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+Date:    Fri, 01 Aug 1997 20:14:52 MDT
+To:      Sam Leffler <sam@cthulhu.engr.sgi.com>
+
+From:    "Conrad J. Poelman (WSAT)" <poelmanc@plk.af.mil>
+Subject: Potential TIFF library additions
+
+Delivery-Date: Fri, 01 Aug 1997 19:21:06 -0700
+
+Sam,
+
+You probably don't remember me, but I sent in a couple of bug fixes
+regarding the TIFF library about a 16 months ago or so...
+
+I just wanted to send you two other additions that I have made to our
+local version of the TIFF library in hopes that you will want to
+incorporate them into your next major release of the TIFF library.
+(These additions are based on TIFF version 3.4beta31, but they sit on
+top of the library so they shouldn't be much trouble to incorporate them
+into any more recent version.) They are internally documented to a
+reasonable extent and we've been successfully using them in our code
+here for over a year. If you think they would make good additions to the
+TIFF library, I'd be happy to clean them up more, document them more,
+and/or integrate them with the latest version of the TIFF library, but I
+figured I'd see if you were interested in using them before I went to
+all that trouble.
+
+TIFF Image Iterator
+-------------------
+Your ReadRGBA() routine works well for reading many different formats
+(TILED, STIP, compressed or not, etc.) of the most basic types of data
+(RGB, 8-bit greyscale, 8-bit colormapped) into an SGI-style data array,
+and serves as a good template for users with other needs. I used it as
+an exmaple of how to make an iterator which, rather than fill a data
+array, calls an arbitrary user-supplied callback function for each
+"chunk" of data - that "chunk" might be a strip or a tile, and might
+have one sample-per-pixel or two, and might be 8-bit data or 16-bit or
+24-bit. The callback function can do whatever it wants with the data -
+store it in a big array, convert it to RGBA, or draw it directly to the
+screen. I was able to use this iterator to read 16-bit greyscale and 32-
+and 64-bit floating point data, which wasn't possible with ReadRGBA().
+
+I have tested this routine with 8- and 16-bit greyscale data as well as
+with 32- and 64-bit floating point data. I believe nearly all of our
+data is organized in strips, so actually I'd appreciate it if you had
+some tiled images that I could test it with.
+
+It should certainly be possible and would be cleanest to reimplement
+ReadRGBA() in terms of the image iterator, but I haven't done that.
+
+
+Private Sub-Directory Read/Write
+--------------------------------
+TIFF-PL is a Phillips Laboratory extension to the TIFF tags that allows
+us to store satellite imaging-specific information in a TIFF format,
+such as the satellite's trajectory, the imaging time, etc. In order to
+give us the flexibility to modify the tag definitions without getting
+approval from the TIFF committee every time, we were given only three
+TIFF tags - a PL signature, a PL version number, and PL directory
+offset, which lists the position in the file at which to find a private
+sub-directory of tags-value pairs. So I wrote two routines:
+TIFFWritePrivateDataSubDirectory(), which takes a list of tags and a
+"get" function and writes the tag values into the TIFF file, returning
+the offset within the file at which it wrote the directory; and
+TIFFReadPrivateDataSubDirectory(), which takes an offset, a list of
+tags, and a "set" function and reads all the data from the private
+directory. The functions themselves are pretty simple. (The files are
+huge because I had to basically copy all of the tif_dirread.c and
+tif_dirwrite.c files in order to access the various fetching routines
+which were all declared static and therefore inaccessible in the TIFF
+library.)
+
+
+I'm including the four source files (tif_imgiter.h, tif_imgiter.c,
+tif_pdsdirread.c, tif_pdsdirwrite.c) in case you want to take a look at
+them. I can also send you some sample code that uses them if you like.
+If you're interested in having them incorporated into the standard TIFF
+library, I'd be happy to do that integration and clean up and document
+the routines. (For example, I've already realized that instead of
+limiting the SEP callback function to three bands (R,G,B) it should take
+an array to enable the handling of n-banded multi-spectral data...) If
+not, I'll just leave them as they are, since they work fine for us now.
+
+Holler if you have any questions.
+
+-- Conrad
+__________________________________________________________________
+  Capt Conrad J. Poelman         PL/WSAT   (Phillips Laboratory)
+    505-846-4347                   3550 Aberdeen Ave SE 
+      (FAX) 505-846-4374             Kirtland AFB, NM 87117-5776
+