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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 N/8 with all N from 1 to 16. (The
1127
library design allows for arbitrary scaling ratios but this is not
1128
likely to be implemented any time soon.) The values are initialized
1129
by jpeg_read_header() with the source DCT size, which is currently
1130
8/8. If you change only the scale_num value while leaving the other
1131
unchanged, then this specifies the DCT scaled size to be applied on
1132
the given input, which is currently equivalent to N/8 scaling, since
1133
the source DCT size is currently always 8. Smaller scaling ratios
1134
permit significantly faster decoding since fewer pixels need be
1135
processed and a simpler IDCT method can be used.
1137
boolean quantize_colors
1138
If set TRUE, colormapped output will be delivered. Default is FALSE,
1139
meaning that full-color output will be delivered.
1141
The next three parameters are relevant only if quantize_colors is TRUE.
1143
int desired_number_of_colors
1144
Maximum number of colors to use in generating a library-supplied color
1145
map (the actual number of colors is returned in a different field).
1146
Default 256. Ignored when the application supplies its own color map.
1148
boolean two_pass_quantize
1149
If TRUE, an extra pass over the image is made to select a custom color
1150
map for the image. This usually looks a lot better than the one-size-
1151
fits-all colormap that is used otherwise. Default is TRUE. Ignored
1152
when the application supplies its own color map.
1154
J_DITHER_MODE dither_mode
1155
Selects color dithering method. Supported values are:
1156
JDITHER_NONE no dithering: fast, very low quality
1157
JDITHER_ORDERED ordered dither: moderate speed and quality
1158
JDITHER_FS Floyd-Steinberg dither: slow, high quality
1159
Default is JDITHER_FS. (At present, ordered dither is implemented
1160
only in the single-pass, standard-colormap case. If you ask for
1161
ordered dither when two_pass_quantize is TRUE or when you supply
1162
an external color map, you'll get F-S dithering.)
1164
When quantize_colors is TRUE, the target color map is described by the next
1165
two fields. colormap is set to NULL by jpeg_read_header(). The application
1166
can supply a color map by setting colormap non-NULL and setting
1167
actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress()
1168
selects a suitable color map and sets these two fields itself.
1169
[Implementation restriction: at present, an externally supplied colormap is
1170
only accepted for 3-component output color spaces.]
1173
The color map, represented as a 2-D pixel array of out_color_components
1174
rows and actual_number_of_colors columns. Ignored if not quantizing.
1175
CAUTION: if the JPEG library creates its own colormap, the storage
1176
pointed to by this field is released by jpeg_finish_decompress().
1177
Copy the colormap somewhere else first, if you want to save it.
1179
int actual_number_of_colors
1180
The number of colors in the color map.
1182
Additional decompression parameters that the application may set include:
1184
J_DCT_METHOD dct_method
1185
Selects the algorithm used for the DCT step. Choices are the same
1186
as described above for compression.
1188
boolean do_fancy_upsampling
1189
If TRUE, use direct DCT scaling with DCT size > 8 for upsampling
1190
of chroma components.
1191
If FALSE, use only DCT size <= 8 and simple separate upsampling.
1193
For better image stability in multiple generation compression cycles
1194
it is preferable that this value matches the corresponding
1195
do_fancy_downsampling value in compression.
1197
boolean do_block_smoothing
1198
If TRUE, interblock smoothing is applied in early stages of decoding
1199
progressive JPEG files; if FALSE, not. Default is TRUE. Early
1200
progression stages look "fuzzy" with smoothing, "blocky" without.
1201
In any case, block smoothing ceases to be applied after the first few
1202
AC coefficients are known to full accuracy, so it is relevant only
1203
when using buffered-image mode for progressive images.
1205
boolean enable_1pass_quant
1206
boolean enable_external_quant
1207
boolean enable_2pass_quant
1208
These are significant only in buffered-image mode, which is
1209
described in its own section below.
1212
The output image dimensions are given by the following fields. These are
1213
computed from the source image dimensions and the decompression parameters
1214
by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions()
1215
to obtain the values that will result from the current parameter settings.
1216
This can be useful if you are trying to pick a scaling ratio that will get
1217
close to a desired target size. It's also important if you are using the
1218
JPEG library's memory manager to allocate output buffer space, because you
1219
are supposed to request such buffers *before* jpeg_start_decompress().
1221
JDIMENSION output_width Actual dimensions of output image.
1222
JDIMENSION output_height
1223
int out_color_components Number of color components in out_color_space.
1224
int output_components Number of color components returned.
1225
int rec_outbuf_height Recommended height of scanline buffer.
1227
When quantizing colors, output_components is 1, indicating a single color map
1228
index per pixel. Otherwise it equals out_color_components. The output arrays
1229
are required to be output_width * output_components JSAMPLEs wide.
1231
rec_outbuf_height is the recommended minimum height (in scanlines) of the
1232
buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the
1233
library will still work, but time will be wasted due to unnecessary data
1234
copying. In high-quality modes, rec_outbuf_height is always 1, but some
1235
faster, lower-quality modes set it to larger values (typically 2 to 4).
1236
If you are going to ask for a high-speed processing mode, you may as well
1237
go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
1238
(An output buffer larger than rec_outbuf_height lines is OK, but won't
1239
provide any material speed improvement over that height.)
1242
Special color spaces
1243
--------------------
1245
The JPEG standard itself is "color blind" and doesn't specify any particular
1246
color space. It is customary to convert color data to a luminance/chrominance
1247
color space before compressing, since this permits greater compression. The
1248
existing de-facto JPEG file format standards specify YCbCr or grayscale data
1249
(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special
1250
applications such as multispectral images, other color spaces can be used,
1251
but it must be understood that such files will be unportable.
1253
The JPEG library can handle the most common colorspace conversions (namely
1254
RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown
1255
color space, passing it through without conversion. If you deal extensively
1256
with an unusual color space, you can easily extend the library to understand
1257
additional color spaces and perform appropriate conversions.
1259
For compression, the source data's color space is specified by field
1260
in_color_space. This is transformed to the JPEG file's color space given
1261
by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color
1262
space depending on in_color_space, but you can override this by calling
1263
jpeg_set_colorspace(). Of course you must select a supported transformation.
1264
jccolor.c currently supports the following transformations:
1269
plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
1270
YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.
1272
The de-facto file format standards (JFIF and Adobe) specify APPn markers that
1273
indicate the color space of the JPEG file. It is important to ensure that
1274
these are written correctly, or omitted if the JPEG file's color space is not
1275
one of the ones supported by the de-facto standards. jpeg_set_colorspace()
1276
will set the compression parameters to include or omit the APPn markers
1277
properly, so long as it is told the truth about the JPEG color space.
1278
For example, if you are writing some random 3-component color space without
1279
conversion, don't try to fake out the library by setting in_color_space and
1280
jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an
1281
APPn marker of your own devising to identify the colorspace --- see "Special
1284
When told that the color space is UNKNOWN, the library will default to using
1285
luminance-quality compression parameters for all color components. You may
1286
well want to change these parameters. See the source code for
1287
jpeg_set_colorspace(), in jcparam.c, for details.
1289
For decompression, the JPEG file's color space is given in jpeg_color_space,
1290
and this is transformed to the output color space out_color_space.
1291
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
1292
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
1293
guess. If you know the JPEG file's color space for certain, you can override
1294
jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also
1295
selects a default output color space based on (its guess of) jpeg_color_space;
1296
set out_color_space to override this. Again, you must select a supported
1297
transformation. jdcolor.c currently supports
1302
as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an
1303
application can force grayscale JPEGs to look like color JPEGs if it only
1304
wants to handle one case.)
1306
The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
1307
(it weights distances appropriately for RGB colors). You'll need to modify
1308
the code if you want to use it for non-RGB output color spaces. Note that
1309
jquant2.c is used to map to an application-supplied colormap as well as for
1310
the normal two-pass colormap selection process.
1312
CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
1313
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
1314
This is arguably a bug in Photoshop, but if you need to work with Photoshop
1315
CMYK files, you will have to deal with it in your application. We cannot
1316
"fix" this in the library by inverting the data during the CMYK<=>YCCK
1317
transform, because that would break other applications, notably Ghostscript.
1318
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
1319
data in the same inverted-YCCK representation used in bare JPEG files, but
1320
the surrounding PostScript code performs an inversion using the PS image
1321
operator. I am told that Photoshop 3.0 will write uninverted YCCK in
1322
EPS/JPEG files, and will omit the PS-level inversion. (But the data
1323
polarity used in bare JPEG files will not change in 3.0.) In either case,
1324
the JPEG library must not invert the data itself, or else Ghostscript would
1325
read these EPS files incorrectly.
1331
When the default error handler is used, any error detected inside the JPEG
1332
routines will cause a message to be printed on stderr, followed by exit().
1333
You can supply your own error handling routines to override this behavior
1334
and to control the treatment of nonfatal warnings and trace/debug messages.
1335
The file example.c illustrates the most common case, which is to have the
1336
application regain control after an error rather than exiting.
1338
The JPEG library never writes any message directly; it always goes through
1339
the error handling routines. Three classes of messages are recognized:
1340
* Fatal errors: the library cannot continue.
1341
* Warnings: the library can continue, but the data is corrupt, and a
1342
damaged output image is likely to result.
1343
* Trace/informational messages. These come with a trace level indicating
1344
the importance of the message; you can control the verbosity of the
1345
program by adjusting the maximum trace level that will be displayed.
1347
You may, if you wish, simply replace the entire JPEG error handling module
1348
(jerror.c) with your own code. However, you can avoid code duplication by
1349
only replacing some of the routines depending on the behavior you need.
1350
This is accomplished by calling jpeg_std_error() as usual, but then overriding
1351
some of the method pointers in the jpeg_error_mgr struct, as illustrated by
1354
All of the error handling routines will receive a pointer to the JPEG object
1355
(a j_common_ptr which points to either a jpeg_compress_struct or a
1356
jpeg_decompress_struct; if you need to tell which, test the is_decompressor
1357
field). This struct includes a pointer to the error manager struct in its
1358
"err" field. Frequently, custom error handler routines will need to access
1359
additional data which is not known to the JPEG library or the standard error
1360
handler. The most convenient way to do this is to embed either the JPEG
1361
object or the jpeg_error_mgr struct in a larger structure that contains
1362
additional fields; then casting the passed pointer provides access to the
1363
additional fields. Again, see example.c for one way to do it. (Beginning
1364
with IJG version 6b, there is also a void pointer "client_data" in each
1365
JPEG object, which the application can also use to find related data.
1366
The library does not touch client_data at all.)
1368
The individual methods that you might wish to override are:
1370
error_exit (j_common_ptr cinfo)
1371
Receives control for a fatal error. Information sufficient to
1372
generate the error message has been stored in cinfo->err; call
1373
output_message to display it. Control must NOT return to the caller;
1374
generally this routine will exit() or longjmp() somewhere.
1375
Typically you would override this routine to get rid of the exit()
1376
default behavior. Note that if you continue processing, you should
1377
clean up the JPEG object with jpeg_abort() or jpeg_destroy().
1379
output_message (j_common_ptr cinfo)
1380
Actual output of any JPEG message. Override this to send messages
1381
somewhere other than stderr. Note that this method does not know
1382
how to generate a message, only where to send it.
1384
format_message (j_common_ptr cinfo, char * buffer)
1385
Constructs a readable error message string based on the error info
1386
stored in cinfo->err. This method is called by output_message. Few
1387
applications should need to override this method. One possible
1388
reason for doing so is to implement dynamic switching of error message
1391
emit_message (j_common_ptr cinfo, int msg_level)
1392
Decide whether or not to emit a warning or trace message; if so,
1393
calls output_message. The main reason for overriding this method
1394
would be to abort on warnings. msg_level is -1 for warnings,
1395
0 and up for trace messages.
1397
Only error_exit() and emit_message() are called from the rest of the JPEG
1398
library; the other two are internal to the error handler.
1400
The actual message texts are stored in an array of strings which is pointed to
1401
by the field err->jpeg_message_table. The messages are numbered from 0 to
1402
err->last_jpeg_message, and it is these code numbers that are used in the
1403
JPEG library code. You could replace the message texts (for instance, with
1404
messages in French or German) by changing the message table pointer. See
1405
jerror.h for the default texts. CAUTION: this table will almost certainly
1406
change or grow from one library version to the next.
1408
It may be useful for an application to add its own message texts that are
1409
handled by the same mechanism. The error handler supports a second "add-on"
1410
message table for this purpose. To define an addon table, set the pointer
1411
err->addon_message_table and the message numbers err->first_addon_message and
1412
err->last_addon_message. If you number the addon messages beginning at 1000
1413
or so, you won't have to worry about conflicts with the library's built-in
1414
messages. See the sample applications cjpeg/djpeg for an example of using
1415
addon messages (the addon messages are defined in cderror.h).
1417
Actual invocation of the error handler is done via macros defined in jerror.h:
1418
ERREXITn(...) for fatal errors
1419
WARNMSn(...) for corrupt-data warnings
1420
TRACEMSn(...) for trace and informational messages.
1421
These macros store the message code and any additional parameters into the
1422
error handler struct, then invoke the error_exit() or emit_message() method.
1423
The variants of each macro are for varying numbers of additional parameters.
1424
The additional parameters are inserted into the generated message using
1425
standard printf() format codes.
1427
See jerror.h and jerror.c for further details.
1430
Compressed data handling (source and destination managers)
1431
----------------------------------------------------------
1433
The JPEG compression library sends its compressed data to a "destination
1434
manager" module. The default destination manager just writes the data to a
1435
stdio stream, but you can provide your own manager to do something else.
1436
Similarly, the decompression library calls a "source manager" to obtain the
1437
compressed data; you can provide your own source manager if you want the data
1438
to come from somewhere other than a stdio stream.
1440
In both cases, compressed data is processed a bufferload at a time: the
1441
destination or source manager provides a work buffer, and the library invokes
1442
the manager only when the buffer is filled or emptied. (You could define a
1443
one-character buffer to force the manager to be invoked for each byte, but
1444
that would be rather inefficient.) The buffer's size and location are
1445
controlled by the manager, not by the library. For example, if you desired to
1446
decompress a JPEG datastream that was all in memory, you could just make the
1447
buffer pointer and length point to the original data in memory. Then the
1448
buffer-reload procedure would be invoked only if the decompressor ran off the
1449
end of the datastream, which would indicate an erroneous datastream.
1451
The work buffer is defined as an array of datatype JOCTET, which is generally
1452
"char" or "unsigned char". On a machine where char is not exactly 8 bits
1453
wide, you must define JOCTET as a wider data type and then modify the data
1454
source and destination modules to transcribe the work arrays into 8-bit units
1455
on external storage.
1457
A data destination manager struct contains a pointer and count defining the
1458
next byte to write in the work buffer and the remaining free space:
1460
JOCTET * next_output_byte; /* => next byte to write in buffer */
1461
size_t free_in_buffer; /* # of byte spaces remaining in buffer */
1463
The library increments the pointer and decrements the count until the buffer
1464
is filled. The manager's empty_output_buffer method must reset the pointer
1465
and count. The manager is expected to remember the buffer's starting address
1466
and total size in private fields not visible to the library.
1468
A data destination manager provides three methods:
1470
init_destination (j_compress_ptr cinfo)
1471
Initialize destination. This is called by jpeg_start_compress()
1472
before any data is actually written. It must initialize
1473
next_output_byte and free_in_buffer. free_in_buffer must be
1474
initialized to a positive value.
1476
empty_output_buffer (j_compress_ptr cinfo)
1477
This is called whenever the buffer has filled (free_in_buffer
1478
reaches zero). In typical applications, it should write out the
1479
*entire* buffer (use the saved start address and buffer length;
1480
ignore the current state of next_output_byte and free_in_buffer).
1481
Then reset the pointer & count to the start of the buffer, and
1482
return TRUE indicating that the buffer has been dumped.
1483
free_in_buffer must be set to a positive value when TRUE is
1484
returned. A FALSE return should only be used when I/O suspension is
1485
desired (this operating mode is discussed in the next section).
1487
term_destination (j_compress_ptr cinfo)
1488
Terminate destination --- called by jpeg_finish_compress() after all
1489
data has been written. In most applications, this must flush any
1490
data remaining in the buffer. Use either next_output_byte or
1491
free_in_buffer to determine how much data is in the buffer.
1493
term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you
1494
want the destination manager to be cleaned up during an abort, you must do it
1497
You will also need code to create a jpeg_destination_mgr struct, fill in its
1498
method pointers, and insert a pointer to the struct into the "dest" field of
1499
the JPEG compression object. This can be done in-line in your setup code if
1500
you like, but it's probably cleaner to provide a separate routine similar to
1501
the jpeg_stdio_dest() routine of the supplied destination manager.
1503
Decompression source managers follow a parallel design, but with some
1504
additional frammishes. The source manager struct contains a pointer and count
1505
defining the next byte to read from the work buffer and the number of bytes
1508
const JOCTET * next_input_byte; /* => next byte to read from buffer */
1509
size_t bytes_in_buffer; /* # of bytes remaining in buffer */
1511
The library increments the pointer and decrements the count until the buffer
1512
is emptied. The manager's fill_input_buffer method must reset the pointer and
1513
count. In most applications, the manager must remember the buffer's starting
1514
address and total size in private fields not visible to the library.
1516
A data source manager provides five methods:
1518
init_source (j_decompress_ptr cinfo)
1519
Initialize source. This is called by jpeg_read_header() before any
1520
data is actually read. Unlike init_destination(), it may leave
1521
bytes_in_buffer set to 0 (in which case a fill_input_buffer() call
1522
will occur immediately).
1524
fill_input_buffer (j_decompress_ptr cinfo)
1525
This is called whenever bytes_in_buffer has reached zero and more
1526
data is wanted. In typical applications, it should read fresh data
1527
into the buffer (ignoring the current state of next_input_byte and
1528
bytes_in_buffer), reset the pointer & count to the start of the
1529
buffer, and return TRUE indicating that the buffer has been reloaded.
1530
It is not necessary to fill the buffer entirely, only to obtain at
1531
least one more byte. bytes_in_buffer MUST be set to a positive value
1532
if TRUE is returned. A FALSE return should only be used when I/O
1533
suspension is desired (this mode is discussed in the next section).
1535
skip_input_data (j_decompress_ptr cinfo, long num_bytes)
1536
Skip num_bytes worth of data. The buffer pointer and count should
1537
be advanced over num_bytes input bytes, refilling the buffer as
1538
needed. This is used to skip over a potentially large amount of
1539
uninteresting data (such as an APPn marker). In some applications
1540
it may be possible to optimize away the reading of the skipped data,
1541
but it's not clear that being smart is worth much trouble; large
1542
skips are uncommon. bytes_in_buffer may be zero on return.
1543
A zero or negative skip count should be treated as a no-op.
1545
resync_to_restart (j_decompress_ptr cinfo, int desired)
1546
This routine is called only when the decompressor has failed to find
1547
a restart (RSTn) marker where one is expected. Its mission is to
1548
find a suitable point for resuming decompression. For most
1549
applications, we recommend that you just use the default resync
1550
procedure, jpeg_resync_to_restart(). However, if you are able to back
1551
up in the input data stream, or if you have a-priori knowledge about
1552
the likely location of restart markers, you may be able to do better.
1553
Read the read_restart_marker() and jpeg_resync_to_restart() routines
1554
in jdmarker.c if you think you'd like to implement your own resync
1557
term_source (j_decompress_ptr cinfo)
1558
Terminate source --- called by jpeg_finish_decompress() after all
1559
data has been read. Often a no-op.
1561
For both fill_input_buffer() and skip_input_data(), there is no such thing
1562
as an EOF return. If the end of the file has been reached, the routine has
1563
a choice of exiting via ERREXIT() or inserting fake data into the buffer.
1564
In most cases, generating a warning message and inserting a fake EOI marker
1565
is the best course of action --- this will allow the decompressor to output
1566
however much of the image is there. In pathological cases, the decompressor
1567
may swallow the EOI and again demand data ... just keep feeding it fake EOIs.
1568
jdatasrc.c illustrates the recommended error recovery behavior.
1570
term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want
1571
the source manager to be cleaned up during an abort, you must do it yourself.
1573
You will also need code to create a jpeg_source_mgr struct, fill in its method
1574
pointers, and insert a pointer to the struct into the "src" field of the JPEG
1575
decompression object. This can be done in-line in your setup code if you
1576
like, but it's probably cleaner to provide a separate routine similar to the
1577
jpeg_stdio_src() routine of the supplied source manager.
1579
For more information, consult the stdio source and destination managers
1580
in jdatasrc.c and jdatadst.c.
1586
Some applications need to use the JPEG library as an incremental memory-to-
1587
memory filter: when the compressed data buffer is filled or emptied, they want
1588
control to return to the outer loop, rather than expecting that the buffer can
1589
be emptied or reloaded within the data source/destination manager subroutine.
1590
The library supports this need by providing an "I/O suspension" mode, which we
1591
describe in this section.
1593
The I/O suspension mode is not a panacea: nothing is guaranteed about the
1594
maximum amount of time spent in any one call to the library, so it will not
1595
eliminate response-time problems in single-threaded applications. If you
1596
need guaranteed response time, we suggest you "bite the bullet" and implement
1597
a real multi-tasking capability.
1599
To use I/O suspension, cooperation is needed between the calling application
1600
and the data source or destination manager; you will always need a custom
1601
source/destination manager. (Please read the previous section if you haven't
1602
already.) The basic idea is that the empty_output_buffer() or
1603
fill_input_buffer() routine is a no-op, merely returning FALSE to indicate
1604
that it has done nothing. Upon seeing this, the JPEG library suspends
1605
operation and returns to its caller. The surrounding application is
1606
responsible for emptying or refilling the work buffer before calling the
1609
Compression suspension:
1611
For compression suspension, use an empty_output_buffer() routine that returns
1612
FALSE; typically it will not do anything else. This will cause the
1613
compressor to return to the caller of jpeg_write_scanlines(), with the return
1614
value indicating that not all the supplied scanlines have been accepted.
1615
The application must make more room in the output buffer, adjust the output
1616
buffer pointer/count appropriately, and then call jpeg_write_scanlines()
1617
again, pointing to the first unconsumed scanline.
1619
When forced to suspend, the compressor will backtrack to a convenient stopping
1620
point (usually the start of the current MCU); it will regenerate some output
1621
data when restarted. Therefore, although empty_output_buffer() is only
1622
called when the buffer is filled, you should NOT write out the entire buffer
1623
after a suspension. Write only the data up to the current position of
1624
next_output_byte/free_in_buffer. The data beyond that point will be
1625
regenerated after resumption.
1627
Because of the backtracking behavior, a good-size output buffer is essential
1628
for efficiency; you don't want the compressor to suspend often. (In fact, an
1629
overly small buffer could lead to infinite looping, if a single MCU required
1630
more data than would fit in the buffer.) We recommend a buffer of at least
1631
several Kbytes. You may want to insert explicit code to ensure that you don't
1632
call jpeg_write_scanlines() unless there is a reasonable amount of space in
1633
the output buffer; in other words, flush the buffer before trying to compress
1636
The compressor does not allow suspension while it is trying to write JPEG
1637
markers at the beginning and end of the file. This means that:
1638
* At the beginning of a compression operation, there must be enough free
1639
space in the output buffer to hold the header markers (typically 600 or
1640
so bytes). The recommended buffer size is bigger than this anyway, so
1641
this is not a problem as long as you start with an empty buffer. However,
1642
this restriction might catch you if you insert large special markers, such
1643
as a JFIF thumbnail image, without flushing the buffer afterwards.
1644
* When you call jpeg_finish_compress(), there must be enough space in the
1645
output buffer to emit any buffered data and the final EOI marker. In the
1646
current implementation, half a dozen bytes should suffice for this, but
1647
for safety's sake we recommend ensuring that at least 100 bytes are free
1648
before calling jpeg_finish_compress().
1650
A more significant restriction is that jpeg_finish_compress() cannot suspend.
1651
This means you cannot use suspension with multi-pass operating modes, namely
1652
Huffman code optimization and multiple-scan output. Those modes write the
1653
whole file during jpeg_finish_compress(), which will certainly result in
1654
buffer overrun. (Note that this restriction applies only to compression,
1655
not decompression. The decompressor supports input suspension in all of its
1658
Decompression suspension:
1660
For decompression suspension, use a fill_input_buffer() routine that simply
1661
returns FALSE (except perhaps during error recovery, as discussed below).
1662
This will cause the decompressor to return to its caller with an indication
1663
that suspension has occurred. This can happen at four places:
1664
* jpeg_read_header(): will return JPEG_SUSPENDED.
1665
* jpeg_start_decompress(): will return FALSE, rather than its usual TRUE.
1666
* jpeg_read_scanlines(): will return the number of scanlines already
1667
completed (possibly 0).
1668
* jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE.
1669
The surrounding application must recognize these cases, load more data into
1670
the input buffer, and repeat the call. In the case of jpeg_read_scanlines(),
1671
increment the passed pointers past any scanlines successfully read.
1673
Just as with compression, the decompressor will typically backtrack to a
1674
convenient restart point before suspending. When fill_input_buffer() is
1675
called, next_input_byte/bytes_in_buffer point to the current restart point,
1676
which is where the decompressor will backtrack to if FALSE is returned.
1677
The data beyond that position must NOT be discarded if you suspend; it needs
1678
to be re-read upon resumption. In most implementations, you'll need to shift
1679
this data down to the start of your work buffer and then load more data after
1680
it. Again, this behavior means that a several-Kbyte work buffer is essential
1681
for decent performance; furthermore, you should load a reasonable amount of
1682
new data before resuming decompression. (If you loaded, say, only one new
1683
byte each time around, you could waste a LOT of cycles.)
1685
The skip_input_data() source manager routine requires special care in a
1686
suspension scenario. This routine is NOT granted the ability to suspend the
1687
decompressor; it can decrement bytes_in_buffer to zero, but no more. If the
1688
requested skip distance exceeds the amount of data currently in the input
1689
buffer, then skip_input_data() must set bytes_in_buffer to zero and record the
1690
additional skip distance somewhere else. The decompressor will immediately
1691
call fill_input_buffer(), which should return FALSE, which will cause a
1692
suspension return. The surrounding application must then arrange to discard
1693
the recorded number of bytes before it resumes loading the input buffer.
1694
(Yes, this design is rather baroque, but it avoids complexity in the far more
1695
common case where a non-suspending source manager is used.)
1697
If the input data has been exhausted, we recommend that you emit a warning
1698
and insert dummy EOI markers just as a non-suspending data source manager
1699
would do. This can be handled either in the surrounding application logic or
1700
within fill_input_buffer(); the latter is probably more efficient. If
1701
fill_input_buffer() knows that no more data is available, it can set the
1702
pointer/count to point to a dummy EOI marker and then return TRUE just as
1703
though it had read more data in a non-suspending situation.
1705
The decompressor does not attempt to suspend within standard JPEG markers;
1706
instead it will backtrack to the start of the marker and reprocess the whole
1707
marker next time. Hence the input buffer must be large enough to hold the
1708
longest standard marker in the file. Standard JPEG markers should normally
1709
not exceed a few hundred bytes each (DHT tables are typically the longest).
1710
We recommend at least a 2K buffer for performance reasons, which is much
1711
larger than any correct marker is likely to be. For robustness against
1712
damaged marker length counts, you may wish to insert a test in your
1713
application for the case that the input buffer is completely full and yet
1714
the decoder has suspended without consuming any data --- otherwise, if this
1715
situation did occur, it would lead to an endless loop. (The library can't
1716
provide this test since it has no idea whether "the buffer is full", or
1717
even whether there is a fixed-size input buffer.)
1719
The input buffer would need to be 64K to allow for arbitrary COM or APPn
1720
markers, but these are handled specially: they are either saved into allocated
1721
memory, or skipped over by calling skip_input_data(). In the former case,
1722
suspension is handled correctly, and in the latter case, the problem of
1723
buffer overrun is placed on skip_input_data's shoulders, as explained above.
1724
Note that if you provide your own marker handling routine for large markers,
1725
you should consider how to deal with buffer overflow.
1727
Multiple-buffer management:
1729
In some applications it is desirable to store the compressed data in a linked
1730
list of buffer areas, so as to avoid data copying. This can be handled by
1731
having empty_output_buffer() or fill_input_buffer() set the pointer and count
1732
to reference the next available buffer; FALSE is returned only if no more
1733
buffers are available. Although seemingly straightforward, there is a
1734
pitfall in this approach: the backtrack that occurs when FALSE is returned
1735
could back up into an earlier buffer. For example, when fill_input_buffer()
1736
is called, the current pointer & count indicate the backtrack restart point.
1737
Since fill_input_buffer() will set the pointer and count to refer to a new
1738
buffer, the restart position must be saved somewhere else. Suppose a second
1739
call to fill_input_buffer() occurs in the same library call, and no
1740
additional input data is available, so fill_input_buffer must return FALSE.
1741
If the JPEG library has not moved the pointer/count forward in the current
1742
buffer, then *the correct restart point is the saved position in the prior
1743
buffer*. Prior buffers may be discarded only after the library establishes
1744
a restart point within a later buffer. Similar remarks apply for output into
1747
The library will never attempt to backtrack over a skip_input_data() call,
1748
so any skipped data can be permanently discarded. You still have to deal
1749
with the case of skipping not-yet-received data, however.
1751
It's much simpler to use only a single buffer; when fill_input_buffer() is
1752
called, move any unconsumed data (beyond the current pointer/count) down to
1753
the beginning of this buffer and then load new data into the remaining buffer
1754
space. This approach requires a little more data copying but is far easier
1758
Progressive JPEG support
1759
------------------------
1761
Progressive JPEG rearranges the stored data into a series of scans of
1762
increasing quality. In situations where a JPEG file is transmitted across a
1763
slow communications link, a decoder can generate a low-quality image very
1764
quickly from the first scan, then gradually improve the displayed quality as
1765
more scans are received. The final image after all scans are complete is
1766
identical to that of a regular (sequential) JPEG file of the same quality
1767
setting. Progressive JPEG files are often slightly smaller than equivalent
1768
sequential JPEG files, but the possibility of incremental display is the main
1769
reason for using progressive JPEG.
1771
The IJG encoder library generates progressive JPEG files when given a
1772
suitable "scan script" defining how to divide the data into scans.
1773
Creation of progressive JPEG files is otherwise transparent to the encoder.
1774
Progressive JPEG files can also be read transparently by the decoder library.
1775
If the decoding application simply uses the library as defined above, it
1776
will receive a final decoded image without any indication that the file was
1777
progressive. Of course, this approach does not allow incremental display.
1778
To perform incremental display, an application needs to use the decoder
1779
library's "buffered-image" mode, in which it receives a decoded image
1782
Each displayed scan requires about as much work to decode as a full JPEG
1783
image of the same size, so the decoder must be fairly fast in relation to the
1784
data transmission rate in order to make incremental display useful. However,
1785
it is possible to skip displaying the image and simply add the incoming bits
1786
to the decoder's coefficient buffer. This is fast because only Huffman
1787
decoding need be done, not IDCT, upsampling, colorspace conversion, etc.
1788
The IJG decoder library allows the application to switch dynamically between
1789
displaying the image and simply absorbing the incoming bits. A properly
1790
coded application can automatically adapt the number of display passes to
1791
suit the time available as the image is received. Also, a final
1792
higher-quality display cycle can be performed from the buffered data after
1793
the end of the file is reached.
1795
Progressive compression:
1797
To create a progressive JPEG file (or a multiple-scan sequential JPEG file),
1798
set the scan_info cinfo field to point to an array of scan descriptors, and
1799
perform compression as usual. Instead of constructing your own scan list,
1800
you can call the jpeg_simple_progression() helper routine to create a
1801
recommended progression sequence; this method should be used by all
1802
applications that don't want to get involved in the nitty-gritty of
1803
progressive scan sequence design. (If you want to provide user control of
1804
scan sequences, you may wish to borrow the scan script reading code found
1805
in rdswitch.c, so that you can read scan script files just like cjpeg's.)
1806
When scan_info is not NULL, the compression library will store DCT'd data
1807
into a buffer array as jpeg_write_scanlines() is called, and will emit all
1808
the requested scans during jpeg_finish_compress(). This implies that
1809
multiple-scan output cannot be created with a suspending data destination
1810
manager, since jpeg_finish_compress() does not support suspension. We
1811
should also note that the compressor currently forces Huffman optimization
1812
mode when creating a progressive JPEG file, because the default Huffman
1813
tables are unsuitable for progressive files.
1815
Progressive decompression:
1817
When buffered-image mode is not used, the decoder library will read all of
1818
a multi-scan file during jpeg_start_decompress(), so that it can provide a
1819
final decoded image. (Here "multi-scan" means either progressive or
1820
multi-scan sequential.) This makes multi-scan files transparent to the
1821
decoding application. However, existing applications that used suspending
1822
input with version 5 of the IJG library will need to be modified to check
1823
for a suspension return from jpeg_start_decompress().
1825
To perform incremental display, an application must use the library's
1826
buffered-image mode. This is described in the next section.
1832
In buffered-image mode, the library stores the partially decoded image in a
1833
coefficient buffer, from which it can be read out as many times as desired.
1834
This mode is typically used for incremental display of progressive JPEG files,
1835
but it can be used with any JPEG file. Each scan of a progressive JPEG file
1836
adds more data (more detail) to the buffered image. The application can
1837
display in lockstep with the source file (one display pass per input scan),
1838
or it can allow input processing to outrun display processing. By making
1839
input and display processing run independently, it is possible for the
1840
application to adapt progressive display to a wide range of data transmission
1843
The basic control flow for buffered-image decoding is
1845
jpeg_create_decompress()
1848
set overall decompression parameters
1849
cinfo.buffered_image = TRUE; /* select buffered-image mode */
1850
jpeg_start_decompress()
1851
for (each output pass) {
1852
adjust output decompression parameters if required
1853
jpeg_start_output() /* start a new output pass */
1854
for (all scanlines in image) {
1855
jpeg_read_scanlines()
1858
jpeg_finish_output() /* terminate output pass */
1860
jpeg_finish_decompress()
1861
jpeg_destroy_decompress()
1863
This differs from ordinary unbuffered decoding in that there is an additional
1864
level of looping. The application can choose how many output passes to make
1865
and how to display each pass.
1867
The simplest approach to displaying progressive images is to do one display
1868
pass for each scan appearing in the input file. In this case the outer loop
1869
condition is typically
1870
while (! jpeg_input_complete(&cinfo))
1871
and the start-output call should read
1872
jpeg_start_output(&cinfo, cinfo.input_scan_number);
1873
The second parameter to jpeg_start_output() indicates which scan of the input
1874
file is to be displayed; the scans are numbered starting at 1 for this
1875
purpose. (You can use a loop counter starting at 1 if you like, but using
1876
the library's input scan counter is easier.) The library automatically reads
1877
data as necessary to complete each requested scan, and jpeg_finish_output()
1878
advances to the next scan or end-of-image marker (hence input_scan_number
1879
will be incremented by the time control arrives back at jpeg_start_output()).
1880
With this technique, data is read from the input file only as needed, and
1881
input and output processing run in lockstep.
1883
After reading the final scan and reaching the end of the input file, the
1884
buffered image remains available; it can be read additional times by
1885
repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output()
1886
sequence. For example, a useful technique is to use fast one-pass color
1887
quantization for display passes made while the image is arriving, followed by
1888
a final display pass using two-pass quantization for highest quality. This
1889
is done by changing the library parameters before the final output pass.
1890
Changing parameters between passes is discussed in detail below.
1892
In general the last scan of a progressive file cannot be recognized as such
1893
until after it is read, so a post-input display pass is the best approach if
1894
you want special processing in the final pass.
1896
When done with the image, be sure to call jpeg_finish_decompress() to release
1897
the buffered image (or just use jpeg_destroy_decompress()).
1899
If input data arrives faster than it can be displayed, the application can
1900
cause the library to decode input data in advance of what's needed to produce
1901
output. This is done by calling the routine jpeg_consume_input().
1902
The return value is one of the following:
1903
JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan)
1904
JPEG_REACHED_EOI: reached the EOI marker (end of image)
1905
JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data
1906
JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan
1907
JPEG_SUSPENDED: suspended before completing any of the above
1908
(JPEG_SUSPENDED can occur only if a suspending data source is used.) This
1909
routine can be called at any time after initializing the JPEG object. It
1910
reads some additional data and returns when one of the indicated significant
1911
events occurs. (If called after the EOI marker is reached, it will
1912
immediately return JPEG_REACHED_EOI without attempting to read more data.)
1914
The library's output processing will automatically call jpeg_consume_input()
1915
whenever the output processing overtakes the input; thus, simple lockstep
1916
display requires no direct calls to jpeg_consume_input(). But by adding
1917
calls to jpeg_consume_input(), you can absorb data in advance of what is
1918
being displayed. This has two benefits:
1919
* You can limit buildup of unprocessed data in your input buffer.
1920
* You can eliminate extra display passes by paying attention to the
1921
state of the library's input processing.
1923
The first of these benefits only requires interspersing calls to
1924
jpeg_consume_input() with your display operations and any other processing
1925
you may be doing. To avoid wasting cycles due to backtracking, it's best to
1926
call jpeg_consume_input() only after a hundred or so new bytes have arrived.
1927
This is discussed further under "I/O suspension", above. (Note: the JPEG
1928
library currently is not thread-safe. You must not call jpeg_consume_input()
1929
from one thread of control if a different library routine is working on the
1930
same JPEG object in another thread.)
1932
When input arrives fast enough that more than one new scan is available
1933
before you start a new output pass, you may as well skip the output pass
1934
corresponding to the completed scan. This occurs for free if you pass
1935
cinfo.input_scan_number as the target scan number to jpeg_start_output().
1936
The input_scan_number field is simply the index of the scan currently being
1937
consumed by the input processor. You can ensure that this is up-to-date by
1938
emptying the input buffer just before calling jpeg_start_output(): call
1939
jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or
1942
The target scan number passed to jpeg_start_output() is saved in the
1943
cinfo.output_scan_number field. The library's output processing calls
1944
jpeg_consume_input() whenever the current input scan number and row within
1945
that scan is less than or equal to the current output scan number and row.
1946
Thus, input processing can "get ahead" of the output processing but is not
1947
allowed to "fall behind". You can achieve several different effects by
1948
manipulating this interlock rule. For example, if you pass a target scan
1949
number greater than the current input scan number, the output processor will
1950
wait until that scan starts to arrive before producing any output. (To avoid
1951
an infinite loop, the target scan number is automatically reset to the last
1952
scan number when the end of image is reached. Thus, if you specify a large
1953
target scan number, the library will just absorb the entire input file and
1954
then perform an output pass. This is effectively the same as what
1955
jpeg_start_decompress() does when you don't select buffered-image mode.)
1956
When you pass a target scan number equal to the current input scan number,
1957
the image is displayed no faster than the current input scan arrives. The
1958
final possibility is to pass a target scan number less than the current input
1959
scan number; this disables the input/output interlock and causes the output
1960
processor to simply display whatever it finds in the image buffer, without
1961
waiting for input. (However, the library will not accept a target scan
1962
number less than one, so you can't avoid waiting for the first scan.)
1964
When data is arriving faster than the output display processing can advance
1965
through the image, jpeg_consume_input() will store data into the buffered
1966
image beyond the point at which the output processing is reading data out
1967
again. If the input arrives fast enough, it may "wrap around" the buffer to
1968
the point where the input is more than one whole scan ahead of the output.
1969
If the output processing simply proceeds through its display pass without
1970
paying attention to the input, the effect seen on-screen is that the lower
1971
part of the image is one or more scans better in quality than the upper part.
1972
Then, when the next output scan is started, you have a choice of what target
1973
scan number to use. The recommended choice is to use the current input scan
1974
number at that time, which implies that you've skipped the output scans
1975
corresponding to the input scans that were completed while you processed the
1976
previous output scan. In this way, the decoder automatically adapts its
1977
speed to the arriving data, by skipping output scans as necessary to keep up
1978
with the arriving data.
1980
When using this strategy, you'll want to be sure that you perform a final
1981
output pass after receiving all the data; otherwise your last display may not
1982
be full quality across the whole screen. So the right outer loop logic is
1983
something like this:
1985
absorb any waiting input by calling jpeg_consume_input()
1986
final_pass = jpeg_input_complete(&cinfo);
1987
adjust output decompression parameters if required
1988
jpeg_start_output(&cinfo, cinfo.input_scan_number);
1990
jpeg_finish_output()
1991
} while (! final_pass);
1992
rather than quitting as soon as jpeg_input_complete() returns TRUE. This
1993
arrangement makes it simple to use higher-quality decoding parameters
1994
for the final pass. But if you don't want to use special parameters for
1995
the final pass, the right loop logic is like this:
1997
absorb any waiting input by calling jpeg_consume_input()
1998
jpeg_start_output(&cinfo, cinfo.input_scan_number);
2000
jpeg_finish_output()
2001
if (jpeg_input_complete(&cinfo) &&
2002
cinfo.input_scan_number == cinfo.output_scan_number)
2005
In this case you don't need to know in advance whether an output pass is to
2006
be the last one, so it's not necessary to have reached EOF before starting
2007
the final output pass; rather, what you want to test is whether the output
2008
pass was performed in sync with the final input scan. This form of the loop
2009
will avoid an extra output pass whenever the decoder is able (or nearly able)
2010
to keep up with the incoming data.
2012
When the data transmission speed is high, you might begin a display pass,
2013
then find that much or all of the file has arrived before you can complete
2014
the pass. (You can detect this by noting the JPEG_REACHED_EOI return code
2015
from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().)
2016
In this situation you may wish to abort the current display pass and start a
2017
new one using the newly arrived information. To do so, just call
2018
jpeg_finish_output() and then start a new pass with jpeg_start_output().
2020
A variant strategy is to abort and restart display if more than one complete
2021
scan arrives during an output pass; this can be detected by noting
2022
JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This
2023
idea should be employed with caution, however, since the display process
2024
might never get to the bottom of the image before being aborted, resulting
2025
in the lower part of the screen being several passes worse than the upper.
2026
In most cases it's probably best to abort an output pass only if the whole
2027
file has arrived and you want to begin the final output pass immediately.
2029
When receiving data across a communication link, we recommend always using
2030
the current input scan number for the output target scan number; if a
2031
higher-quality final pass is to be done, it should be started (aborting any
2032
incomplete output pass) as soon as the end of file is received. However,
2033
many other strategies are possible. For example, the application can examine
2034
the parameters of the current input scan and decide whether to display it or
2035
not. If the scan contains only chroma data, one might choose not to use it
2036
as the target scan, expecting that the scan will be small and will arrive
2037
quickly. To skip to the next scan, call jpeg_consume_input() until it
2038
returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher
2039
number as the target scan for jpeg_start_output(); but that method doesn't
2040
let you inspect the next scan's parameters before deciding to display it.
2043
In buffered-image mode, jpeg_start_decompress() never performs input and
2044
thus never suspends. An application that uses input suspension with
2045
buffered-image mode must be prepared for suspension returns from these
2047
* jpeg_start_output() performs input only if you request 2-pass quantization
2048
and the target scan isn't fully read yet. (This is discussed below.)
2049
* jpeg_read_scanlines(), as always, returns the number of scanlines that it
2050
was able to produce before suspending.
2051
* jpeg_finish_output() will read any markers following the target scan,
2052
up to the end of the file or the SOS marker that begins another scan.
2053
(But it reads no input if jpeg_consume_input() has already reached the
2054
end of the file or a SOS marker beyond the target output scan.)
2055
* jpeg_finish_decompress() will read until the end of file, and thus can
2056
suspend if the end hasn't already been reached (as can be tested by
2057
calling jpeg_input_complete()).
2058
jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress()
2059
all return TRUE if they completed their tasks, FALSE if they had to suspend.
2060
In the event of a FALSE return, the application must load more input data
2061
and repeat the call. Applications that use non-suspending data sources need
2062
not check the return values of these three routines.
2065
It is possible to change decoding parameters between output passes in the
2066
buffered-image mode. The decoder library currently supports only very
2067
limited changes of parameters. ONLY THE FOLLOWING parameter changes are
2068
allowed after jpeg_start_decompress() is called:
2069
* dct_method can be changed before each call to jpeg_start_output().
2070
For example, one could use a fast DCT method for early scans, changing
2071
to a higher quality method for the final scan.
2072
* dither_mode can be changed before each call to jpeg_start_output();
2073
of course this has no impact if not using color quantization. Typically
2074
one would use ordered dither for initial passes, then switch to
2075
Floyd-Steinberg dither for the final pass. Caution: changing dither mode
2076
can cause more memory to be allocated by the library. Although the amount
2077
of memory involved is not large (a scanline or so), it may cause the
2078
initial max_memory_to_use specification to be exceeded, which in the worst
2079
case would result in an out-of-memory failure.
2080
* do_block_smoothing can be changed before each call to jpeg_start_output().
2081
This setting is relevant only when decoding a progressive JPEG image.
2082
During the first DC-only scan, block smoothing provides a very "fuzzy" look
2083
instead of the very "blocky" look seen without it; which is better seems a
2084
matter of personal taste. But block smoothing is nearly always a win
2085
during later stages, especially when decoding a successive-approximation
2086
image: smoothing helps to hide the slight blockiness that otherwise shows
2087
up on smooth gradients until the lowest coefficient bits are sent.
2088
* Color quantization mode can be changed under the rules described below.
2089
You *cannot* change between full-color and quantized output (because that
2090
would alter the required I/O buffer sizes), but you can change which
2091
quantization method is used.
2093
When generating color-quantized output, changing quantization method is a
2094
very useful way of switching between high-speed and high-quality display.
2095
The library allows you to change among its three quantization methods:
2096
1. Single-pass quantization to a fixed color cube.
2097
Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL.
2098
2. Single-pass quantization to an application-supplied colormap.
2099
Selected by setting cinfo.colormap to point to the colormap (the value of
2100
two_pass_quantize is ignored); also set cinfo.actual_number_of_colors.
2101
3. Two-pass quantization to a colormap chosen specifically for the image.
2102
Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.
2103
(This is the default setting selected by jpeg_read_header, but it is
2104
probably NOT what you want for the first pass of progressive display!)
2105
These methods offer successively better quality and lesser speed. However,
2106
only the first method is available for quantizing in non-RGB color spaces.
2108
IMPORTANT: because the different quantizer methods have very different
2109
working-storage requirements, the library requires you to indicate which
2110
one(s) you intend to use before you call jpeg_start_decompress(). (If we did
2111
not require this, the max_memory_to_use setting would be a complete fiction.)
2112
You do this by setting one or more of these three cinfo fields to TRUE:
2113
enable_1pass_quant Fixed color cube colormap
2114
enable_external_quant Externally-supplied colormap
2115
enable_2pass_quant Two-pass custom colormap
2116
All three are initialized FALSE by jpeg_read_header(). But
2117
jpeg_start_decompress() automatically sets TRUE the one selected by the
2118
current two_pass_quantize and colormap settings, so you only need to set the
2119
enable flags for any other quantization methods you plan to change to later.
2121
After setting the enable flags correctly at jpeg_start_decompress() time, you
2122
can change to any enabled quantization method by setting two_pass_quantize
2123
and colormap properly just before calling jpeg_start_output(). The following
2124
special rules apply:
2125
1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass
2126
or 2-pass mode from a different mode, or when you want the 2-pass
2127
quantizer to be re-run to generate a new colormap.
2128
2. To switch to an external colormap, or to change to a different external
2129
colormap than was used on the prior pass, you must call
2130
jpeg_new_colormap() after setting cinfo.colormap.
2131
NOTE: if you want to use the same colormap as was used in the prior pass,
2132
you should not do either of these things. This will save some nontrivial
2134
(These requirements exist because cinfo.colormap will always be non-NULL
2135
after completing a prior output pass, since both the 1-pass and 2-pass
2136
quantizers set it to point to their output colormaps. Thus you have to
2137
do one of these two things to notify the library that something has changed.
2138
Yup, it's a bit klugy, but it's necessary to do it this way for backwards
2141
Note that in buffered-image mode, the library generates any requested colormap
2142
during jpeg_start_output(), not during jpeg_start_decompress().
2144
When using two-pass quantization, jpeg_start_output() makes a pass over the
2145
buffered image to determine the optimum color map; it therefore may take a
2146
significant amount of time, whereas ordinarily it does little work. The
2147
progress monitor hook is called during this pass, if defined. It is also
2148
important to realize that if the specified target scan number is greater than
2149
or equal to the current input scan number, jpeg_start_output() will attempt
2150
to consume input as it makes this pass. If you use a suspending data source,
2151
you need to check for a FALSE return from jpeg_start_output() under these
2152
conditions. The combination of 2-pass quantization and a not-yet-fully-read
2153
target scan is the only case in which jpeg_start_output() will consume input.
2156
Application authors who support buffered-image mode may be tempted to use it
2157
for all JPEG images, even single-scan ones. This will work, but it is
2158
inefficient: there is no need to create an image-sized coefficient buffer for
2159
single-scan images. Requesting buffered-image mode for such an image wastes
2160
memory. Worse, it can cost time on large images, since the buffered data has
2161
to be swapped out or written to a temporary file. If you are concerned about
2162
maximum performance on baseline JPEG files, you should use buffered-image
2163
mode only when the incoming file actually has multiple scans. This can be
2164
tested by calling jpeg_has_multiple_scans(), which will return a correct
2165
result at any time after jpeg_read_header() completes.
2167
It is also worth noting that when you use jpeg_consume_input() to let input
2168
processing get ahead of output processing, the resulting pattern of access to
2169
the coefficient buffer is quite nonsequential. It's best to use the memory
2170
manager jmemnobs.c if you can (ie, if you have enough real or virtual main
2171
memory). If not, at least make sure that max_memory_to_use is set as high as
2172
possible. If the JPEG memory manager has to use a temporary file, you will
2173
probably see a lot of disk traffic and poor performance. (This could be
2174
improved with additional work on the memory manager, but we haven't gotten
2177
In some applications it may be convenient to use jpeg_consume_input() for all
2178
input processing, including reading the initial markers; that is, you may
2179
wish to call jpeg_consume_input() instead of jpeg_read_header() during
2180
startup. This works, but note that you must check for JPEG_REACHED_SOS and
2181
JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes.
2182
Once the first SOS marker has been reached, you must call
2183
jpeg_start_decompress() before jpeg_consume_input() will consume more input;
2184
it'll just keep returning JPEG_REACHED_SOS until you do. If you read a
2185
tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI
2186
without ever returning JPEG_REACHED_SOS; be sure to check for this case.
2187
If this happens, the decompressor will not read any more input until you call
2188
jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not
2189
using buffered-image mode, but in that case it's basically a no-op after the
2190
initial markers have been read: it will just return JPEG_SUSPENDED.
2193
Abbreviated datastreams and multiple images
2194
-------------------------------------------
2196
A JPEG compression or decompression object can be reused to process multiple
2197
images. This saves a small amount of time per image by eliminating the
2198
"create" and "destroy" operations, but that isn't the real purpose of the
2199
feature. Rather, reuse of an object provides support for abbreviated JPEG
2200
datastreams. Object reuse can also simplify processing a series of images in
2201
a single input or output file. This section explains these features.
2203
A JPEG file normally contains several hundred bytes worth of quantization
2204
and Huffman tables. In a situation where many images will be stored or
2205
transmitted with identical tables, this may represent an annoying overhead.
2206
The JPEG standard therefore permits tables to be omitted. The standard
2207
defines three classes of JPEG datastreams:
2208
* "Interchange" datastreams contain an image and all tables needed to decode
2209
the image. These are the usual kind of JPEG file.
2210
* "Abbreviated image" datastreams contain an image, but are missing some or
2211
all of the tables needed to decode that image.
2212
* "Abbreviated table specification" (henceforth "tables-only") datastreams
2213
contain only table specifications.
2214
To decode an abbreviated image, it is necessary to load the missing table(s)
2215
into the decoder beforehand. This can be accomplished by reading a separate
2216
tables-only file. A variant scheme uses a series of images in which the first
2217
image is an interchange (complete) datastream, while subsequent ones are
2218
abbreviated and rely on the tables loaded by the first image. It is assumed
2219
that once the decoder has read a table, it will remember that table until a
2220
new definition for the same table number is encountered.
2222
It is the application designer's responsibility to figure out how to associate
2223
the correct tables with an abbreviated image. While abbreviated datastreams
2224
can be useful in a closed environment, their use is strongly discouraged in
2225
any situation where data exchange with other applications might be needed.
2228
The JPEG library provides support for reading and writing any combination of
2229
tables-only datastreams and abbreviated images. In both compression and
2230
decompression objects, a quantization or Huffman table will be retained for
2231
the lifetime of the object, unless it is overwritten by a new table definition.
2234
To create abbreviated image datastreams, it is only necessary to tell the
2235
compressor not to emit some or all of the tables it is using. Each
2236
quantization and Huffman table struct contains a boolean field "sent_table",
2237
which normally is initialized to FALSE. For each table used by the image, the
2238
header-writing process emits the table and sets sent_table = TRUE unless it is
2239
already TRUE. (In normal usage, this prevents outputting the same table
2240
definition multiple times, as would otherwise occur because the chroma
2241
components typically share tables.) Thus, setting this field to TRUE before
2242
calling jpeg_start_compress() will prevent the table from being written at
2245
If you want to create a "pure" abbreviated image file containing no tables,
2246
just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the
2247
tables. If you want to emit some but not all tables, you'll need to set the
2248
individual sent_table fields directly.
2250
To create an abbreviated image, you must also call jpeg_start_compress()
2251
with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress()
2252
will force all the sent_table fields to FALSE. (This is a safety feature to
2253
prevent abbreviated images from being created accidentally.)
2255
To create a tables-only file, perform the same parameter setup that you
2256
normally would, but instead of calling jpeg_start_compress() and so on, call
2257
jpeg_write_tables(&cinfo). This will write an abbreviated datastream
2258
containing only SOI, DQT and/or DHT markers, and EOI. All the quantization
2259
and Huffman tables that are currently defined in the compression object will
2260
be emitted unless their sent_tables flag is already TRUE, and then all the
2261
sent_tables flags will be set TRUE.
2263
A sure-fire way to create matching tables-only and abbreviated image files
2264
is to proceed as follows:
2266
create JPEG compression object
2268
set destination to tables-only file
2269
jpeg_write_tables(&cinfo);
2270
set destination to image file
2271
jpeg_start_compress(&cinfo, FALSE);
2273
jpeg_finish_compress(&cinfo);
2275
Since the JPEG parameters are not altered between writing the table file and
2276
the abbreviated image file, the same tables are sure to be used. Of course,
2277
you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence
2278
many times to produce many abbreviated image files matching the table file.
2280
You cannot suppress output of the computed Huffman tables when Huffman
2281
optimization is selected. (If you could, there'd be no way to decode the
2282
image...) Generally, you don't want to set optimize_coding = TRUE when
2283
you are trying to produce abbreviated files.
2285
In some cases you might want to compress an image using tables which are
2286
not stored in the application, but are defined in an interchange or
2287
tables-only file readable by the application. This can be done by setting up
2288
a JPEG decompression object to read the specification file, then copying the
2289
tables into your compression object. See jpeg_copy_critical_parameters()
2290
for an example of copying quantization tables.
2293
To read abbreviated image files, you simply need to load the proper tables
2294
into the decompression object before trying to read the abbreviated image.
2295
If the proper tables are stored in the application program, you can just
2296
allocate the table structs and fill in their contents directly. For example,
2297
to load a fixed quantization table into table slot "n":
2299
if (cinfo.quant_tbl_ptrs[n] == NULL)
2300
cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo);
2301
quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */
2302
for (i = 0; i < 64; i++) {
2303
/* Qtable[] is desired quantization table, in natural array order */
2304
quant_ptr->quantval[i] = Qtable[i];
2307
Code to load a fixed Huffman table is typically (for AC table "n"):
2309
if (cinfo.ac_huff_tbl_ptrs[n] == NULL)
2310
cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo);
2311
huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */
2312
for (i = 1; i <= 16; i++) {
2313
/* counts[i] is number of Huffman codes of length i bits, i=1..16 */
2314
huff_ptr->bits[i] = counts[i];
2316
for (i = 0; i < 256; i++) {
2317
/* symbols[] is the list of Huffman symbols, in code-length order */
2318
huff_ptr->huffval[i] = symbols[i];
2321
(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a
2322
constant JQUANT_TBL object is not safe. If the incoming file happened to
2323
contain a quantization table definition, your master table would get
2324
overwritten! Instead allocate a working table copy and copy the master table
2325
into it, as illustrated above. Ditto for Huffman tables, of course.)
2327
You might want to read the tables from a tables-only file, rather than
2328
hard-wiring them into your application. The jpeg_read_header() call is
2329
sufficient to read a tables-only file. You must pass a second parameter of
2330
FALSE to indicate that you do not require an image to be present. Thus, the
2333
create JPEG decompression object
2334
set source to tables-only file
2335
jpeg_read_header(&cinfo, FALSE);
2336
set source to abbreviated image file
2337
jpeg_read_header(&cinfo, TRUE);
2338
set decompression parameters
2339
jpeg_start_decompress(&cinfo);
2341
jpeg_finish_decompress(&cinfo);
2343
In some cases, you may want to read a file without knowing whether it contains
2344
an image or just tables. In that case, pass FALSE and check the return value
2345
from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found,
2346
JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value,
2347
JPEG_SUSPENDED, is possible when using a suspending data source manager.)
2348
Note that jpeg_read_header() will not complain if you read an abbreviated
2349
image for which you haven't loaded the missing tables; the missing-table check
2350
occurs later, in jpeg_start_decompress().
2353
It is possible to read a series of images from a single source file by
2354
repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence,
2355
without releasing/recreating the JPEG object or the data source module.
2356
(If you did reinitialize, any partial bufferload left in the data source
2357
buffer at the end of one image would be discarded, causing you to lose the
2358
start of the next image.) When you use this method, stored tables are
2359
automatically carried forward, so some of the images can be abbreviated images
2360
that depend on tables from earlier images.
2362
If you intend to write a series of images into a single destination file,
2363
you might want to make a specialized data destination module that doesn't
2364
flush the output buffer at term_destination() time. This would speed things
2365
up by some trifling amount. Of course, you'd need to remember to flush the
2366
buffer after the last image. You can make the later images be abbreviated
2367
ones by passing FALSE to jpeg_start_compress().
2373
Some applications may need to insert or extract special data in the JPEG
2374
datastream. The JPEG standard provides marker types "COM" (comment) and
2375
"APP0" through "APP15" (application) to hold application-specific data.
2376
Unfortunately, the use of these markers is not specified by the standard.
2377
COM markers are fairly widely used to hold user-supplied text. The JFIF file
2378
format spec uses APP0 markers with specified initial strings to hold certain
2379
data. Adobe applications use APP14 markers beginning with the string "Adobe"
2380
for miscellaneous data. Other APPn markers are rarely seen, but might
2381
contain almost anything.
2383
If you wish to store user-supplied text, we recommend you use COM markers
2384
and place readable 7-bit ASCII text in them. Newline conventions are not
2385
standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR
2386
(Mac style). A robust COM reader should be able to cope with random binary
2387
garbage, including nulls, since some applications generate COM markers
2388
containing non-ASCII junk. (But yours should not be one of them.)
2390
For program-supplied data, use an APPn marker, and be sure to begin it with an
2391
identifying string so that you can tell whether the marker is actually yours.
2392
It's probably best to avoid using APP0 or APP14 for any private markers.
2393
(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you
2394
not use APP8 markers for any private purposes, either.)
2396
Keep in mind that at most 65533 bytes can be put into one marker, but you
2397
can have as many markers as you like.
2399
By default, the IJG compression library will write a JFIF APP0 marker if the
2400
selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if
2401
the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but
2402
we don't recommend it. The decompression library will recognize JFIF and
2403
Adobe markers and will set the JPEG colorspace properly when one is found.
2406
You can write special markers immediately following the datastream header by
2407
calling jpeg_write_marker() after jpeg_start_compress() and before the first
2408
call to jpeg_write_scanlines(). When you do this, the markers appear after
2409
the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before
2410
all else. Specify the marker type parameter as "JPEG_COM" for COM or
2411
"JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write
2412
any marker type, but we don't recommend writing any other kinds of marker.)
2413
For example, to write a user comment string pointed to by comment_text:
2414
jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));
2416
If it's not convenient to store all the marker data in memory at once,
2417
you can instead call jpeg_write_m_header() followed by multiple calls to
2418
jpeg_write_m_byte(). If you do it this way, it's your responsibility to
2419
call jpeg_write_m_byte() exactly the number of times given in the length
2420
parameter to jpeg_write_m_header(). (This method lets you empty the
2421
output buffer partway through a marker, which might be important when
2422
using a suspending data destination module. In any case, if you are using
2423
a suspending destination, you should flush its buffer after inserting
2424
any special markers. See "I/O suspension".)
2426
Or, if you prefer to synthesize the marker byte sequence yourself,
2427
you can just cram it straight into the data destination module.
2429
If you are writing JFIF 1.02 extension markers (thumbnail images), don't
2430
forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the
2431
correct JFIF version number in the JFIF header marker. The library's default
2432
is to write version 1.01, but that's wrong if you insert any 1.02 extension
2433
markers. (We could probably get away with just defaulting to 1.02, but there
2434
used to be broken decoders that would complain about unknown minor version
2435
numbers. To reduce compatibility risks it's safest not to write 1.02 unless
2436
you are actually using 1.02 extensions.)
2439
When reading, two methods of handling special markers are available:
2440
1. You can ask the library to save the contents of COM and/or APPn markers
2441
into memory, and then examine them at your leisure afterwards.
2442
2. You can supply your own routine to process COM and/or APPn markers
2443
on-the-fly as they are read.
2444
The first method is simpler to use, especially if you are using a suspending
2445
data source; writing a marker processor that copes with input suspension is
2446
not easy (consider what happens if the marker is longer than your available
2447
input buffer). However, the second method conserves memory since the marker
2448
data need not be kept around after it's been processed.
2450
For either method, you'd normally set up marker handling after creating a
2451
decompression object and before calling jpeg_read_header(), because the
2452
markers of interest will typically be near the head of the file and so will
2453
be scanned by jpeg_read_header. Once you've established a marker handling
2454
method, it will be used for the life of that decompression object
2455
(potentially many datastreams), unless you change it. Marker handling is
2456
determined separately for COM markers and for each APPn marker code.
2459
To save the contents of special markers in memory, call
2460
jpeg_save_markers(cinfo, marker_code, length_limit)
2461
where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.
2462
(To arrange to save all the special marker types, you need to call this
2463
routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer
2464
than length_limit data bytes, only length_limit bytes will be saved; this
2465
parameter allows you to avoid chewing up memory when you only need to see the
2466
first few bytes of a potentially large marker. If you want to save all the
2467
data, set length_limit to 0xFFFF; that is enough since marker lengths are only
2468
16 bits. As a special case, setting length_limit to 0 prevents that marker
2469
type from being saved at all. (That is the default behavior, in fact.)
2471
After jpeg_read_header() completes, you can examine the special markers by
2472
following the cinfo->marker_list pointer chain. All the special markers in
2473
the file appear in this list, in order of their occurrence in the file (but
2474
omitting any markers of types you didn't ask for). Both the original data
2475
length and the saved data length are recorded for each list entry; the latter
2476
will not exceed length_limit for the particular marker type. Note that these
2477
lengths exclude the marker length word, whereas the stored representation
2478
within the JPEG file includes it. (Hence the maximum data length is really
2481
It is possible that additional special markers appear in the file beyond the
2482
SOS marker at which jpeg_read_header stops; if so, the marker list will be
2483
extended during reading of the rest of the file. This is not expected to be
2484
common, however. If you are short on memory you may want to reset the length
2485
limit to zero for all marker types after finishing jpeg_read_header, to
2486
ensure that the max_memory_to_use setting cannot be exceeded due to addition
2489
The marker list remains stored until you call jpeg_finish_decompress or
2490
jpeg_abort, at which point the memory is freed and the list is set to empty.
2491
(jpeg_destroy also releases the storage, of course.)
2493
Note that the library is internally interested in APP0 and APP14 markers;
2494
if you try to set a small nonzero length limit on these types, the library
2495
will silently force the length up to the minimum it wants. (But you can set
2496
a zero length limit to prevent them from being saved at all.) Also, in a
2497
16-bit environment, the maximum length limit may be constrained to less than
2498
65533 by malloc() limitations. It is therefore best not to assume that the
2499
effective length limit is exactly what you set it to be.
2502
If you want to supply your own marker-reading routine, you do it by calling
2503
jpeg_set_marker_processor(). A marker processor routine must have the
2505
boolean jpeg_marker_parser_method (j_decompress_ptr cinfo)
2506
Although the marker code is not explicitly passed, the routine can find it
2507
in cinfo->unread_marker. At the time of call, the marker proper has been
2508
read from the data source module. The processor routine is responsible for
2509
reading the marker length word and the remaining parameter bytes, if any.
2510
Return TRUE to indicate success. (FALSE should be returned only if you are
2511
using a suspending data source and it tells you to suspend. See the standard
2512
marker processors in jdmarker.c for appropriate coding methods if you need to
2513
use a suspending data source.)
2515
If you override the default APP0 or APP14 processors, it is up to you to
2516
recognize JFIF and Adobe markers if you want colorspace recognition to occur
2517
properly. We recommend copying and extending the default processors if you
2518
want to do that. (A better idea is to save these marker types for later
2519
examination by calling jpeg_save_markers(); that method doesn't interfere
2520
with the library's own processing of these markers.)
2522
jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive
2523
--- if you call one it overrides any previous call to the other, for the
2524
particular marker type specified.
2526
A simple example of an external COM processor can be found in djpeg.c.
2527
Also, see jpegtran.c for an example of using jpeg_save_markers.
2530
Raw (downsampled) image data
2531
----------------------------
2533
Some applications need to supply already-downsampled image data to the JPEG
2534
compressor, or to receive raw downsampled data from the decompressor. The
2535
library supports this requirement by allowing the application to write or
2536
read raw data, bypassing the normal preprocessing or postprocessing steps.
2537
The interface is different from the standard one and is somewhat harder to
2538
use. If your interest is merely in bypassing color conversion, we recommend
2539
that you use the standard interface and simply set jpeg_color_space =
2540
in_color_space (or jpeg_color_space = out_color_space for decompression).
2541
The mechanism described in this section is necessary only to supply or
2542
receive downsampled image data, in which not all components have the same
2546
To compress raw data, you must supply the data in the colorspace to be used
2547
in the JPEG file (please read the earlier section on Special color spaces)
2548
and downsampled to the sampling factors specified in the JPEG parameters.
2549
You must supply the data in the format used internally by the JPEG library,
2550
namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional
2551
arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one
2552
color component. This structure is necessary since the components are of
2553
different sizes. If the image dimensions are not a multiple of the MCU size,
2554
you must also pad the data correctly (usually, this is done by replicating
2555
the last column and/or row). The data must be padded to a multiple of a DCT
2556
block in each component: that is, each downsampled row must contain a
2557
multiple of 8 valid samples, and there must be a multiple of 8 sample rows
2558
for each component. (For applications such as conversion of digital TV
2559
images, the standard image size is usually a multiple of the DCT block size,
2560
so that no padding need actually be done.)
2562
The procedure for compression of raw data is basically the same as normal
2563
compression, except that you call jpeg_write_raw_data() in place of
2564
jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do
2566
* Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().)
2567
This notifies the library that you will be supplying raw data.
2568
Furthermore, set cinfo->do_fancy_downsampling to FALSE if you want to use
2569
real downsampled data. (It is set TRUE by jpeg_set_defaults().)
2570
* Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()
2571
call is a good idea. Note that since color conversion is bypassed,
2572
in_color_space is ignored, except that jpeg_set_defaults() uses it to
2573
choose the default jpeg_color_space setting.
2574
* Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and
2575
cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the
2576
dimensions of the data you are supplying, it's wise to set them
2577
explicitly, rather than assuming the library's defaults are what you want.
2579
To pass raw data to the library, call jpeg_write_raw_data() in place of
2580
jpeg_write_scanlines(). The two routines work similarly except that
2581
jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.
2582
The scanlines count passed to and returned from jpeg_write_raw_data is
2583
measured in terms of the component with the largest v_samp_factor.
2585
jpeg_write_raw_data() processes one MCU row per call, which is to say
2586
v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines
2587
value must be at least max_v_samp_factor*DCTSIZE, and the return value will
2588
be exactly that amount (or possibly some multiple of that amount, in future
2589
library versions). This is true even on the last call at the bottom of the
2590
image; don't forget to pad your data as necessary.
2592
The required dimensions of the supplied data can be computed for each
2594
cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row
2595
cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image
2596
after jpeg_start_compress() has initialized those fields. If the valid data
2597
is smaller than this, it must be padded appropriately. For some sampling
2598
factors and image sizes, additional dummy DCT blocks are inserted to make
2599
the image a multiple of the MCU dimensions. The library creates such dummy
2600
blocks itself; it does not read them from your supplied data. Therefore you
2601
need never pad by more than DCTSIZE samples. An example may help here.
2602
Assume 2h2v downsampling of YCbCr data, that is
2603
cinfo->comp_info[0].h_samp_factor = 2 for Y
2604
cinfo->comp_info[0].v_samp_factor = 2
2605
cinfo->comp_info[1].h_samp_factor = 1 for Cb
2606
cinfo->comp_info[1].v_samp_factor = 1
2607
cinfo->comp_info[2].h_samp_factor = 1 for Cr
2608
cinfo->comp_info[2].v_samp_factor = 1
2609
and suppose that the nominal image dimensions (cinfo->image_width and
2610
cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will
2611
compute downsampled_width = 101 and width_in_blocks = 13 for Y,
2612
downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same
2613
for the height fields). You must pad the Y data to at least 13*8 = 104
2614
columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The
2615
MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16
2616
scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual
2617
sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed,
2618
so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row
2619
of Y data is dummy, so it doesn't matter what you pass for it in the data
2620
arrays, but the scanlines count must total up to 112 so that all of the Cb
2621
and Cr data gets passed.
2623
Output suspension is supported with raw-data compression: if the data
2624
destination module suspends, jpeg_write_raw_data() will return 0.
2625
In this case the same data rows must be passed again on the next call.
2628
Decompression with raw data output implies bypassing all postprocessing.
2629
You must deal with the color space and sampling factors present in the
2630
incoming file. If your application only handles, say, 2h1v YCbCr data,
2631
you must check for and fail on other color spaces or other sampling factors.
2632
The library will not convert to a different color space for you.
2634
To obtain raw data output, set cinfo->raw_data_out = TRUE before
2635
jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to
2636
verify that the color space and sampling factors are ones you can handle.
2637
Furthermore, set cinfo->do_fancy_upsampling = FALSE if you want to get real
2638
downsampled data (it is set TRUE by jpeg_read_header()).
2639
Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The
2640
decompression process is otherwise the same as usual.
2642
jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a
2643
buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is
2644
the same as for raw-data compression). The buffer you pass must be large
2645
enough to hold the actual data plus padding to DCT-block boundaries. As with
2646
compression, any entirely dummy DCT blocks are not processed so you need not
2647
allocate space for them, but the total scanline count includes them. The
2648
above example of computing buffer dimensions for raw-data compression is
2649
equally valid for decompression.
2651
Input suspension is supported with raw-data decompression: if the data source
2652
module suspends, jpeg_read_raw_data() will return 0. You can also use
2653
buffered-image mode to read raw data in multiple passes.
2656
Really raw data: DCT coefficients
2657
---------------------------------
2659
It is possible to read or write the contents of a JPEG file as raw DCT
2660
coefficients. This facility is mainly intended for use in lossless
2661
transcoding between different JPEG file formats. Other possible applications
2662
include lossless cropping of a JPEG image, lossless reassembly of a
2663
multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc.
2665
To read the contents of a JPEG file as DCT coefficients, open the file and do
2666
jpeg_read_header() as usual. But instead of calling jpeg_start_decompress()
2667
and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the
2668
entire image into a set of virtual coefficient-block arrays, one array per
2669
component. The return value is a pointer to an array of virtual-array
2670
descriptors. Each virtual array can be accessed directly using the JPEG
2671
memory manager's access_virt_barray method (see Memory management, below,
2672
and also read structure.txt's discussion of virtual array handling). Or,
2673
for simple transcoding to a different JPEG file format, the array list can
2674
just be handed directly to jpeg_write_coefficients().
2676
Each block in the block arrays contains quantized coefficient values in
2677
normal array order (not JPEG zigzag order). The block arrays contain only
2678
DCT blocks containing real data; any entirely-dummy blocks added to fill out
2679
interleaved MCUs at the right or bottom edges of the image are discarded
2680
during reading and are not stored in the block arrays. (The size of each
2681
block array can be determined from the width_in_blocks and height_in_blocks
2682
fields of the component's comp_info entry.) This is also the data format
2683
expected by jpeg_write_coefficients().
2685
When you are done using the virtual arrays, call jpeg_finish_decompress()
2686
to release the array storage and return the decompression object to an idle
2687
state; or just call jpeg_destroy() if you don't need to reuse the object.
2689
If you use a suspending data source, jpeg_read_coefficients() will return
2690
NULL if it is forced to suspend; a non-NULL return value indicates successful
2691
completion. You need not test for a NULL return value when using a
2692
non-suspending data source.
2694
It is also possible to call jpeg_read_coefficients() to obtain access to the
2695
decoder's coefficient arrays during a normal decode cycle in buffered-image
2696
mode. This frammish might be useful for progressively displaying an incoming
2697
image and then re-encoding it without loss. To do this, decode in buffered-
2698
image mode as discussed previously, then call jpeg_read_coefficients() after
2699
the last jpeg_finish_output() call. The arrays will be available for your use
2700
until you call jpeg_finish_decompress().
2703
To write the contents of a JPEG file as DCT coefficients, you must provide
2704
the DCT coefficients stored in virtual block arrays. You can either pass
2705
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
2706
allocate virtual arrays from the JPEG compression object and fill them
2707
yourself. In either case, jpeg_write_coefficients() is substituted for
2708
jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is
2709
* Create compression object
2710
* Set all compression parameters as necessary
2711
* Request virtual arrays if needed
2712
* jpeg_write_coefficients()
2713
* jpeg_finish_compress()
2714
* Destroy or re-use compression object
2715
jpeg_write_coefficients() is passed a pointer to an array of virtual block
2716
array descriptors; the number of arrays is equal to cinfo.num_components.
2718
The virtual arrays need only have been requested, not realized, before
2719
jpeg_write_coefficients() is called. A side-effect of
2720
jpeg_write_coefficients() is to realize any virtual arrays that have been
2721
requested from the compression object's memory manager. Thus, when obtaining
2722
the virtual arrays from the compression object, you should fill the arrays
2723
after calling jpeg_write_coefficients(). The data is actually written out
2724
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
2727
When writing raw DCT coefficients, it is crucial that the JPEG quantization
2728
tables and sampling factors match the way the data was encoded, or the
2729
resulting file will be invalid. For transcoding from an existing JPEG file,
2730
we recommend using jpeg_copy_critical_parameters(). This routine initializes
2731
all the compression parameters to default values (like jpeg_set_defaults()),
2732
then copies the critical information from a source decompression object.
2733
The decompression object should have just been used to read the entire
2734
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().
2736
jpeg_write_coefficients() marks all tables stored in the compression object
2737
as needing to be written to the output file (thus, it acts like
2738
jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid
2739
emitting abbreviated JPEG files by accident. If you really want to emit an
2740
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
2741
individual sent_table flags, between calling jpeg_write_coefficients() and
2742
jpeg_finish_compress().
2748
Some applications may need to regain control from the JPEG library every so
2749
often. The typical use of this feature is to produce a percent-done bar or
2750
other progress display. (For a simple example, see cjpeg.c or djpeg.c.)
2751
Although you do get control back frequently during the data-transferring pass
2752
(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes
2753
will occur inside jpeg_finish_compress or jpeg_start_decompress; those
2754
routines may take a long time to execute, and you don't get control back
2755
until they are done.
2757
You can define a progress-monitor routine which will be called periodically
2758
by the library. No guarantees are made about how often this call will occur,
2759
so we don't recommend you use it for mouse tracking or anything like that.
2760
At present, a call will occur once per MCU row, scanline, or sample row
2761
group, whichever unit is convenient for the current processing mode; so the
2762
wider the image, the longer the time between calls. During the data
2763
transferring pass, only one call occurs per call of jpeg_read_scanlines or
2764
jpeg_write_scanlines, so don't pass a large number of scanlines at once if
2765
you want fine resolution in the progress count. (If you really need to use
2766
the callback mechanism for time-critical tasks like mouse tracking, you could
2767
insert additional calls inside some of the library's inner loops.)
2769
To establish a progress-monitor callback, create a struct jpeg_progress_mgr,
2770
fill in its progress_monitor field with a pointer to your callback routine,
2771
and set cinfo->progress to point to the struct. The callback will be called
2772
whenever cinfo->progress is non-NULL. (This pointer is set to NULL by
2773
jpeg_create_compress or jpeg_create_decompress; the library will not change
2774
it thereafter. So if you allocate dynamic storage for the progress struct,
2775
make sure it will live as long as the JPEG object does. Allocating from the
2776
JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You
2777
can use the same callback routine for both compression and decompression.
2779
The jpeg_progress_mgr struct contains four fields which are set by the library:
2780
long pass_counter; /* work units completed in this pass */
2781
long pass_limit; /* total number of work units in this pass */
2782
int completed_passes; /* passes completed so far */
2783
int total_passes; /* total number of passes expected */
2784
During any one pass, pass_counter increases from 0 up to (not including)
2785
pass_limit; the step size is usually but not necessarily 1. The pass_limit
2786
value may change from one pass to another. The expected total number of
2787
passes is in total_passes, and the number of passes already completed is in
2788
completed_passes. Thus the fraction of work completed may be estimated as
2789
completed_passes + (pass_counter/pass_limit)
2790
--------------------------------------------
2792
ignoring the fact that the passes may not be equal amounts of work.
2794
When decompressing, pass_limit can even change within a pass, because it
2795
depends on the number of scans in the JPEG file, which isn't always known in
2796
advance. The computed fraction-of-work-done may jump suddenly (if the library
2797
discovers it has overestimated the number of scans) or even decrease (in the
2798
opposite case). It is not wise to put great faith in the work estimate.
2800
When using the decompressor's buffered-image mode, the progress monitor work
2801
estimate is likely to be completely unhelpful, because the library has no way
2802
to know how many output passes will be demanded of it. Currently, the library
2803
sets total_passes based on the assumption that there will be one more output
2804
pass if the input file end hasn't yet been read (jpeg_input_complete() isn't
2805
TRUE), but no more output passes if the file end has been reached when the
2806
output pass is started. This means that total_passes will rise as additional
2807
output passes are requested. If you have a way of determining the input file
2808
size, estimating progress based on the fraction of the file that's been read
2809
will probably be more useful than using the library's value.
2815
This section covers some key facts about the JPEG library's built-in memory
2816
manager. For more info, please read structure.txt's section about the memory
2817
manager, and consult the source code if necessary.
2819
All memory and temporary file allocation within the library is done via the
2820
memory manager. If necessary, you can replace the "back end" of the memory
2821
manager to control allocation yourself (for example, if you don't want the
2822
library to use malloc() and free() for some reason).
2824
Some data is allocated "permanently" and will not be freed until the JPEG
2825
object is destroyed. Most data is allocated "per image" and is freed by
2826
jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the
2827
memory manager yourself to allocate structures that will automatically be
2828
freed at these times. Typical code for this is
2829
ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size);
2830
Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object.
2831
Use alloc_large instead of alloc_small for anything bigger than a few Kbytes.
2832
There are also alloc_sarray and alloc_barray routines that automatically
2833
build 2-D sample or block arrays.
2835
The library's minimum space requirements to process an image depend on the
2836
image's width, but not on its height, because the library ordinarily works
2837
with "strip" buffers that are as wide as the image but just a few rows high.
2838
Some operating modes (eg, two-pass color quantization) require full-image
2839
buffers. Such buffers are treated as "virtual arrays": only the current strip
2840
need be in memory, and the rest can be swapped out to a temporary file.
2842
If you use the simplest memory manager back end (jmemnobs.c), then no
2843
temporary files are used; virtual arrays are simply malloc()'d. Images bigger
2844
than memory can be processed only if your system supports virtual memory.
2845
The other memory manager back ends support temporary files of various flavors
2846
and thus work in machines without virtual memory. They may also be useful on
2847
Unix machines if you need to process images that exceed available swap space.
2849
When using temporary files, the library will make the in-memory buffers for
2850
its virtual arrays just big enough to stay within a "maximum memory" setting.
2851
Your application can set this limit by setting cinfo->mem->max_memory_to_use
2852
after creating the JPEG object. (Of course, there is still a minimum size for
2853
the buffers, so the max-memory setting is effective only if it is bigger than
2854
the minimum space needed.) If you allocate any large structures yourself, you
2855
must allocate them before jpeg_start_compress() or jpeg_start_decompress() in
2856
order to have them counted against the max memory limit. Also keep in mind
2857
that space allocated with alloc_small() is ignored, on the assumption that
2858
it's too small to be worth worrying about; so a reasonable safety margin
2859
should be left when setting max_memory_to_use.
2861
If you use the jmemname.c or jmemdos.c memory manager back end, it is
2862
important to clean up the JPEG object properly to ensure that the temporary
2863
files get deleted. (This is especially crucial with jmemdos.c, where the
2864
"temporary files" may be extended-memory segments; if they are not freed,
2865
DOS will require a reboot to recover the memory.) Thus, with these memory
2866
managers, it's a good idea to provide a signal handler that will trap any
2867
early exit from your program. The handler should call either jpeg_abort()
2868
or jpeg_destroy() for any active JPEG objects. A handler is not needed with
2869
jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either,
2870
since the C library is supposed to take care of deleting files made with
2877
Working memory requirements while performing compression or decompression
2878
depend on image dimensions, image characteristics (such as colorspace and
2879
JPEG process), and operating mode (application-selected options).
2881
As of v6b, the decompressor requires:
2882
1. About 24K in more-or-less-fixed-size data. This varies a bit depending
2883
on operating mode and image characteristics (particularly color vs.
2884
grayscale), but it doesn't depend on image dimensions.
2885
2. Strip buffers (of size proportional to the image width) for IDCT and
2886
upsampling results. The worst case for commonly used sampling factors
2887
is about 34 bytes * width in pixels for a color image. A grayscale image
2888
only needs about 8 bytes per pixel column.
2889
3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG
2890
file (including progressive JPEGs), or whenever you select buffered-image
2891
mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's
2892
3 bytes per pixel for a color image. Worst case (1x1 sampling) requires
2893
6 bytes/pixel. For grayscale, figure 2 bytes/pixel.
2894
4. To perform 2-pass color quantization, the decompressor also needs a
2895
128K color lookup table and a full-image pixel buffer (3 bytes/pixel).
2896
This does not count any memory allocated by the application, such as a
2897
buffer to hold the final output image.
2899
The above figures are valid for 8-bit JPEG data precision and a machine with
2900
32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and
2901
quantization pixel buffer. The "fixed-size" data will be somewhat smaller
2902
with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual
2903
color spaces will require different amounts of space.
2905
The full-image coefficient and pixel buffers, if needed at all, do not
2906
have to be fully RAM resident; you can have the library use temporary
2907
files instead when the total memory usage would exceed a limit you set.
2908
(But if your OS supports virtual memory, it's probably better to just use
2909
jmemnobs and let the OS do the swapping.)
2911
The compressor's memory requirements are similar, except that it has no need
2912
for color quantization. Also, it needs a full-image DCT coefficient buffer
2913
if Huffman-table optimization is asked for, even if progressive mode is not
2916
If you need more detailed information about memory usage in a particular
2917
situation, you can enable the MEM_STATS code in jmemmgr.c.
2920
Library compile-time options
2921
----------------------------
2923
A number of compile-time options are available by modifying jmorecfg.h.
2925
The JPEG standard provides for both the baseline 8-bit DCT process and
2926
a 12-bit DCT process. The IJG code supports 12-bit lossy JPEG if you define
2927
BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be
2928
larger than a char, so it affects the surrounding application's image data.
2929
The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
2930
and GIF file formats; you must disable the other file formats to compile a
2931
12-bit cjpeg or djpeg. (install.txt has more information about that.)
2932
At present, a 12-bit library can handle *only* 12-bit images, not both
2933
precisions. (If you need to include both 8- and 12-bit libraries in a single
2934
application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES
2935
for just one of the copies. You'd have to access the 8-bit and 12-bit copies
2936
from separate application source files. This is untested ... if you try it,
2937
we'd like to hear whether it works!)
2939
Note that a 12-bit library always compresses in Huffman optimization mode,
2940
in order to generate valid Huffman tables. This is necessary because our
2941
default Huffman tables only cover 8-bit data. If you need to output 12-bit
2942
files in one pass, you'll have to supply suitable default Huffman tables.
2943
You may also want to supply your own DCT quantization tables; the existing
2944
quality-scaling code has been developed for 8-bit use, and probably doesn't
2945
generate especially good tables for 12-bit.
2947
The maximum number of components (color channels) in the image is determined
2948
by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we
2949
expect that few applications will need more than four or so.
2951
On machines with unusual data type sizes, you may be able to improve
2952
performance or reduce memory space by tweaking the various typedefs in
2953
jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s
2954
is quite slow; consider trading memory for speed by making JCOEF, INT16, and
2955
UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int.
2956
You probably don't want to make JSAMPLE be int unless you have lots of memory
2959
You can reduce the size of the library by compiling out various optional
2960
functions. To do this, undefine xxx_SUPPORTED symbols as necessary.
2962
You can also save a few K by not having text error messages in the library;
2963
the standard error message table occupies about 5Kb. This is particularly
2964
reasonable for embedded applications where there's no good way to display
2965
a message anyway. To do this, remove the creation of the message table
2966
(jpeg_std_message_table[]) from jerror.c, and alter format_message to do
2967
something reasonable without it. You could output the numeric value of the
2968
message code number, for example. If you do this, you can also save a couple
2969
more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing;
2970
you don't need trace capability anyway, right?
2973
Portability considerations
2974
--------------------------
2976
The JPEG library has been written to be extremely portable; the sample
2977
applications cjpeg and djpeg are slightly less so. This section summarizes
2978
the design goals in this area. (If you encounter any bugs that cause the
2979
library to be less portable than is claimed here, we'd appreciate hearing
2982
The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of
2983
the popular system include file setups, and some not-so-popular ones too.
2984
See install.txt for configuration procedures.
2986
The code is not dependent on the exact sizes of the C data types. As
2987
distributed, we make the assumptions that
2988
char is at least 8 bits wide
2989
short is at least 16 bits wide
2990
int is at least 16 bits wide
2991
long is at least 32 bits wide
2992
(These are the minimum requirements of the ANSI C standard.) Wider types will
2993
work fine, although memory may be used inefficiently if char is much larger
2994
than 8 bits or short is much bigger than 16 bits. The code should work
2995
equally well with 16- or 32-bit ints.
2997
In a system where these assumptions are not met, you may be able to make the
2998
code work by modifying the typedefs in jmorecfg.h. However, you will probably
2999
have difficulty if int is less than 16 bits wide, since references to plain
3000
int abound in the code.
3002
char can be either signed or unsigned, although the code runs faster if an
3003
unsigned char type is available. If char is wider than 8 bits, you will need
3004
to redefine JOCTET and/or provide custom data source/destination managers so
3005
that JOCTET represents exactly 8 bits of data on external storage.
3007
The JPEG library proper does not assume ASCII representation of characters.
3008
But some of the image file I/O modules in cjpeg/djpeg do have ASCII
3009
dependencies in file-header manipulation; so does cjpeg's select_file_type()
3012
The JPEG library does not rely heavily on the C library. In particular, C
3013
stdio is used only by the data source/destination modules and the error
3014
handler, all of which are application-replaceable. (cjpeg/djpeg are more
3015
heavily dependent on stdio.) malloc and free are called only from the memory
3016
manager "back end" module, so you can use a different memory allocator by
3017
replacing that one file.
3019
The code generally assumes that C names must be unique in the first 15
3020
characters. However, global function names can be made unique in the
3021
first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES.
3023
More info about porting the code may be gleaned by reading jconfig.txt,
3024
jmorecfg.h, and jinclude.h.
3027
Notes for MS-DOS implementors
3028
-----------------------------
3030
The IJG code is designed to work efficiently in 80x86 "small" or "medium"
3031
memory models (i.e., data pointers are 16 bits unless explicitly declared
3032
"far"; code pointers can be either size). You may be able to use small
3033
model to compile cjpeg or djpeg by itself, but you will probably have to use
3034
medium model for any larger application. This won't make much difference in
3035
performance. You *will* take a noticeable performance hit if you use a
3036
large-data memory model (perhaps 10%-25%), and you should avoid "huge" model
3039
The JPEG library typically needs 2Kb-3Kb of stack space. It will also
3040
malloc about 20K-30K of near heap space while executing (and lots of far
3041
heap, but that doesn't count in this calculation). This figure will vary
3042
depending on selected operating mode, and to a lesser extent on image size.
3043
There is also about 5Kb-6Kb of constant data which will be allocated in the
3044
near data segment (about 4Kb of this is the error message table).
3045
Thus you have perhaps 20K available for other modules' static data and near
3046
heap space before you need to go to a larger memory model. The C library's
3047
static data will account for several K of this, but that still leaves a good
3048
deal for your needs. (If you are tight on space, you could reduce the sizes
3049
of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to
3050
1K. Another possibility is to move the error message table to far memory;
3051
this should be doable with only localized hacking on jerror.c.)
3053
About 2K of the near heap space is "permanent" memory that will not be
3054
released until you destroy the JPEG object. This is only an issue if you
3055
save a JPEG object between compression or decompression operations.
3057
Far data space may also be a tight resource when you are dealing with large
3058
images. The most memory-intensive case is decompression with two-pass color
3059
quantization, or single-pass quantization to an externally supplied color
3060
map. This requires a 128Kb color lookup table plus strip buffers amounting
3061
to about 40 bytes per column for typical sampling ratios (eg, about 25600
3062
bytes for a 640-pixel-wide image). You may not be able to process wide
3063
images if you have large data structures of your own.
3065
Of course, all of these concerns vanish if you use a 32-bit flat-memory-model
3066
compiler, such as DJGPP or Watcom C. We highly recommend flat model if you
3067
can use it; the JPEG library is significantly faster in flat model.
added
removed
Source/LibJPEG/libjpeg.txt
