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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
-