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The following described approach to managing tag extensions has been
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mostly superceeded since libtiff 3.6.0. The described approach requires
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internal knowledge of the libtiff API and tends to be very fragile
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in the face of libtiff upgrades.
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Please read over the html/addingtags.html in preference to the below
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==================================
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Client module for adding to LIBTIFF tagset
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-------------------------------------------
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In the past, users of the "libtiff" package had to modify the
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source code of the library if they required additional private tags
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or codes not recognized by libtiff. Thus, whenever
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a new revision of libtiff came out the client would have to
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perform modifications to six or seven different files to re-install
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The latest versions of libtiff now provide client software new routines,
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giving them the opportunity to install private extensions at runtime,
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rather than compile-time. This means that the client may encapsulate
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all of their private tags into a separate module, which need only
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be recompiled when new versions of libtiff are released; no manual
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editing of files is required.
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The mechanism for overriding the tag access has been enabled with
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a single new routine, which has the following calling sequence:
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TIFFExtendProc old_extender;
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old_extender = TIFFSetTagExtender(tag_extender);
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which must be called prior to opening or creating TIFF files.
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This routine sets a static pointer to the user-specified function
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<tag_extender>, which in turn is called by TIFFDefaultDirectory(),
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just after the usual TIFFSetField() and TIFFGetField() methods
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are defined, and just before the compression tag is set. It also
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returns a pointer to the previously-defined value of the tag-extender,
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so that multiple clients may be installed.
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The TIFFExtendProc method that you define should be used to override
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the TIFF file's "vsetfield" and "vgetfield" methods, so that you
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can trap your new, private tags, and install their values into
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a private directory structure. For your convienience, a new pointer
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has also been added to the "TIFF" file structure:
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tidata_t tif_clientdir; /* client TIFF directory */
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into which you may install whatever private directory structures you like.
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You should also override the tag-printing method from within your
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"vsetfield" method, to permit the symbolic printing of your new tags.
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An example module has been provided as a template for installing
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your own tags into a client tag extender. The module is called
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"xtif_dir.c", and defines all of the interface routines, tag field
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access, tag printing, etc. for most purpose.
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To see how the client module operates, there are three "fake"
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tags currently installed. If you use the existing makefile you can
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make all -f Makefile.gcc !or Makefile.mpw
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This will build two example programs called "maketif" and "listtif"
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and then run them. These programs do nothing more than create a small
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file called "newtif.tif", install the fake tags, and then list them out
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using TIFFPrintDirectory().
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Installing Private Tags
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-----------------------
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To use this module for installing your own tags, edit each of the files
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and search for the string "XXX". At these locations the comments
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will direct you how to install your own tag values, define their
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types, etc. Three examples tags are currently installed, demonstrating
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how to implement multi-valued tags, single-valued tags, and ASCII tags.
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The examples are not valid, registered tags, so you must replace them with
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To test the routines, also edit the test programs "maketif.c" and
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"listtif.c" and replace the portions of the code that set the
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private tag values and list them.
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Once you have edited these files, you may build the client module
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with the Makefile provided, and run the test programs.
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To use these files in your own code, the "xtif_dir.c" module defines
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replacement routines for the standard "TIFFOpen()" "TIFFFdOpen",
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and "TIFFClose()" routines, called XTIFFOpen, XTIFFFdOpen and XTIFFClose.
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You must use these routines in order to have the extended tag handlers
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installed. Once installed, the standard TIFFGetField() and TIFFSetField
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routines may be used as before.
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Adding Extended Tags to "tools"
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-------------------------------
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To create an extended-tag savvy "tiffinfo" program or other utility, you may
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simply recompile and link the tools to your "libxtiff" library, adding
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-DTIFFOpen=XTIFFOpen -DTIFFClose=XTIFFClose -DTIFFFdOpen=XTIFFFdOpen
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to the compile statement.
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Send all reports and suggestions to ndr@tazboy.jpl.nasa.gov
added
removed
contrib/tags/README
