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+
+NOTE:  Sept/2004 
+
+The following described approach to managing tag extensions has been
+mostly superceeded since libtiff 3.6.0.  The described approach requires
+internal knowledge of the libtiff API and tends to be very fragile 
+in the face of libtiff upgrades.  
+
+Please read over the html/addingtags.html in preference to the below
+described approach.
+
+
+
+==================================
+
+Client module for adding to LIBTIFF tagset
+-------------------------------------------
+  Author: Niles Ritter
+
+
+  
+
+In the past, users of the "libtiff" package had to modify the
+source code of the library if they required additional private tags
+or codes not recognized by libtiff. Thus, whenever
+a new revision of libtiff came out the client would have to
+perform modifications to six or seven different files to re-install
+their tags.
+
+The latest versions of libtiff now provide client software new routines, 
+giving them the opportunity to install private extensions at runtime,
+rather than compile-time. This means that the client may encapsulate
+all of their private tags into a separate module, which need only
+be recompiled when new versions of libtiff are released; no manual
+editing of files is required.
+
+How it works
+------------
+
+The mechanism for overriding the tag access has been enabled with
+a single new routine, which has the following calling sequence:
+
+  TIFFExtendProc old_extender;
+  
+  old_extender = TIFFSetTagExtender(tag_extender);
+
+which must be called prior to opening or creating TIFF files.
+
+This routine sets a static pointer to the user-specified function
+<tag_extender>, which in turn is called by TIFFDefaultDirectory(),
+just after the usual TIFFSetField() and TIFFGetField() methods
+are defined, and just before the compression tag is set. It also
+returns a pointer to the previously-defined value of the tag-extender,
+so that multiple clients may be installed.
+
+The TIFFExtendProc method that you define should be used to override
+the TIFF file's "vsetfield" and "vgetfield" methods, so that you
+can trap your new, private tags, and install their values into
+a private directory structure. For your convienience, a new pointer
+has also been added to the "TIFF" file structure:
+
+	tidata_t	tif_clientdir;	/* client TIFF directory */
+
+into which you may install whatever private directory structures you like.
+You should also override the tag-printing method from within your
+"vsetfield" method, to permit the symbolic printing of your new tags.
+
+
+Example Client Code:
+--------------------
+
+An example module has been provided as a template for installing
+your own tags into a client tag extender. The module is called
+"xtif_dir.c", and defines all of the interface routines, tag field
+access, tag printing, etc. for most purpose. 
+
+To see how the client module operates, there are three "fake"
+tags currently installed. If you use the existing makefile you can
+build them with:
+
+     make all -f Makefile.gcc  !or Makefile.mpw
+     maketif
+     listtif
+  
+This will build two example programs called "maketif" and "listtif" 
+and then run them. These programs do nothing more than create a small
+file called "newtif.tif", install the fake tags, and then list them out
+using TIFFPrintDirectory().
+
+Installing Private Tags
+-----------------------
+
+To use this module for installing your own tags, edit each of the files
+
+    xtif_dir.c
+    xtiffio.h
+    xtiffiop.h
+    
+and search for the string "XXX". At these locations the comments
+will direct you how to install your own tag values, define their
+types, etc. Three examples tags are currently installed, demonstrating
+how to implement multi-valued tags, single-valued tags, and ASCII tags.
+The examples are not valid, registered tags, so you must replace them with
+your own.
+
+To test the routines, also edit the test programs "maketif.c" and
+"listtif.c" and replace the portions of the code that set the
+private tag values and list them.
+
+Once you have edited these files, you may build the client module
+with the Makefile provided, and run the test programs.
+
+To use these files in your own code, the "xtif_dir.c" module defines
+replacement routines for the standard "TIFFOpen()" "TIFFFdOpen",
+and "TIFFClose()" routines, called XTIFFOpen, XTIFFFdOpen and XTIFFClose.
+You must use these routines in order to have the extended tag handlers
+installed. Once installed, the standard TIFFGetField() and TIFFSetField
+routines may be used as before.
+
+Adding Extended Tags to "tools"
+-------------------------------
+To create an extended-tag savvy "tiffinfo" program or other utility, you may
+simply recompile and link the tools to your "libxtiff" library, adding 
+
+   -DTIFFOpen=XTIFFOpen -DTIFFClose=XTIFFClose -DTIFFFdOpen=XTIFFFdOpen
+
+to the compile statement.
+
+Bugs, Comments Etc:
+------------------
+ Send all reports and suggestions to ndr@tazboy.jpl.nasa.gov
+ (Niles Ritter).