "Special
1285
When told that the color space is UNKNOWN, the library will default to using
1286
luminance-quality compression parameters for all color components. You may
1287
well want to change these parameters. See the source code for
1288
jpeg_set_colorspace(), in jcparam.c, for details.
1290
For decompression, the JPEG file's color space is given in jpeg_color_space,
1291
and this is transformed to the output color space out_color_space.
1292
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
1293
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
1294
guess. If you know the JPEG file's color space for certain, you can override
1295
jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also
1296
selects a default output color space based on (its guess of) jpeg_color_space;
1297
set out_color_space to override this. Again, you must select a supported
1298
transformation. jdcolor.c currently supports
1303
as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an
1304
application can force grayscale JPEGs to look like color JPEGs if it only
1305
wants to handle one case.)
1307
The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
1308
(it weights distances appropriately for RGB colors). You'll need to modify
1309
the code if you want to use it for non-RGB output color spaces. Note that
1310
jquant2.c is used to map to an application-supplied colormap as well as for
1311
the normal two-pass colormap selection process.
1313
CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
1314
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
1315
This is arguably a bug in Photoshop, but if you need to work with Photoshop
1316
CMYK files, you will have to deal with it in your application. We cannot
1317
"fix" this in the library by inverting the data during the CMYK<=>YCCK
1318
transform, because that would break other applications, notably Ghostscript.
1319
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
1320
data in the same inverted-YCCK representation used in bare JPEG files, but
1321
the surrounding PostScript code performs an inversion using the PS image
1322
operator. I am told that Photoshop 3.0 will write uninverted YCCK in
1323
EPS/JPEG files, and will omit the PS-level inversion. (But the data
1324
polarity used in bare JPEG files will not change in 3.0.) In either case,
1325
the JPEG library must not invert the data itself, or else Ghostscript would
1326
read these EPS files incorrectly.
1332
When the default error handler is used, any error detected inside the JPEG
1333
routines will cause a message to be printed on stderr, followed by exit().
1334
You can supply your own error handling routines to override this behavior
1335
and to control the treatment of nonfatal warnings and trace/debug messages.
1336
The file example.c illustrates the most common case, which is to have the
1337
application regain control after an error rather than exiting.
1339
The JPEG library never writes any message directly; it always goes through
1340
the error handling routines. Three classes of messages are recognized:
1341
* Fatal errors: the library cannot continue.
1342
* Warnings: the library can continue, but the data is corrupt, and a
1343
damaged output image is likely to result.
1344
* Trace/informational messages. These come with a trace level indicating
1345
the importance of the message; you can control the verbosity of the
1346
program by adjusting the maximum trace level that will be displayed.
1348
You may, if you wish, simply replace the entire JPEG error handling module
1349
(jerror.c) with your own code. However, you can avoid code duplication by
1350
only replacing some of the routines depending on the behavior you need.
1351
This is accomplished by calling jpeg_std_error() as usual, but then overriding
1352
some of the method pointers in the jpeg_error_mgr struct, as illustrated by
1355
All of the error handling routines will receive a pointer to the JPEG object
1356
(a j_common_ptr which points to either a jpeg_compress_struct or a
1357
jpeg_decompress_struct; if you need to tell which, test the is_decompressor
1358
field). This struct includes a pointer to the error manager struct in its
1359
"err" field. Frequently, custom error handler routines will need to access
1360
additional data which is not known to the JPEG library or the standard error
1361
handler. The most convenient way to do this is to embed either the JPEG
1362
object or the jpeg_error_mgr struct in a larger structure that contains
1363
additional fields; then casting the passed pointer provides access to the
1364
additional fields. Again, see example.c for one way to do it. (Beginning
1365
with IJG version 6b, there is also a void pointer "client_data" in each
1366
JPEG object, which the application can also use to find related data.
1367
The library does not touch client_data at all.)
1369
The individual methods that you might wish to override are:
1371
error_exit (j_common_ptr cinfo)
1372
Receives control for a fatal error. Information sufficient to
1373
generate the error message has been stored in cinfo->err; call
1374
output_message to display it. Control must NOT return to the caller;
1375
generally this routine will exit() or longjmp() somewhere.
1376
Typically you would override this routine to get rid of the exit()
1377
default behavior. Note that if you continue processing, you should
1378
clean up the JPEG object with jpeg_abort() or jpeg_destroy().
1380
output_message (j_common_ptr cinfo)
1381
Actual output of any JPEG message. Override this to send messages
1382
somewhere other than stderr. Note that this method does not know
1383
how to generate a message, only where to send it.
1385
format_message (j_common_ptr cinfo, char * buffer)
1386
Constructs a readable error message string based on the error info
1387
stored in cinfo->err. This method is called by output_message. Few
1388
applications should need to override this method. One possible
1389
reason for doing so is to implement dynamic switching of error message
1392
emit_message (j_common_ptr cinfo, int msg_level)
1393
Decide whether or not to emit a warning or trace message; if so,
1394
calls output_message. The main reason for overriding this method
1395
would be to abort on warnings. msg_level is -1 for warnings,
1396
0 and up for trace messages.
1398
Only error_exit() and emit_message() are called from the rest of the JPEG
1399
library; the other two are internal to the error handler.
1401
The actual message texts are stored in an array of strings which is pointed to
1402
by the field err->jpeg_message_table. The messages are numbered from 0 to
1403
err->last_jpeg_message, and it is these code numbers that are used in the
1404
JPEG library code. You could replace the message texts (for instance, with
1405
messages in French or German) by changing the message table pointer. See
1406
jerror.h for the default texts. CAUTION: this table will almost certainly
1407
change or grow from one library version to the next.
1409
It may be useful for an application to add its own message texts that are
1410
handled by the same mechanism. The error handler supports a second "add-on"
1411
message table for this purpose. To define an addon table, set the pointer
1412
err->addon_message_table and the message numbers err->first_addon_message and
1413
err->last_addon_message. If you number the addon messages beginning at 1000
1414
or so, you won't have to worry about conflicts with the library's built-in
1415
messages. See the sample applications cjpeg/djpeg for an example of using
1416
addon messages (the addon messages are defined in cderror.h).
1418
Actual invocation of the error handler is done via macros defined in jerror.h:
1419
ERREXITn(...) for fatal errors
1420
WARNMSn(...) for corrupt-data warnings
1421
TRACEMSn(...) for trace and informational messages.
1422
These macros store the message code and any additional parameters into the
1423
error handler struct, then invoke the error_exit() or emit_message() method.
1424
The variants of each macro are for varying numbers of additional parameters.
1425
The additional parameters are inserted into the generated message using
1426
standard printf() format codes.
1428
See jerror.h and jerror.c for further details.
1431
Compressed data handling (source and destination managers)
1432
----------------------------------------------------------
1434
The JPEG compression library sends its compressed data to a "destination
1435
manager" module. The default destination manager just writes the data to a
1436
memory buffer or to a stdio stream, but you can provide your own manager to
1437
do something else. Similarly, the decompression library calls a "source
1438
manager" to obtain the compressed data; you can provide your own source
1439
manager if you want the data to come from somewhere other than a memory
1440
buffer or a stdio stream.
1442
In both cases, compressed data is processed a bufferload at a time: the
1443
destination or source manager provides a work buffer, and the library invokes
1444
the manager only when the buffer is filled or emptied. (You could define a
1445
one-character buffer to force the manager to be invoked for each byte, but
1446
that would be rather inefficient.) The buffer's size and location are
1447
controlled by the manager, not by the library. For example, the memory
1448
source manager just makes the buffer pointer and length point to the original
1449
data in memory. In this case the buffer-reload procedure will be invoked
1450
only if the decompressor ran off the end of the datastream, which would
1451
indicate an erroneous datastream.
1453
The work buffer is defined as an array of datatype JOCTET, which is generally
1454
"char" or "unsigned char". On a machine where char is not exactly 8 bits
1455
wide, you must define JOCTET as a wider data type and then modify the data
1456
source and destination modules to transcribe the work arrays into 8-bit units
1457
on external storage.
1459
A data destination manager struct contains a pointer and count defining the
1460
next byte to write in the work buffer and the remaining free space:
1462
JOCTET * next_output_byte; /* => next byte to write in buffer */
1463
size_t free_in_buffer; /* # of byte spaces remaining in buffer */
1465
The library increments the pointer and decrements the count until the buffer
1466
is filled. The manager's empty_output_buffer method must reset the pointer
1467
and count. The manager is expected to remember the buffer's starting address
1468
and total size in private fields not visible to the library.
1470
A data destination manager provides three methods:
1472
init_destination (j_compress_ptr cinfo)
1473
Initialize destination. This is called by jpeg_start_compress()
1474
before any data is actually written. It must initialize
1475
next_output_byte and free_in_buffer. free_in_buffer must be
1476
initialized to a positive value.
1478
empty_output_buffer (j_compress_ptr cinfo)
1479
This is called whenever the buffer has filled (free_in_buffer
1480
reaches zero). In typical applications, it should write out the
1481
*entire* buffer (use the saved start address and buffer length;
1482
ignore the current state of next_output_byte and free_in_buffer).
1483
Then reset the pointer & count to the start of the buffer, and
1484
return TRUE indicating that the buffer has been dumped.
1485
free_in_buffer must be set to a positive value when TRUE is
1486
returned. A FALSE return should only be used when I/O suspension is
1487
desired (this operating mode is discussed in the next section).
1489
term_destination (j_compress_ptr cinfo)
1490
Terminate destination --- called by jpeg_finish_compress() after all
1491
data has been written. In most applications, this must flush any
1492
data remaining in the buffer. Use either next_output_byte or
1493
free_in_buffer to determine how much data is in the buffer.
1495
term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you
1496
want the destination manager to be cleaned up during an abort, you must do it
1499
You will also need code to create a jpeg_destination_mgr struct, fill in its
1500
method pointers, and insert a pointer to the struct into the "dest" field of
1501
the JPEG compression object. This can be done in-line in your setup code if
1502
you like, but it's probably cleaner to provide a separate routine similar to
1503
the jpeg_stdio_dest() or jpeg_mem_dest() routines of the supplied destination
1506
Decompression source managers follow a parallel design, but with some
1507
additional frammishes. The source manager struct contains a pointer and count
1508
defining the next byte to read from the work buffer and the number of bytes
1511
const JOCTET * next_input_byte; /* => next byte to read from buffer */
1512
size_t bytes_in_buffer; /* # of bytes remaining in buffer */
1514
The library increments the pointer and decrements the count until the buffer
1515
is emptied. The manager's fill_input_buffer method must reset the pointer and
1516
count. In most applications, the manager must remember the buffer's starting
1517
address and total size in private fields not visible to the library.
1519
A data source manager provides five methods:
1521
init_source (j_decompress_ptr cinfo)
1522
Initialize source. This is called by jpeg_read_header() before any
1523
data is actually read. Unlike init_destination(), it may leave
1524
bytes_in_buffer set to 0 (in which case a fill_input_buffer() call
1525
will occur immediately).
1527
fill_input_buffer (j_decompress_ptr cinfo)
1528
This is called whenever bytes_in_buffer has reached zero and more
1529
data is wanted. In typical applications, it should read fresh data
1530
into the buffer (ignoring the current state of next_input_byte and
1531
bytes_in_buffer), reset the pointer & count to the start of the
1532
buffer, and return TRUE indicating that the buffer has been reloaded.
1533
It is not necessary to fill the buffer entirely, only to obtain at
1534
least one more byte. bytes_in_buffer MUST be set to a positive value
1535
if TRUE is returned. A FALSE return should only be used when I/O
1536
suspension is desired (this mode is discussed in the next section).
1538
skip_input_data (j_decompress_ptr cinfo, long num_bytes)
1539
Skip num_bytes worth of data. The buffer pointer and count should
1540
be advanced over num_bytes input bytes, refilling the buffer as
1541
needed. This is used to skip over a potentially large amount of
1542
uninteresting data (such as an APPn marker). In some applications
1543
it may be possible to optimize away the reading of the skipped data,
1544
but it's not clear that being smart is worth much trouble; large
1545
skips are uncommon. bytes_in_buffer may be zero on return.
1546
A zero or negative skip count should be treated as a no-op.
1548
resync_to_restart (j_decompress_ptr cinfo, int desired)
1549
This routine is called only when the decompressor has failed to find
1550
a restart (RSTn) marker where one is expected. Its mission is to
1551
find a suitable point for resuming decompression. For most
1552
applications, we recommend that you just use the default resync
1553
procedure, jpeg_resync_to_restart(). However, if you are able to back
1554
up in the input data stream, or if you have a-priori knowledge about
1555
the likely location of restart markers, you may be able to do better.
1556
Read the read_restart_marker() and jpeg_resync_to_restart() routines
1557
in jdmarker.c if you think you'd like to implement your own resync
1560
term_source (j_decompress_ptr cinfo)
1561
Terminate source --- called by jpeg_finish_decompress() after all
1562
data has been read. Often a no-op.
1564
For both fill_input_buffer() and skip_input_data(), there is no such thing
1565
as an EOF return. If the end of the file has been reached, the routine has
1566
a choice of exiting via ERREXIT() or inserting fake data into the buffer.
1567
In most cases, generating a warning message and inserting a fake EOI marker
1568
is the best course of action --- this will allow the decompressor to output
1569
however much of the image is there. In pathological cases, the decompressor
1570
may swallow the EOI and again demand data ... just keep feeding it fake EOIs.
1571
jdatasrc.c illustrates the recommended error recovery behavior.
1573
term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want
1574
the source manager to be cleaned up during an abort, you must do it yourself.
1576
You will also need code to create a jpeg_source_mgr struct, fill in its method
1577
pointers, and insert a pointer to the struct into the "src" field of the JPEG
1578
decompression object. This can be done in-line in your setup code if you
1579
like, but it's probably cleaner to provide a separate routine similar to the
1580
jpeg_stdio_src() or jpeg_mem_src() routines of the supplied source managers.
1582
For more information, consult the memory and stdio source and destination
1583
managers in jdatasrc.c and jdatadst.c.
1589
Some applications need to use the JPEG library as an incremental memory-to-
1590
memory filter: when the compressed data buffer is filled or emptied, they want
1591
control to return to the outer loop, rather than expecting that the buffer can
1592
be emptied or reloaded within the data source/destination manager subroutine.
1593
The library supports this need by providing an "I/O suspension" mode, which we
1594
describe in this section.
1596
The I/O suspension mode is not a panacea: nothing is guaranteed about the
1597
maximum amount of time spent in any one call to the library, so it will not
1598
eliminate response-time problems in single-threaded applications. If you
1599
need guaranteed response time, we suggest you "bite the bullet" and implement
1600
a real multi-tasking capability.
1602
To use I/O suspension, cooperation is needed between the calling application
1603
and the data source or destination manager; you will always need a custom
1604
source/destination manager. (Please read the previous section if you haven't
1605
already.) The basic idea is that the empty_output_buffer() or
1606
fill_input_buffer() routine is a no-op, merely returning FALSE to indicate
1607
that it has done nothing. Upon seeing this, the JPEG library suspends
1608
operation and returns to its caller. The surrounding application is
1609
responsible for emptying or refilling the work buffer before calling the
1612
Compression suspension:
1614
For compression suspension, use an empty_output_buffer() routine that returns
1615
FALSE; typically it will not do anything else. This will cause the
1616
compressor to return to the caller of jpeg_write_scanlines(), with the return
1617
value indicating that not all the supplied scanlines have been accepted.
1618
The application must make more room in the output buffer, adjust the output
1619
buffer pointer/count appropriately, and then call jpeg_write_scanlines()
1620
again, pointing to the first unconsumed scanline.
1622
When forced to suspend, the compressor will backtrack to a convenient stopping
1623
point (usually the start of the current MCU); it will regenerate some output
1624
data when restarted. Therefore, although empty_output_buffer() is only
1625
called when the buffer is filled, you should NOT write out the entire buffer
1626
after a suspension. Write only the data up to the current position of
1627
next_output_byte/free_in_buffer. The data beyond that point will be
1628
regenerated after resumption.
1630
Because of the backtracking behavior, a good-size output buffer is essential
1631
for efficiency; you don't want the compressor to suspend often. (In fact, an
1632
overly small buffer could lead to infinite looping, if a single MCU required
1633
more data than would fit in the buffer.) We recommend a buffer of at least
1634
several Kbytes. You may want to insert explicit code to ensure that you don't
1635
call jpeg_write_scanlines() unless there is a reasonable amount of space in
1636
the output buffer; in other words, flush the buffer before trying to compress
1639
The compressor does not allow suspension while it is trying to write JPEG
1640
markers at the beginning and end of the file. This means that:
1641
* At the beginning of a compression operation, there must be enough free
1642
space in the output buffer to hold the header markers (typically 600 or
1643
so bytes). The recommended buffer size is bigger than this anyway, so
1644
this is not a problem as long as you start with an empty buffer. However,
1645
this restriction might catch you if you insert large special markers, such
1646
as a JFIF thumbnail image, without flushing the buffer afterwards.
1647
* When you call jpeg_finish_compress(), there must be enough space in the
1648
output buffer to emit any buffered data and the final EOI marker. In the
1649
current implementation, half a dozen bytes should suffice for this, but
1650
for safety's sake we recommend ensuring that at least 100 bytes are free
1651
before calling jpeg_finish_compress().
1653
A more significant restriction is that jpeg_finish_compress() cannot suspend.
1654
This means you cannot use suspension with multi-pass operating modes, namely
1655
Huffman code optimization and multiple-scan output. Those modes write the
1656
whole file during jpeg_finish_compress(), which will certainly result in
1657
buffer overrun. (Note that this restriction applies only to compression,
1658
not decompression. The decompressor supports input suspension in all of its
1661
Decompression suspension:
1663
For decompression suspension, use a fill_input_buffer() routine that simply
1664
returns FALSE (except perhaps during error recovery, as discussed below).
1665
This will cause the decompressor to return to its caller with an indication
1666
that suspension has occurred. This can happen at four places:
1667
* jpeg_read_header(): will return JPEG_SUSPENDED.
1668
* jpeg_start_decompress(): will return FALSE, rather than its usual TRUE.
1669
* jpeg_read_scanlines(): will return the number of scanlines already
1670
completed (possibly 0).
1671
* jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE.
1672
The surrounding application must recognize these cases, load more data into
1673
the input buffer, and repeat the call. In the case of jpeg_read_scanlines(),
1674
increment the passed pointers past any scanlines successfully read.
1676
Just as with compression, the decompressor will typically backtrack to a
1677
convenient restart point before suspending. When fill_input_buffer() is
1678
called, next_input_byte/bytes_in_buffer point to the current restart point,
1679
which is where the decompressor will backtrack to if FALSE is returned.
1680
The data beyond that position must NOT be discarded if you suspend; it needs
1681
to be re-read upon resumption. In most implementations, you'll need to shift
1682
this data down to the start of your work buffer and then load more data after
1683
it. Again, this behavior means that a several-Kbyte work buffer is essential
1684
for decent performance; furthermore, you should load a reasonable amount of
1685
new data before resuming decompression. (If you loaded, say, only one new
1686
byte each time around, you could waste a LOT of cycles.)
1688
The skip_input_data() source manager routine requires special care in a
1689
suspension scenario. This routine is NOT granted the ability to suspend the
1690
decompressor; it can decrement bytes_in_buffer to zero, but no more. If the
1691
requested skip distance exceeds the amount of data currently in the input
1692
buffer, then skip_input_data() must set bytes_in_buffer to zero and record the
1693
additional skip distance somewhere else. The decompressor will immediately
1694
call fill_input_buffer(), which should return FALSE, which will cause a
1695
suspension return. The surrounding application must then arrange to discard
1696
the recorded number of bytes before it resumes loading the input buffer.
1697
(Yes, this design is rather baroque, but it avoids complexity in the far more
1698
common case where a non-suspending source manager is used.)
1700
If the input data has been exhausted, we recommend that you emit a warning
1701
and insert dummy EOI markers just as a non-suspending data source manager
1702
would do. This can be handled either in the surrounding application logic or
1703
within fill_input_buffer(); the latter is probably more efficient. If
1704
fill_input_buffer() knows that no more data is available, it can set the
1705
pointer/count to point to a dummy EOI marker and then return TRUE just as
1706
though it had read more data in a non-suspending situation.
1708
The decompressor does not attempt to suspend within standard JPEG markers;
1709
instead it will backtrack to the start of the marker and reprocess the whole
1710
marker next time. Hence the input buffer must be large enough to hold the
1711
longest standard marker in the file. Standard JPEG markers should normally
1712
not exceed a few hundred bytes each (DHT tables are typically the longest).
1713
We recommend at least a 2K buffer for performance reasons, which is much
1714
larger than any correct marker is likely to be. For robustness against
1715
damaged marker length counts, you may wish to insert a test in your
1716
application for the case that the input buffer is completely full and yet
1717
the decoder has suspended without consuming any data --- otherwise, if this
1718
situation did occur, it would lead to an endless loop. (The library can't
1719
provide this test since it has no idea whether "the buffer is full", or
1720
even whether there is a fixed-size input buffer.)
1722
The input buffer would need to be 64K to allow for arbitrary COM or APPn
1723
markers, but these are handled specially: they are either saved into allocated
1724
memory, or skipped over by calling skip_input_data(). In the former case,
1725
suspension is handled correctly, and in the latter case, the problem of
1726
buffer overrun is placed on skip_input_data's shoulders, as explained above.
1727
Note that if you provide your own marker handling routine for large markers,
1728
you should consider how to deal with buffer overflow.
1730
Multiple-buffer management:
1732
In some applications it is desirable to store the compressed data in a linked
1733
list of buffer areas, so as to avoid data copying. This can be handled by
1734
having empty_output_buffer() or fill_input_buffer() set the pointer and count
1735
to reference the next available buffer; FALSE is returned only if no more
1736
buffers are available. Although seemingly straightforward, there is a
1737
pitfall in this approach: the backtrack that occurs when FALSE is returned
1738
could back up into an earlier buffer. For example, when fill_input_buffer()
1739
is called, the current pointer & count indicate the backtrack restart point.
1740
Since fill_input_buffer() will set the pointer and count to refer to a new
1741
buffer, the restart position must be saved somewhere else. Suppose a second
1742
call to fill_input_buffer() occurs in the same library call, and no
1743
additional input data is available, so fill_input_buffer must return FALSE.
1744
If the JPEG library has not moved the pointer/count forward in the current
1745
buffer, then *the correct restart point is the saved position in the prior
1746
buffer*. Prior buffers may be discarded only after the library establishes
1747
a restart point within a later buffer. Similar remarks apply for output into
1750
The library will never attempt to backtrack over a skip_input_data() call,
1751
so any skipped data can be permanently discarded. You still have to deal
1752
with the case of skipping not-yet-received data, however.
1754
It's much simpler to use only a single buffer; when fill_input_buffer() is
1755
called, move any unconsumed data (beyond the current pointer/count) down to
1756
the beginning of this buffer and then load new data into the remaining buffer
1757
space. This approach requires a little more data copying but is far easier
1761
Progressive JPEG support
1762
------------------------
1764
Progressive JPEG rearranges the stored data into a series of scans of
1765
increasing quality. In situations where a JPEG file is transmitted across a
1766
slow communications link, a decoder can generate a low-quality image very
1767
quickly from the first scan, then gradually improve the displayed quality as
1768
more scans are received. The final image after all scans are complete is
1769
identical to that of a regular (sequential) JPEG file of the same quality
1770
setting. Progressive JPEG files are often slightly smaller than equivalent
1771
sequential JPEG files, but the possibility of incremental display is the main
1772
reason for using progressive JPEG.
1774
The IJG encoder library generates progressive JPEG files when given a
1775
suitable "scan script" defining how to divide the data into scans.
1776
Creation of progressive JPEG files is otherwise transparent to the encoder.
1777
Progressive JPEG files can also be read transparently by the decoder library.
1778
If the decoding application simply uses the library as defined above, it
1779
will receive a final decoded image without any indication that the file was
1780
progressive. Of course, this approach does not allow incremental display.
1781
To perform incremental display, an application needs to use the decoder
1782
library's "buffered-image" mode, in which it receives a decoded image
1785
Each displayed scan requires about as much work to decode as a full JPEG
1786
image of the same size, so the decoder must be fairly fast in relation to the
1787
data transmission rate in order to make incremental display useful. However,
1788
it is possible to skip displaying the image and simply add the incoming bits
1789
to the decoder's coefficient buffer. This is fast because only Huffman
1790
decoding need be done, not IDCT, upsampling, colorspace conversion, etc.
1791
The IJG decoder library allows the application to switch dynamically between
1792
displaying the image and simply absorbing the incoming bits. A properly
1793
coded application can automatically adapt the number of display passes to
1794
suit the time available as the image is received. Also, a final
1795
higher-quality display cycle can be performed from the buffered data after
1796
the end of the file is reached.
1798
Progressive compression:
1800
To create a progressive JPEG file (or a multiple-scan sequential JPEG file),
1801
set the scan_info cinfo field to point to an array of scan descriptors, and
1802
perform compression as usual. Instead of constructing your own scan list,
1803
you can call the jpeg_simple_progression() helper routine to create a
1804
recommended progression sequence; this method should be used by all
1805
applications that don't want to get involved in the nitty-gritty of
1806
progressive scan sequence design. (If you want to provide user control of
1807
scan sequences, you may wish to borrow the scan script reading code found
1808
in rdswitch.c, so that you can read scan script files just like cjpeg's.)
1809
When scan_info is not NULL, the compression library will store DCT'd data
1810
into a buffer array as jpeg_write_scanlines() is called, and will emit all
1811
the requested scans during jpeg_finish_compress(). This implies that
1812
multiple-scan output cannot be created with a suspending data destination
1813
manager, since jpeg_finish_compress() does not support suspension. We
1814
should also note that the compressor currently forces Huffman optimization
1815
mode when creating a progressive JPEG file, because the default Huffman
1816
tables are unsuitable for progressive files.
1818
Progressive decompression:
1820
When buffered-image mode is not used, the decoder library will read all of
1821
a multi-scan file during jpeg_start_decompress(), so that it can provide a
1822
final decoded image. (Here "multi-scan" means either progressive or
1823
multi-scan sequential.) This makes multi-scan files transparent to the
1824
decoding application. However, existing applications that used suspending
1825
input with version 5 of the IJG library will need to be modified to check
1826
for a suspension return from jpeg_start_decompress().
1828
To perform incremental display, an application must use the library's
1829
buffered-image mode. This is described in the next section.
1835
In buffered-image mode, the library stores the partially decoded image in a
1836
coefficient buffer, from which it can be read out as many times as desired.
1837
This mode is typically used for incremental display of progressive JPEG files,
1838
but it can be used with any JPEG file. Each scan of a progressive JPEG file
1839
adds more data (more detail) to the buffered image. The application can
1840
display in lockstep with the source file (one display pass per input scan),
1841
or it can allow input processing to outrun display processing. By making
1842
input and display processing run independently, it is possible for the
1843
application to adapt progressive display to a wide range of data transmission
1846
The basic control flow for buffered-image decoding is
1848
jpeg_create_decompress()
1851
set overall decompression parameters
1852
cinfo.buffered_image = TRUE; /* select buffered-image mode */
1853
jpeg_start_decompress()
1854
for (each output pass) {
1855
adjust output decompression parameters if required
1856
jpeg_start_output() /* start a new output pass */
1857
for (all scanlines in image) {
1858
jpeg_read_scanlines()
1861
jpeg_finish_output() /* terminate output pass */
1863
jpeg_finish_decompress()
1864
jpeg_destroy_decompress()
1866
This differs from ordinary unbuffered decoding in that there is an additional
1867
level of looping. The application can choose how many output passes to make
1868
and how to display each pass.
1870
The simplest approach to displaying progressive images is to do one display
1871
pass for each scan appearing in the input file. In this case the outer loop
1872
condition is typically
1873
while (! jpeg_input_complete(&cinfo))
1874
and the start-output call should read
1875
jpeg_start_output(&cinfo, cinfo.input_scan_number);
1876
The second parameter to jpeg_start_output() indicates which scan of the input
1877
file is to be displayed; the scans are numbered starting at 1 for this
1878
purpose. (You can use a loop counter starting at 1 if you like, but using
1879
the library's input scan counter is easier.) The library automatically reads
1880
data as necessary to complete each requested scan, and jpeg_finish_output()
1881
advances to the next scan or end-of-image marker (hence input_scan_number
1882
will be incremented by the time control arrives back at jpeg_start_output()).
1883
With this technique, data is read from the input file only as needed, and
1884
input and output processing run in lockstep.
1886
After reading the final scan and reaching the end of the input file, the
1887
buffered image remains available; it can be read additional times by
1888
repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output()
1889
sequence. For example, a useful technique is to use fast one-pass color
1890
quantization for display passes made while the image is arriving, followed by
1891
a final display pass using two-pass quantization for highest quality. This
1892
is done by changing the library parameters before the final output pass.
1893
Changing parameters between passes is discussed in detail below.
1895
In general the last scan of a progressive file cannot be recognized as such
1896
until after it is read, so a post-input display pass is the best approach if
1897
you want special processing in the final pass.
1899
When done with the image, be sure to call jpeg_finish_decompress() to release
1900
the buffered image (or just use jpeg_destroy_decompress()).
1902
If input data arrives faster than it can be displayed, the application can
1903
cause the library to decode input data in advance of what's needed to produce
1904
output. This is done by calling the routine jpeg_consume_input().
1905
The return value is one of the following:
1906
JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan)
1907
JPEG_REACHED_EOI: reached the EOI marker (end of image)
1908
JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data
1909
JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan
1910
JPEG_SUSPENDED: suspended before completing any of the above
1911
(JPEG_SUSPENDED can occur only if a suspending data source is used.) This
1912
routine can be called at any time after initializing the JPEG object. It
1913
reads some additional data and returns when one of the indicated significant
1914
events occurs. (If called after the EOI marker is reached, it will
1915
immediately return JPEG_REACHED_EOI without attempting to read more data.)
1917
The library's output processing will automatically call jpeg_consume_input()
1918
whenever the output processing overtakes the input; thus, simple lockstep
1919
display requires no direct calls to jpeg_consume_input(). But by adding
1920
calls to jpeg_consume_input(), you can absorb data in advance of what is
1921
being displayed. This has two benefits:
1922
* You can limit buildup of unprocessed data in your input buffer.
1923
* You can eliminate extra display passes by paying attention to the
1924
state of the library's input processing.
1926
The first of these benefits only requires interspersing calls to
1927
jpeg_consume_input() with your display operations and any other processing
1928
you may be doing. To avoid wasting cycles due to backtracking, it's best to
1929
call jpeg_consume_input() only after a hundred or so new bytes have arrived.
1930
This is discussed further under "I/O suspension", above. (Note: the JPEG
1931
library currently is not thread-safe. You must not call jpeg_consume_input()
1932
from one thread of control if a different library routine is working on the
1933
same JPEG object in another thread.)
1935
When input arrives fast enough that more than one new scan is available
1936
before you start a new output pass, you may as well skip the output pass
1937
corresponding to the completed scan. This occurs for free if you pass
1938
cinfo.input_scan_number as the target scan number to jpeg_start_output().
1939
The input_scan_number field is simply the index of the scan currently being
1940
consumed by the input processor. You can ensure that this is up-to-date by
1941
emptying the input buffer just before calling jpeg_start_output(): call
1942
jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or
1945
The target scan number passed to jpeg_start_output() is saved in the
1946
cinfo.output_scan_number field. The library's output processing calls
1947
jpeg_consume_input() whenever the current input scan number and row within
1948
that scan is less than or equal to the current output scan number and row.
1949
Thus, input processing can "get ahead" of the output processing but is not
1950
allowed to "fall behind". You can achieve several different effects by
1951
manipulating this interlock rule. For example, if you pass a target scan
1952
number greater than the current input scan number, the output processor will
1953
wait until that scan starts to arrive before producing any output. (To avoid
1954
an infinite loop, the target scan number is automatically reset to the last
1955
scan number when the end of image is reached. Thus, if you specify a large
1956
target scan number, the library will just absorb the entire input file and
1957
then perform an output pass. This is effectively the same as what
1958
jpeg_start_decompress() does when you don't select buffered-image mode.)
1959
When you pass a target scan number equal to the current input scan number,
1960
the image is displayed no faster than the current input scan arrives. The
1961
final possibility is to pass a target scan number less than the current input
1962
scan number; this disables the input/output interlock and causes the output
1963
processor to simply display whatever it finds in the image buffer, without
1964
waiting for input. (However, the library will not accept a target scan
1965
number less than one, so you can't avoid waiting for the first scan.)
1967
When data is arriving faster than the output display processing can advance
1968
through the image, jpeg_consume_input() will store data into the buffered
1969
image beyond the point at which the output processing is reading data out
1970
again. If the input arrives fast enough, it may "wrap around" the buffer to
1971
the point where the input is more than one whole scan ahead of the output.
1972
If the output processing simply proceeds through its display pass without
1973
paying attention to the input, the effect seen on-screen is that the lower
1974
part of the image is one or more scans better in quality than the upper part.
1975
Then, when the next output scan is started, you have a choice of what target
1976
scan number to use. The recommended choice is to use the current input scan
1977
number at that time, which implies that you've skipped the output scans
1978
corresponding to the input scans that were completed while you processed the
1979
previous output scan. In this way, the decoder automatically adapts its
1980
speed to the arriving data, by skipping output scans as necessary to keep up
1981
with the arriving data.
1983
When using this strategy, you'll want to be sure that you perform a final
1984
output pass after receiving all the data; otherwise your last display may not
1985
be full quality across the whole screen. So the right outer loop logic is
1986
something like this:
1988
absorb any waiting input by calling jpeg_consume_input()
1989
final_pass = jpeg_input_complete(&cinfo);
1990
adjust output decompression parameters if required
1991
jpeg_start_output(&cinfo, cinfo.input_scan_number);
1993
jpeg_finish_output()
1994
} while (! final_pass);
1995
rather than quitting as soon as jpeg_input_complete() returns TRUE. This
1996
arrangement makes it simple to use higher-quality decoding parameters
1997
for the final pass. But if you don't want to use special parameters for
1998
the final pass, the right loop logic is like this:
2000
absorb any waiting input by calling jpeg_consume_input()
2001
jpeg_start_output(&cinfo, cinfo.input_scan_number);
2003
jpeg_finish_output()
2004
if (jpeg_input_complete(&cinfo) &&
2005
cinfo.input_scan_number == cinfo.output_scan_number)
2008
In this case you don't need to know in advance whether an output pass is to
2009
be the last one, so it's not necessary to have reached EOF before starting
2010
the final output pass; rather, what you want to test is whether the output
2011
pass was performed in sync with the final input scan. This form of the loop
2012
will avoid an extra output pass whenever the decoder is able (or nearly able)
2013
to keep up with the incoming data.
2015
When the data transmission speed is high, you might begin a display pass,
2016
then find that much or all of the file has arrived before you can complete
2017
the pass. (You can detect this by noting the JPEG_REACHED_EOI return code
2018
from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().)
2019
In this situation you may wish to abort the current display pass and start a
2020
new one using the newly arrived information. To do so, just call
2021
jpeg_finish_output() and then start a new pass with jpeg_start_output().
2023
A variant strategy is to abort and restart display if more than one complete
2024
scan arrives during an output pass; this can be detected by noting
2025
JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This
2026
idea should be employed with caution, however, since the display process
2027
might never get to the bottom of the image before being aborted, resulting
2028
in the lower part of the screen being several passes worse than the upper.
2029
In most cases it's probably best to abort an output pass only if the whole
2030
file has arrived and you want to begin the final output pass immediately.
2032
When receiving data across a communication link, we recommend always using
2033
the current input scan number for the output target scan number; if a
2034
higher-quality final pass is to be done, it should be started (aborting any
2035
incomplete output pass) as soon as the end of file is received. However,
2036
many other strategies are possible. For example, the application can examine
2037
the parameters of the current input scan and decide whether to display it or
2038
not. If the scan contains only chroma data, one might choose not to use it
2039
as the target scan, expecting that the scan will be small and will arrive
2040
quickly. To skip to the next scan, call jpeg_consume_input() until it
2041
returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher
2042
number as the target scan for jpeg_start_output(); but that method doesn't
2043
let you inspect the next scan's parameters before deciding to display it.
2046
In buffered-image mode, jpeg_start_decompress() never performs input and
2047
thus never suspends. An application that uses input suspension with
2048
buffered-image mode must be prepared for suspension returns from these
2050
* jpeg_start_output() performs input only if you request 2-pass quantization
2051
and the target scan isn't fully read yet. (This is discussed below.)
2052
* jpeg_read_scanlines(), as always, returns the number of scanlines that it
2053
was able to produce before suspending.
2054
* jpeg_finish_output() will read any markers following the target scan,
2055
up to the end of the file or the SOS marker that begins another scan.
2056
(But it reads no input if jpeg_consume_input() has already reached the
2057
end of the file or a SOS marker beyond the target output scan.)
2058
* jpeg_finish_decompress() will read until the end of file, and thus can
2059
suspend if the end hasn't already been reached (as can be tested by
2060
calling jpeg_input_complete()).
2061
jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress()
2062
all return TRUE if they completed their tasks, FALSE if they had to suspend.
2063
In the event of a FALSE return, the application must load more input data
2064
and repeat the call. Applications that use non-suspending data sources need
2065
not check the return values of these three routines.
2068
It is possible to change decoding parameters between output passes in the
2069
buffered-image mode. The decoder library currently supports only very
2070
limited changes of parameters. ONLY THE FOLLOWING parameter changes are
2071
allowed after jpeg_start_decompress() is called:
2072
* dct_method can be changed before each call to jpeg_start_output().
2073
For example, one could use a fast DCT method for early scans, changing
2074
to a higher quality method for the final scan.
2075
* dither_mode can be changed before each call to jpeg_start_output();
2076
of course this has no impact if not using color quantization. Typically
2077
one would use ordered dither for initial passes, then switch to
2078
Floyd-Steinberg dither for the final pass. Caution: changing dither mode
2079
can cause more memory to be allocated by the library. Although the amount
2080
of memory involved is not large (a scanline or so), it may cause the
2081
initial max_memory_to_use specification to be exceeded, which in the worst
2082
case would result in an out-of-memory failure.
2083
* do_block_smoothing can be changed before each call to jpeg_start_output().
2084
This setting is relevant only when decoding a progressive JPEG image.
2085
During the first DC-only scan, block smoothing provides a very "fuzzy" look
2086
instead of the very "blocky" look seen without it; which is better seems a
2087
matter of personal taste. But block smoothing is nearly always a win
2088
during later stages, especially when decoding a successive-approximation
2089
image: smoothing helps to hide the slight blockiness that otherwise shows
2090
up on smooth gradients until the lowest coefficient bits are sent.
2091
* Color quantization mode can be changed under the rules described below.
2092
You *cannot* change between full-color and quantized output (because that
2093
would alter the required I/O buffer sizes), but you can change which
2094
quantization method is used.
2096
When generating color-quantized output, changing quantization method is a
2097
very useful way of switching between high-speed and high-quality display.
2098
The library allows you to change among its three quantization methods:
2099
1. Single-pass quantization to a fixed color cube.
2100
Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL.
2101
2. Single-pass quantization to an application-supplied colormap.
2102
Selected by setting cinfo.colormap to point to the colormap (the value of
2103
two_pass_quantize is ignored); also set cinfo.actual_number_of_colors.
2104
3. Two-pass quantization to a colormap chosen specifically for the image.
2105
Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.
2106
(This is the default setting selected by jpeg_read_header, but it is
2107
probably NOT what you want for the first pass of progressive display!)
2108
These methods offer successively better quality and lesser speed. However,
2109
only the first method is available for quantizing in non-RGB color spaces.
2111
IMPORTANT: because the different quantizer methods have very different
2112
working-storage requirements, the library requires you to indicate which
2113
one(s) you intend to use before you call jpeg_start_decompress(). (If we did
2114
not require this, the max_memory_to_use setting would be a complete fiction.)
2115
You do this by setting one or more of these three cinfo fields to TRUE:
2116
enable_1pass_quant Fixed color cube colormap
2117
enable_external_quant Externally-supplied colormap
2118
enable_2pass_quant Two-pass custom colormap
2119
All three are initialized FALSE by jpeg_read_header(). But
2120
jpeg_start_decompress() automatically sets TRUE the one selected by the
2121
current two_pass_quantize and colormap settings, so you only need to set the
2122
enable flags for any other quantization methods you plan to change to later.
2124
After setting the enable flags correctly at jpeg_start_decompress() time, you
2125
can change to any enabled quantization method by setting two_pass_quantize
2126
and colormap properly just before calling jpeg_start_output(). The following
2127
special rules apply:
2128
1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass
2129
or 2-pass mode from a different mode, or when you want the 2-pass
2130
quantizer to be re-run to generate a new colormap.
2131
2. To switch to an external colormap, or to change to a different external
2132
colormap than was used on the prior pass, you must call
2133
jpeg_new_colormap() after setting cinfo.colormap.
2134
NOTE: if you want to use the same colormap as was used in the prior pass,
2135
you should not do either of these things. This will save some nontrivial
2137
(These requirements exist because cinfo.colormap will always be non-NULL
2138
after completing a prior output pass, since both the 1-pass and 2-pass
2139
quantizers set it to point to their output colormaps. Thus you have to
2140
do one of these two things to notify the library that something has changed.
2141
Yup, it's a bit klugy, but it's necessary to do it this way for backwards
2144
Note that in buffered-image mode, the library generates any requested colormap
2145
during jpeg_start_output(), not during jpeg_start_decompress().
2147
When using two-pass quantization, jpeg_start_output() makes a pass over the
2148
buffered image to determine the optimum color map; it therefore may take a
2149
significant amount of time, whereas ordinarily it does little work. The
2150
progress monitor hook is called during this pass, if defined. It is also
2151
important to realize that if the specified target scan number is greater than
2152
or equal to the current input scan number, jpeg_start_output() will attempt
2153
to consume input as it makes this pass. If you use a suspending data source,
2154
you need to check for a FALSE return from jpeg_start_output() under these
2155
conditions. The combination of 2-pass quantization and a not-yet-fully-read
2156
target scan is the only case in which jpeg_start_output() will consume input.
2159
Application authors who support buffered-image mode may be tempted to use it
2160
for all JPEG images, even single-scan ones. This will work, but it is
2161
inefficient: there is no need to create an image-sized coefficient buffer for
2162
single-scan images. Requesting buffered-image mode for such an image wastes
2163
memory. Worse, it can cost time on large images, since the buffered data has
2164
to be swapped out or written to a temporary file. If you are concerned about
2165
maximum performance on baseline JPEG files, you should use buffered-image
2166
mode only when the incoming file actually has multiple scans. This can be
2167
tested by calling jpeg_has_multiple_scans(), which will return a correct
2168
result at any time after jpeg_read_header() completes.
2170
It is also worth noting that when you use jpeg_consume_input() to let input
2171
processing get ahead of output processing, the resulting pattern of access to
2172
the coefficient buffer is quite nonsequential. It's best to use the memory
2173
manager jmemnobs.c if you can (ie, if you have enough real or virtual main
2174
memory). If not, at least make sure that max_memory_to_use is set as high as
2175
possible. If the JPEG memory manager has to use a temporary file, you will
2176
probably see a lot of disk traffic and poor performance. (This could be
2177
improved with additional work on the memory manager, but we haven't gotten
2180
In some applications it may be convenient to use jpeg_consume_input() for all
2181
input processing, including reading the initial markers; that is, you may
2182
wish to call jpeg_consume_input() instead of jpeg_read_header() during
2183
startup. This works, but note that you must check for JPEG_REACHED_SOS and
2184
JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes.
2185
Once the first SOS marker has been reached, you must call
2186
jpeg_start_decompress() before jpeg_consume_input() will consume more input;
2187
it'll just keep returning JPEG_REACHED_SOS until you do. If you read a
2188
tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI
2189
without ever returning JPEG_REACHED_SOS; be sure to check for this case.
2190
If this happens, the decompressor will not read any more input until you call
2191
jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not
2192
using buffered-image mode, but in that case it's basically a no-op after the
2193
initial markers have been read: it will just return JPEG_SUSPENDED.
2196
Abbreviated datastreams and multiple images
2197
-------------------------------------------
2199
A JPEG compression or decompression object can be reused to process multiple
2200
images. This saves a small amount of time per image by eliminating the
2201
"create" and "destroy" operations, but that isn't the real purpose of the
2202
feature. Rather, reuse of an object provides support for abbreviated JPEG
2203
datastreams. Object reuse can also simplify processing a series of images in
2204
a single input or output file. This section explains these features.
2206
A JPEG file normally contains several hundred bytes worth of quantization
2207
and Huffman tables. In a situation where many images will be stored or
2208
transmitted with identical tables, this may represent an annoying overhead.
2209
The JPEG standard therefore permits tables to be omitted. The standard
2210
defines three classes of JPEG datastreams:
2211
* "Interchange" datastreams contain an image and all tables needed to decode
2212
the image. These are the usual kind of JPEG file.
2213
* "Abbreviated image" datastreams contain an image, but are missing some or
2214
all of the tables needed to decode that image.
2215
* "Abbreviated table specification" (henceforth "tables-only") datastreams
2216
contain only table specifications.
2217
To decode an abbreviated image, it is necessary to load the missing table(s)
2218
into the decoder beforehand. This can be accomplished by reading a separate
2219
tables-only file. A variant scheme uses a series of images in which the first
2220
image is an interchange (complete) datastream, while subsequent ones are
2221
abbreviated and rely on the tables loaded by the first image. It is assumed
2222
that once the decoder has read a table, it will remember that table until a
2223
new definition for the same table number is encountered.
2225
It is the application designer's responsibility to figure out how to associate
2226
the correct tables with an abbreviated image. While abbreviated datastreams
2227
can be useful in a closed environment, their use is strongly discouraged in
2228
any situation where data exchange with other applications might be needed.
2231
The JPEG library provides support for reading and writing any combination of
2232
tables-only datastreams and abbreviated images. In both compression and
2233
decompression objects, a quantization or Huffman table will be retained for
2234
the lifetime of the object, unless it is overwritten by a new table definition.
2237
To create abbreviated image datastreams, it is only necessary to tell the
2238
compressor not to emit some or all of the tables it is using. Each
2239
quantization and Huffman table struct contains a boolean field "sent_table",
2240
which normally is initialized to FALSE. For each table used by the image, the
2241
header-writing process emits the table and sets sent_table = TRUE unless it is
2242
already TRUE. (In normal usage, this prevents outputting the same table
2243
definition multiple times, as would otherwise occur because the chroma
2244
components typically share tables.) Thus, setting this field to TRUE before
2245
calling jpeg_start_compress() will prevent the table from being written at
2248
If you want to create a "pure" abbreviated image file containing no tables,
2249
just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the
2250
tables. If you want to emit some but not all tables, you'll need to set the
2251
individual sent_table fields directly.
2253
To create an abbreviated image, you must also call jpeg_start_compress()
2254
with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress()
2255
will force all the sent_table fields to FALSE. (This is a safety feature to
2256
prevent abbreviated images from being created accidentally.)
2258
To create a tables-only file, perform the same parameter setup that you
2259
normally would, but instead of calling jpeg_start_compress() and so on, call
2260
jpeg_write_tables(&cinfo). This will write an abbreviated datastream
2261
containing only SOI, DQT and/or DHT markers, and EOI. All the quantization
2262
and Huffman tables that are currently defined in the compression object will
2263
be emitted unless their sent_tables flag is already TRUE, and then all the
2264
sent_tables flags will be set TRUE.
2266
A sure-fire way to create matching tables-only and abbreviated image files
2267
is to proceed as follows:
2269
create JPEG compression object
2271
set destination to tables-only file
2272
jpeg_write_tables(&cinfo);
2273
set destination to image file
2274
jpeg_start_compress(&cinfo, FALSE);
2276
jpeg_finish_compress(&cinfo);
2278
Since the JPEG parameters are not altered between writing the table file and
2279
the abbreviated image file, the same tables are sure to be used. Of course,
2280
you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence
2281
many times to produce many abbreviated image files matching the table file.
2283
You cannot suppress output of the computed Huffman tables when Huffman
2284
optimization is selected. (If you could, there'd be no way to decode the
2285
image...) Generally, you don't want to set optimize_coding = TRUE when
2286
you are trying to produce abbreviated files.
2288
In some cases you might want to compress an image using tables which are
2289
not stored in the application, but are defined in an interchange or
2290
tables-only file readable by the application. This can be done by setting up
2291
a JPEG decompression object to read the specification file, then copying the
2292
tables into your compression object. See jpeg_copy_critical_parameters()
2293
for an example of copying quantization tables.
2296
To read abbreviated image files, you simply need to load the proper tables
2297
into the decompression object before trying to read the abbreviated image.
2298
If the proper tables are stored in the application program, you can just
2299
allocate the table structs and fill in their contents directly. For example,
2300
to load a fixed quantization table into table slot "n":
2302
if (cinfo.quant_tbl_ptrs[n] == NULL)
2303
cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo);
2304
quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */
2305
for (i = 0; i < 64; i++) {
2306
/* Qtable[] is desired quantization table, in natural array order */
2307
quant_ptr->quantval[i] = Qtable[i];
2310
Code to load a fixed Huffman table is typically (for AC table "n"):
2312
if (cinfo.ac_huff_tbl_ptrs[n] == NULL)
2313
cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo);
2314
huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */
2315
for (i = 1; i <= 16; i++) {
2316
/* counts[i] is number of Huffman codes of length i bits, i=1..16 */
2317
huff_ptr->bits[i] = counts[i];
2319
for (i = 0; i < 256; i++) {
2320
/* symbols[] is the list of Huffman symbols, in code-length order */
2321
huff_ptr->huffval[i] = symbols[i];
2324
(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a
2325
constant JQUANT_TBL object is not safe. If the incoming file happened to
2326
contain a quantization table definition, your master table would get
2327
overwritten! Instead allocate a working table copy and copy the master table
2328
into it, as illustrated above. Ditto for Huffman tables, of course.)
2330
You might want to read the tables from a tables-only file, rather than
2331
hard-wiring them into your application. The jpeg_read_header() call is
2332
sufficient to read a tables-only file. You must pass a second parameter of
2333
FALSE to indicate that you do not require an image to be present. Thus, the
2336
create JPEG decompression object
2337
set source to tables-only file
2338
jpeg_read_header(&cinfo, FALSE);
2339
set source to abbreviated image file
2340
jpeg_read_header(&cinfo, TRUE);
2341
set decompression parameters
2342
jpeg_start_decompress(&cinfo);
2344
jpeg_finish_decompress(&cinfo);
2346
In some cases, you may want to read a file without knowing whether it contains
2347
an image or just tables. In that case, pass FALSE and check the return value
2348
from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found,
2349
JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value,
2350
JPEG_SUSPENDED, is possible when using a suspending data source manager.)
2351
Note that jpeg_read_header() will not complain if you read an abbreviated
2352
image for which you haven't loaded the missing tables; the missing-table check
2353
occurs later, in jpeg_start_decompress().
2356
It is possible to read a series of images from a single source file by
2357
repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence,
2358
without releasing/recreating the JPEG object or the data source module.
2359
(If you did reinitialize, any partial bufferload left in the data source
2360
buffer at the end of one image would be discarded, causing you to lose the
2361
start of the next image.) When you use this method, stored tables are
2362
automatically carried forward, so some of the images can be abbreviated images
2363
that depend on tables from earlier images.
2365
If you intend to write a series of images into a single destination file,
2366
you might want to make a specialized data destination module that doesn't
2367
flush the output buffer at term_destination() time. This would speed things
2368
up by some trifling amount. Of course, you'd need to remember to flush the
2369
buffer after the last image. You can make the later images be abbreviated
2370
ones by passing FALSE to jpeg_start_compress().
2376
Some applications may need to insert or extract special data in the JPEG
2377
datastream. The JPEG standard provides marker types "COM" (comment) and
2378
"APP0" through "APP15" (application) to hold application-specific data.
2379
Unfortunately, the use of these markers is not specified by the standard.
2380
COM markers are fairly widely used to hold user-supplied text. The JFIF file
2381
format spec uses APP0 markers with specified initial strings to hold certain
2382
data. Adobe applications use APP14 markers beginning with the string "Adobe"
2383
for miscellaneous data. Other APPn markers are rarely seen, but might
2384
contain almost anything.
2386
If you wish to store user-supplied text, we recommend you use COM markers
2387
and place readable 7-bit ASCII text in them. Newline conventions are not
2388
standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR
2389
(Mac style). A robust COM reader should be able to cope with random binary
2390
garbage, including nulls, since some applications generate COM markers
2391
containing non-ASCII junk. (But yours should not be one of them.)
2393
For program-supplied data, use an APPn marker, and be sure to begin it with an
2394
identifying string so that you can tell whether the marker is actually yours.
2395
It's probably best to avoid using APP0 or APP14 for any private markers.
2396
(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you
2397
not use APP8 markers for any private purposes, either.)
2399
Keep in mind that at most 65533 bytes can be put into one marker, but you
2400
can have as many markers as you like.
2402
By default, the IJG compression library will write a JFIF APP0 marker if the
2403
selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if
2404
the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but
2405
we don't recommend it. The decompression library will recognize JFIF and
2406
Adobe markers and will set the JPEG colorspace properly when one is found.
2409
You can write special markers immediately following the datastream header by
2410
calling jpeg_write_marker() after jpeg_start_compress() and before the first
2411
call to jpeg_write_scanlines(). When you do this, the markers appear after
2412
the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before
2413
all else. Specify the marker type parameter as "JPEG_COM" for COM or
2414
"JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write
2415
any marker type, but we don't recommend writing any other kinds of marker.)
2416
For example, to write a user comment string pointed to by comment_text:
2417
jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));
2419
If it's not convenient to store all the marker data in memory at once,
2420
you can instead call jpeg_write_m_header() followed by multiple calls to
2421
jpeg_write_m_byte(). If you do it this way, it's your responsibility to
2422
call jpeg_write_m_byte() exactly the number of times given in the length
2423
parameter to jpeg_write_m_header(). (This method lets you empty the
2424
output buffer partway through a marker, which might be important when
2425
using a suspending data destination module. In any case, if you are using
2426
a suspending destination, you should flush its buffer after inserting
2427
any special markers. See "I/O suspension".)
2429
Or, if you prefer to synthesize the marker byte sequence yourself,
2430
you can just cram it straight into the data destination module.
2432
If you are writing JFIF 1.02 extension markers (thumbnail images), don't
2433
forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the
2434
correct JFIF version number in the JFIF header marker. The library's default
2435
is to write version 1.01, but that's wrong if you insert any 1.02 extension
2436
markers. (We could probably get away with just defaulting to 1.02, but there
2437
used to be broken decoders that would complain about unknown minor version
2438
numbers. To reduce compatibility risks it's safest not to write 1.02 unless
2439
you are actually using 1.02 extensions.)
2442
When reading, two methods of handling special markers are available:
2443
1. You can ask the library to save the contents of COM and/or APPn markers
2444
into memory, and then examine them at your leisure afterwards.
2445
2. You can supply your own routine to process COM and/or APPn markers
2446
on-the-fly as they are read.
2447
The first method is simpler to use, especially if you are using a suspending
2448
data source; writing a marker processor that copes with input suspension is
2449
not easy (consider what happens if the marker is longer than your available
2450
input buffer). However, the second method conserves memory since the marker
2451
data need not be kept around after it's been processed.
2453
For either method, you'd normally set up marker handling after creating a
2454
decompression object and before calling jpeg_read_header(), because the
2455
markers of interest will typically be near the head of the file and so will
2456
be scanned by jpeg_read_header. Once you've established a marker handling
2457
method, it will be used for the life of that decompression object
2458
(potentially many datastreams), unless you change it. Marker handling is
2459
determined separately for COM markers and for each APPn marker code.
2462
To save the contents of special markers in memory, call
2463
jpeg_save_markers(cinfo, marker_code, length_limit)
2464
where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.
2465
(To arrange to save all the special marker types, you need to call this
2466
routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer
2467
than length_limit data bytes, only length_limit bytes will be saved; this
2468
parameter allows you to avoid chewing up memory when you only need to see the
2469
first few bytes of a potentially large marker. If you want to save all the
2470
data, set length_limit to 0xFFFF; that is enough since marker lengths are only
2471
16 bits. As a special case, setting length_limit to 0 prevents that marker
2472
type from being saved at all. (That is the default behavior, in fact.)
2474
After jpeg_read_header() completes, you can examine the special markers by
2475
following the cinfo->marker_list pointer chain. All the special markers in
2476
the file appear in this list, in order of their occurrence in the file (but
2477
omitting any markers of types you didn't ask for). Both the original data
2478
length and the saved data length are recorded for each list entry; the latter
2479
will not exceed length_limit for the particular marker type. Note that these
2480
lengths exclude the marker length word, whereas the stored representation
2481
within the JPEG file includes it. (Hence the maximum data length is really
2484
It is possible that additional special markers appear in the file beyond the
2485
SOS marker at which jpeg_read_header stops; if so, the marker list will be
2486
extended during reading of the rest of the file. This is not expected to be
2487
common, however. If you are short on memory you may want to reset the length
2488
limit to zero for all marker types after finishing jpeg_read_header, to
2489
ensure that the max_memory_to_use setting cannot be exceeded due to addition
2492
The marker list remains stored until you call jpeg_finish_decompress or
2493
jpeg_abort, at which point the memory is freed and the list is set to empty.
2494
(jpeg_destroy also releases the storage, of course.)
2496
Note that the library is internally interested in APP0 and APP14 markers;
2497
if you try to set a small nonzero length limit on these types, the library
2498
will silently force the length up to the minimum it wants. (But you can set
2499
a zero length limit to prevent them from being saved at all.) Also, in a
2500
16-bit environment, the maximum length limit may be constrained to less than
2501
65533 by malloc() limitations. It is therefore best not to assume that the
2502
effective length limit is exactly what you set it to be.
2505
If you want to supply your own marker-reading routine, you do it by calling
2506
jpeg_set_marker_processor(). A marker processor routine must have the
2508
boolean jpeg_marker_parser_method (j_decompress_ptr cinfo)
2509
Although the marker code is not explicitly passed, the routine can find it
2510
in cinfo->unread_marker. At the time of call, the marker proper has been
2511
read from the data source module. The processor routine is responsible for
2512
reading the marker length word and the remaining parameter bytes, if any.
2513
Return TRUE to indicate success. (FALSE should be returned only if you are
2514
using a suspending data source and it tells you to suspend. See the standard
2515
marker processors in jdmarker.c for appropriate coding methods if you need to
2516
use a suspending data source.)
2518
If you override the default APP0 or APP14 processors, it is up to you to
2519
recognize JFIF and Adobe markers if you want colorspace recognition to occur
2520
properly. We recommend copying and extending the default processors if you
2521
want to do that. (A better idea is to save these marker types for later
2522
examination by calling jpeg_save_markers(); that method doesn't interfere
2523
with the library's own processing of these markers.)
2525
jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive
2526
--- if you call one it overrides any previous call to the other, for the
2527
particular marker type specified.
2529
A simple example of an external COM processor can be found in djpeg.c.
2530
Also, see jpegtran.c for an example of using jpeg_save_markers.
2533
Raw (downsampled) image data
2534
----------------------------
2536
Some applications need to supply already-downsampled image data to the JPEG
2537
compressor, or to receive raw downsampled data from the decompressor. The
2538
library supports this requirement by allowing the application to write or
2539
read raw data, bypassing the normal preprocessing or postprocessing steps.
2540
The interface is different from the standard one and is somewhat harder to
2541
use. If your interest is merely in bypassing color conversion, we recommend
2542
that you use the standard interface and simply set jpeg_color_space =
2543
in_color_space (or jpeg_color_space = out_color_space for decompression).
2544
The mechanism described in this section is necessary only to supply or
2545
receive downsampled image data, in which not all components have the same
2549
To compress raw data, you must supply the data in the colorspace to be used
2550
in the JPEG file (please read the earlier section on Special color spaces)
2551
and downsampled to the sampling factors specified in the JPEG parameters.
2552
You must supply the data in the format used internally by the JPEG library,
2553
namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional
2554
arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one
2555
color component. This structure is necessary since the components are of
2556
different sizes. If the image dimensions are not a multiple of the MCU size,
2557
you must also pad the data correctly (usually, this is done by replicating
2558
the last column and/or row). The data must be padded to a multiple of a DCT
2559
block in each component: that is, each downsampled row must contain a
2560
multiple of 8 valid samples, and there must be a multiple of 8 sample rows
2561
for each component. (For applications such as conversion of digital TV
2562
images, the standard image size is usually a multiple of the DCT block size,
2563
so that no padding need actually be done.)
2565
The procedure for compression of raw data is basically the same as normal
2566
compression, except that you call jpeg_write_raw_data() in place of
2567
jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do
2569
* Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().)
2570
This notifies the library that you will be supplying raw data.
2571
Furthermore, set cinfo->do_fancy_downsampling to FALSE if you want to use
2572
real downsampled data. (It is set TRUE by jpeg_set_defaults().)
2573
* Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()
2574
call is a good idea. Note that since color conversion is bypassed,
2575
in_color_space is ignored, except that jpeg_set_defaults() uses it to
2576
choose the default jpeg_color_space setting.
2577
* Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and
2578
cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the
2579
dimensions of the data you are supplying, it's wise to set them
2580
explicitly, rather than assuming the library's defaults are what you want.
2582
To pass raw data to the library, call jpeg_write_raw_data() in place of
2583
jpeg_write_scanlines(). The two routines work similarly except that
2584
jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.
2585
The scanlines count passed to and returned from jpeg_write_raw_data is
2586
measured in terms of the component with the largest v_samp_factor.
2588
jpeg_write_raw_data() processes one MCU row per call, which is to say
2589
v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines
2590
value must be at least max_v_samp_factor*DCTSIZE, and the return value will
2591
be exactly that amount (or possibly some multiple of that amount, in future
2592
library versions). This is true even on the last call at the bottom of the
2593
image; don't forget to pad your data as necessary.
2595
The required dimensions of the supplied data can be computed for each
2597
cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row
2598
cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image
2599
after jpeg_start_compress() has initialized those fields. If the valid data
2600
is smaller than this, it must be padded appropriately. For some sampling
2601
factors and image sizes, additional dummy DCT blocks are inserted to make
2602
the image a multiple of the MCU dimensions. The library creates such dummy
2603
blocks itself; it does not read them from your supplied data. Therefore you
2604
need never pad by more than DCTSIZE samples. An example may help here.
2605
Assume 2h2v downsampling of YCbCr data, that is
2606
cinfo->comp_info[0].h_samp_factor = 2 for Y
2607
cinfo->comp_info[0].v_samp_factor = 2
2608
cinfo->comp_info[1].h_samp_factor = 1 for Cb
2609
cinfo->comp_info[1].v_samp_factor = 1
2610
cinfo->comp_info[2].h_samp_factor = 1 for Cr
2611
cinfo->comp_info[2].v_samp_factor = 1
2612
and suppose that the nominal image dimensions (cinfo->image_width and
2613
cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will
2614
compute downsampled_width = 101 and width_in_blocks = 13 for Y,
2615
downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same
2616
for the height fields). You must pad the Y data to at least 13*8 = 104
2617
columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The
2618
MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16
2619
scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual
2620
sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed,
2621
so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row
2622
of Y data is dummy, so it doesn't matter what you pass for it in the data
2623
arrays, but the scanlines count must total up to 112 so that all of the Cb
2624
and Cr data gets passed.
2626
Output suspension is supported with raw-data compression: if the data
2627
destination module suspends, jpeg_write_raw_data() will return 0.
2628
In this case the same data rows must be passed again on the next call.
2631
Decompression with raw data output implies bypassing all postprocessing.
2632
You must deal with the color space and sampling factors present in the
2633
incoming file. If your application only handles, say, 2h1v YCbCr data,
2634
you must check for and fail on other color spaces or other sampling factors.
2635
The library will not convert to a different color space for you.
2637
To obtain raw data output, set cinfo->raw_data_out = TRUE before
2638
jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to
2639
verify that the color space and sampling factors are ones you can handle.
2640
Furthermore, set cinfo->do_fancy_upsampling = FALSE if you want to get real
2641
downsampled data (it is set TRUE by jpeg_read_header()).
2642
Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The
2643
decompression process is otherwise the same as usual.
2645
jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a
2646
buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is
2647
the same as for raw-data compression). The buffer you pass must be large
2648
enough to hold the actual data plus padding to DCT-block boundaries. As with
2649
compression, any entirely dummy DCT blocks are not processed so you need not
2650
allocate space for them, but the total scanline count includes them. The
2651
above example of computing buffer dimensions for raw-data compression is
2652
equally valid for decompression.
2654
Input suspension is supported with raw-data decompression: if the data source
2655
module suspends, jpeg_read_raw_data() will return 0. You can also use
2656
buffered-image mode to read raw data in multiple passes.
2659
Really raw data: DCT coefficients
2660
---------------------------------
2662
It is possible to read or write the contents of a JPEG file as raw DCT
2663
coefficients. This facility is mainly intended for use in lossless
2664
transcoding between different JPEG file formats. Other possible applications
2665
include lossless cropping of a JPEG image, lossless reassembly of a
2666
multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc.
2668
To read the contents of a JPEG file as DCT coefficients, open the file and do
2669
jpeg_read_header() as usual. But instead of calling jpeg_start_decompress()
2670
and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the
2671
entire image into a set of virtual coefficient-block arrays, one array per
2672
component. The return value is a pointer to an array of virtual-array
2673
descriptors. Each virtual array can be accessed directly using the JPEG
2674
memory manager's access_virt_barray method (see Memory management, below,
2675
and also read structure.txt's discussion of virtual array handling). Or,
2676
for simple transcoding to a different JPEG file format, the array list can
2677
just be handed directly to jpeg_write_coefficients().
2679
Each block in the block arrays contains quantized coefficient values in
2680
normal array order (not JPEG zigzag order). The block arrays contain only
2681
DCT blocks containing real data; any entirely-dummy blocks added to fill out
2682
interleaved MCUs at the right or bottom edges of the image are discarded
2683
during reading and are not stored in the block arrays. (The size of each
2684
block array can be determined from the width_in_blocks and height_in_blocks
2685
fields of the component's comp_info entry.) This is also the data format
2686
expected by jpeg_write_coefficients().
2688
When you are done using the virtual arrays, call jpeg_finish_decompress()
2689
to release the array storage and return the decompression object to an idle
2690
state; or just call jpeg_destroy() if you don't need to reuse the object.
2692
If you use a suspending data source, jpeg_read_coefficients() will return
2693
NULL if it is forced to suspend; a non-NULL return value indicates successful
2694
completion. You need not test for a NULL return value when using a
2695
non-suspending data source.
2697
It is also possible to call jpeg_read_coefficients() to obtain access to the
2698
decoder's coefficient arrays during a normal decode cycle in buffered-image
2699
mode. This frammish might be useful for progressively displaying an incoming
2700
image and then re-encoding it without loss. To do this, decode in buffered-
2701
image mode as discussed previously, then call jpeg_read_coefficients() after
2702
the last jpeg_finish_output() call. The arrays will be available for your use
2703
until you call jpeg_finish_decompress().
2706
To write the contents of a JPEG file as DCT coefficients, you must provide
2707
the DCT coefficients stored in virtual block arrays. You can either pass
2708
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
2709
allocate virtual arrays from the JPEG compression object and fill them
2710
yourself. In either case, jpeg_write_coefficients() is substituted for
2711
jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is
2712
* Create compression object
2713
* Set all compression parameters as necessary
2714
* Request virtual arrays if needed
2715
* jpeg_write_coefficients()
2716
* jpeg_finish_compress()
2717
* Destroy or re-use compression object
2718
jpeg_write_coefficients() is passed a pointer to an array of virtual block
2719
array descriptors; the number of arrays is equal to cinfo.num_components.
2721
The virtual arrays need only have been requested, not realized, before
2722
jpeg_write_coefficients() is called. A side-effect of
2723
jpeg_write_coefficients() is to realize any virtual arrays that have been
2724
requested from the compression object's memory manager. Thus, when obtaining
2725
the virtual arrays from the compression object, you should fill the arrays
2726
after calling jpeg_write_coefficients(). The data is actually written out
2727
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
2730
When writing raw DCT coefficients, it is crucial that the JPEG quantization
2731
tables and sampling factors match the way the data was encoded, or the
2732
resulting file will be invalid. For transcoding from an existing JPEG file,
2733
we recommend using jpeg_copy_critical_parameters(). This routine initializes
2734
all the compression parameters to default values (like jpeg_set_defaults()),
2735
then copies the critical information from a source decompression object.
2736
The decompression object should have just been used to read the entire
2737
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().
2739
jpeg_write_coefficients() marks all tables stored in the compression object
2740
as needing to be written to the output file (thus, it acts like
2741
jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid
2742
emitting abbreviated JPEG files by accident. If you really want to emit an
2743
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
2744
individual sent_table flags, between calling jpeg_write_coefficients() and
2745
jpeg_finish_compress().
2751
Some applications may need to regain control from the JPEG library every so
2752
often. The typical use of this feature is to produce a percent-done bar or
2753
other progress display. (For a simple example, see cjpeg.c or djpeg.c.)
2754
Although you do get control back frequently during the data-transferring pass
2755
(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes
2756
will occur inside jpeg_finish_compress or jpeg_start_decompress; those
2757
routines may take a long time to execute, and you don't get control back
2758
until they are done.
2760
You can define a progress-monitor routine which will be called periodically
2761
by the library. No guarantees are made about how often this call will occur,
2762
so we don't recommend you use it for mouse tracking or anything like that.
2763
At present, a call will occur once per MCU row, scanline, or sample row
2764
group, whichever unit is convenient for the current processing mode; so the
2765
wider the image, the longer the time between calls. During the data
2766
transferring pass, only one call occurs per call of jpeg_read_scanlines or
2767
jpeg_write_scanlines, so don't pass a large number of scanlines at once if
2768
you want fine resolution in the progress count. (If you really need to use
2769
the callback mechanism for time-critical tasks like mouse tracking, you could
2770
insert additional calls inside some of the library's inner loops.)
2772
To establish a progress-monitor callback, create a struct jpeg_progress_mgr,
2773
fill in its progress_monitor field with a pointer to your callback routine,
2774
and set cinfo->progress to point to the struct. The callback will be called
2775
whenever cinfo->progress is non-NULL. (This pointer is set to NULL by
2776
jpeg_create_compress or jpeg_create_decompress; the library will not change
2777
it thereafter. So if you allocate dynamic storage for the progress struct,
2778
make sure it will live as long as the JPEG object does. Allocating from the
2779
JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You
2780
can use the same callback routine for both compression and decompression.
2782
The jpeg_progress_mgr struct contains four fields which are set by the library:
2783
long pass_counter; /* work units completed in this pass */
2784
long pass_limit; /* total number of work units in this pass */
2785
int completed_passes; /* passes completed so far */
2786
int total_passes; /* total number of passes expected */
2787
During any one pass, pass_counter increases from 0 up to (not including)
2788
pass_limit; the step size is usually but not necessarily 1. The pass_limit
2789
value may change from one pass to another. The expected total number of
2790
passes is in total_passes, and the number of passes already completed is in
2791
completed_passes. Thus the fraction of work completed may be estimated as
2792
completed_passes + (pass_counter/pass_limit)
2793
--------------------------------------------
2795
ignoring the fact that the passes may not be equal amounts of work.
2797
When decompressing, pass_limit can even change within a pass, because it
2798
depends on the number of scans in the JPEG file, which isn't always known in
2799
advance. The computed fraction-of-work-done may jump suddenly (if the library
2800
discovers it has overestimated the number of scans) or even decrease (in the
2801
opposite case). It is not wise to put great faith in the work estimate.
2803
When using the decompressor's buffered-image mode, the progress monitor work
2804
estimate is likely to be completely unhelpful, because the library has no way
2805
to know how many output passes will be demanded of it. Currently, the library
2806
sets total_passes based on the assumption that there will be one more output
2807
pass if the input file end hasn't yet been read (jpeg_input_complete() isn't
2808
TRUE), but no more output passes if the file end has been reached when the
2809
output pass is started. This means that total_passes will rise as additional
2810
output passes are requested. If you have a way of determining the input file
2811
size, estimating progress based on the fraction of the file that's been read
2812
will probably be more useful than using the library's value.
2818
This section covers some key facts about the JPEG library's built-in memory
2819
manager. For more info, please read structure.txt's section about the memory
2820
manager, and consult the source code if necessary.
2822
All memory and temporary file allocation within the library is done via the
2823
memory manager. If necessary, you can replace the "back end" of the memory
2824
manager to control allocation yourself (for example, if you don't want the
2825
library to use malloc() and free() for some reason).
2827
Some data is allocated "permanently" and will not be freed until the JPEG
2828
object is destroyed. Most data is allocated "per image" and is freed by
2829
jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the
2830
memory manager yourself to allocate structures that will automatically be
2831
freed at these times. Typical code for this is
2832
ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size);
2833
Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object.
2834
Use alloc_large instead of alloc_small for anything bigger than a few Kbytes.
2835
There are also alloc_sarray and alloc_barray routines that automatically
2836
build 2-D sample or block arrays.
2838
The library's minimum space requirements to process an image depend on the
2839
image's width, but not on its height, because the library ordinarily works
2840
with "strip" buffers that are as wide as the image but just a few rows high.
2841
Some operating modes (eg, two-pass color quantization) require full-image
2842
buffers. Such buffers are treated as "virtual arrays": only the current strip
2843
need be in memory, and the rest can be swapped out to a temporary file.
2845
If you use the simplest memory manager back end (jmemnobs.c), then no
2846
temporary files are used; virtual arrays are simply malloc()'d. Images bigger
2847
than memory can be processed only if your system supports virtual memory.
2848
The other memory manager back ends support temporary files of various flavors
2849
and thus work in machines without virtual memory. They may also be useful on
2850
Unix machines if you need to process images that exceed available swap space.
2852
When using temporary files, the library will make the in-memory buffers for
2853
its virtual arrays just big enough to stay within a "maximum memory" setting.
2854
Your application can set this limit by setting cinfo->mem->max_memory_to_use
2855
after creating the JPEG object. (Of course, there is still a minimum size for
2856
the buffers, so the max-memory setting is effective only if it is bigger than
2857
the minimum space needed.) If you allocate any large structures yourself, you
2858
must allocate them before jpeg_start_compress() or jpeg_start_decompress() in
2859
order to have them counted against the max memory limit. Also keep in mind
2860
that space allocated with alloc_small() is ignored, on the assumption that
2861
it's too small to be worth worrying about; so a reasonable safety margin
2862
should be left when setting max_memory_to_use.
2864
If you use the jmemname.c or jmemdos.c memory manager back end, it is
2865
important to clean up the JPEG object properly to ensure that the temporary
2866
files get deleted. (This is especially crucial with jmemdos.c, where the
2867
"temporary files" may be extended-memory segments; if they are not freed,
2868
DOS will require a reboot to recover the memory.) Thus, with these memory
2869
managers, it's a good idea to provide a signal handler that will trap any
2870
early exit from your program. The handler should call either jpeg_abort()
2871
or jpeg_destroy() for any active JPEG objects. A handler is not needed with
2872
jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either,
2873
since the C library is supposed to take care of deleting files made with
2880
Working memory requirements while performing compression or decompression
2881
depend on image dimensions, image characteristics (such as colorspace and
2882
JPEG process), and operating mode (application-selected options).
2884
As of v6b, the decompressor requires:
2885
1. About 24K in more-or-less-fixed-size data. This varies a bit depending
2886
on operating mode and image characteristics (particularly color vs.
2887
grayscale), but it doesn't depend on image dimensions.
2888
2. Strip buffers (of size proportional to the image width) for IDCT and
2889
upsampling results. The worst case for commonly used sampling factors
2890
is about 34 bytes * width in pixels for a color image. A grayscale image
2891
only needs about 8 bytes per pixel column.
2892
3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG
2893
file (including progressive JPEGs), or whenever you select buffered-image
2894
mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's
2895
3 bytes per pixel for a color image. Worst case (1x1 sampling) requires
2896
6 bytes/pixel. For grayscale, figure 2 bytes/pixel.
2897
4. To perform 2-pass color quantization, the decompressor also needs a
2898
128K color lookup table and a full-image pixel buffer (3 bytes/pixel).
2899
This does not count any memory allocated by the application, such as a
2900
buffer to hold the final output image.
2902
The above figures are valid for 8-bit JPEG data precision and a machine with
2903
32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and
2904
quantization pixel buffer. The "fixed-size" data will be somewhat smaller
2905
with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual
2906
color spaces will require different amounts of space.
2908
The full-image coefficient and pixel buffers, if needed at all, do not
2909
have to be fully RAM resident; you can have the library use temporary
2910
files instead when the total memory usage would exceed a limit you set.
2911
(But if your OS supports virtual memory, it's probably better to just use
2912
jmemnobs and let the OS do the swapping.)
2914
The compressor's memory requirements are similar, except that it has no need
2915
for color quantization. Also, it needs a full-image DCT coefficient buffer
2916
if Huffman-table optimization is asked for, even if progressive mode is not
2919
If you need more detailed information about memory usage in a particular
2920
situation, you can enable the MEM_STATS code in jmemmgr.c.
2923
Library compile-time options
2924
----------------------------
2926
A number of compile-time options are available by modifying jmorecfg.h.
2928
The JPEG standard provides for both the baseline 8-bit DCT process and
2929
a 12-bit DCT process. The IJG code supports 12-bit lossy JPEG if you define
2930
BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be
2931
larger than a char, so it affects the surrounding application's image data.
2932
The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
2933
and GIF file formats; you must disable the other file formats to compile a
2934
12-bit cjpeg or djpeg. (install.txt has more information about that.)
2935
At present, a 12-bit library can handle *only* 12-bit images, not both
2936
precisions. (If you need to include both 8- and 12-bit libraries in a single
2937
application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES
2938
for just one of the copies. You'd have to access the 8-bit and 12-bit copies
2939
from separate application source files. This is untested ... if you try it,
2940
we'd like to hear whether it works!)
2942
Note that a 12-bit library always compresses in Huffman optimization mode,
2943
in order to generate valid Huffman tables. This is necessary because our
2944
default Huffman tables only cover 8-bit data. If you need to output 12-bit
2945
files in one pass, you'll have to supply suitable default Huffman tables.
2946
You may also want to supply your own DCT quantization tables; the existing
2947
quality-scaling code has been developed for 8-bit use, and probably doesn't
2948
generate especially good tables for 12-bit.
2950
The maximum number of components (color channels) in the image is determined
2951
by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we
2952
expect that few applications will need more than four or so.
2954
On machines with unusual data type sizes, you may be able to improve
2955
performance or reduce memory space by tweaking the various typedefs in
2956
jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s
2957
is quite slow; consider trading memory for speed by making JCOEF, INT16, and
2958
UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int.
2959
You probably don't want to make JSAMPLE be int unless you have lots of memory
2962
You can reduce the size of the library by compiling out various optional
2963
functions. To do this, undefine xxx_SUPPORTED symbols as necessary.
2965
You can also save a few K by not having text error messages in the library;
2966
the standard error message table occupies about 5Kb. This is particularly
2967
reasonable for embedded applications where there's no good way to display
2968
a message anyway. To do this, remove the creation of the message table
2969
(jpeg_std_message_table[]) from jerror.c, and alter format_message to do
2970
something reasonable without it. You could output the numeric value of the
2971
message code number, for example. If you do this, you can also save a couple
2972
more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing;
2973
you don't need trace capability anyway, right?
2976
Portability considerations
2977
--------------------------
2979
The JPEG library has been written to be extremely portable; the sample
2980
applications cjpeg and djpeg are slightly less so. This section summarizes
2981
the design goals in this area. (If you encounter any bugs that cause the
2982
library to be less portable than is claimed here, we'd appreciate hearing
2985
The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of
2986
the popular system include file setups, and some not-so-popular ones too.
2987
See install.txt for configuration procedures.
2989
The code is not dependent on the exact sizes of the C data types. As
2990
distributed, we make the assumptions that
2991
char is at least 8 bits wide
2992
short is at least 16 bits wide
2993
int is at least 16 bits wide
2994
long is at least 32 bits wide
2995
(These are the minimum requirements of the ANSI C standard.) Wider types will
2996
work fine, although memory may be used inefficiently if char is much larger
2997
than 8 bits or short is much bigger than 16 bits. The code should work
2998
equally well with 16- or 32-bit ints.
3000
In a system where these assumptions are not met, you may be able to make the
3001
code work by modifying the typedefs in jmorecfg.h. However, you will probably
3002
have difficulty if int is less than 16 bits wide, since references to plain
3003
int abound in the code.
3005
char can be either signed or unsigned, although the code runs faster if an
3006
unsigned char type is available. If char is wider than 8 bits, you will need
3007
to redefine JOCTET and/or provide custom data source/destination managers so
3008
that JOCTET represents exactly 8 bits of data on external storage.
3010
The JPEG library proper does not assume ASCII representation of characters.
3011
But some of the image file I/O modules in cjpeg/djpeg do have ASCII
3012
dependencies in file-header manipulation; so does cjpeg's select_file_type()
3015
The JPEG library does not rely heavily on the C library. In particular, C
3016
stdio is used only by the data source/destination modules and the error
3017
handler, all of which are application-replaceable. (cjpeg/djpeg are more
3018
heavily dependent on stdio.) malloc and free are called only from the memory
3019
manager "back end" module, so you can use a different memory allocator by
3020
replacing that one file.
3022
The code generally assumes that C names must be unique in the first 15
3023
characters. However, global function names can be made unique in the
3024
first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES.
3026
More info about porting the code may be gleaned by reading jconfig.txt,
3027
jmorecfg.h, and jinclude.h.
3030
Notes for MS-DOS implementors
3031
-----------------------------
3033
The IJG code is designed to work efficiently in 80x86 "small" or "medium"
3034
memory models (i.e., data pointers are 16 bits unless explicitly declared
3035
"far"; code pointers can be either size). You may be able to use small
3036
model to compile cjpeg or djpeg by itself, but you will probably have to use
3037
medium model for any larger application. This won't make much difference in
3038
performance. You *will* take a noticeable performance hit if you use a
3039
large-data memory model (perhaps 10%-25%), and you should avoid "huge" model
3042
The JPEG library typically needs 2Kb-3Kb of stack space. It will also
3043
malloc about 20K-30K of near heap space while executing (and lots of far
3044
heap, but that doesn't count in this calculation). This figure will vary
3045
depending on selected operating mode, and to a lesser extent on image size.
3046
There is also about 5Kb-6Kb of constant data which will be allocated in the
3047
near data segment (about 4Kb of this is the error message table).
3048
Thus you have perhaps 20K available for other modules' static data and near
3049
heap space before you need to go to a larger memory model. The C library's
3050
static data will account for several K of this, but that still leaves a good
3051
deal for your needs. (If you are tight on space, you could reduce the sizes
3052
of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to
3053
1K. Another possibility is to move the error message table to far memory;
3054
this should be doable with only localized hacking on jerror.c.)
3056
About 2K of the near heap space is "permanent" memory that will not be
3057
released until you destroy the JPEG object. This is only an issue if you
3058
save a JPEG object between compression or decompression operations.
3060
Far data space may also be a tight resource when you are dealing with large
3061
images. The most memory-intensive case is decompression with two-pass color
3062
quantization, or single-pass quantization to an externally supplied color
3063
map. This requires a 128Kb color lookup table plus strip buffers amounting
3064
to about 40 bytes per column for typical sampling ratios (eg, about 25600
3065
bytes for a 640-pixel-wide image). You may not be able to process wide
3066
images if you have large data structures of your own.
3068
Of course, all of these concerns vanish if you use a 32-bit flat-memory-model
3069
compiler, such as DJGPP or Watcom C. We highly recommend flat model if you
3070
can use it; the JPEG library is significantly faster in flat model.
added
removed
source/Irrlicht/jpeglib/libjpeg.txt
