2011.03.22 04:01 "Re: [Tiff] Lab based colormaps", by Steve Underwood

On 03/22/2011 11:29 AM, Bob Friesenhahn wrote:

If I use libtiff to read the colormap it gives me three arrays, each 256 elements long, just as would happen for an RGB colour map. If I try to interpret these as the L, a, and b arrays, I get crazy colours. If, however, I reorder the data as if the TIFF file contains the colour table in the order L[0], a[0], b[0], L[1], a[1], b[1], etc. the resulting image looks OK. I can't see in the TIFF spec, or the extension specs, where the order of Lab colormap data is defined. However, it would seem bizarre if it is not in the same order as RGB data.

Has anyone had similar experiences? The test files are supposed to have been used for interworking tests with several FAX makers, so I would have expected anything as blatantly wrong as this to have been caught and fixed.

What does 'tiffinfo' say for one of these files? Are these files available for download from somewhere?

You can find these test files in several place on the internet, one of

which is

http://www.itu.int/itudoc/itu-t/com16/tiff-fx/t-cases/index.html. As far

as I am aware there is only one revision of these files.

The following is the tiffinfo report for the first page in this file:

TIFFReadDirectory: Warning, /home/steveu/telephony/TIFF-FX/l04x_02x.tif:

unknown field with tag 346 (0x15a) encountered.

TIFFReadDirectory: Warning, /home/steveu/telephony/TIFF-FX/l04x_02x.tif:

unknown field with tag 400 (0x190) encountered.
TIFF Directory at offset 0x8 (8)
   Subfile Type: multi-page document (2 = 0x2)
   Image Width: 1728 Image Length: 2342
   Resolution: 200, 200 pixels/inch
   Bits/Sample: 8
   Compression Scheme: 10 (0xa)
   Photometric Interpretation: 10 (0xa)
   FillOrder: msb-to-lsb
   Samples/Pixel: 1
   Rows/Strip: 2342
   Planar Configuration: single image plane
   Page Number: 0-2
   Color Map: (present)
   DocumentName: Test case L4
   ImageDescription: Page 1: Color test chart Page 2: Bar chart
   Software: Xerox TIFF-FX Reference Implementation Mon Feb 08 08:58:02
PM 99
   Tag 346: 1
   Tag 400: 1946

Tag 346 says its a color-map indexed image. Tag 400 is TIFF-FX specific,

and points to a global IFD.

Compression scheme 10 is T.43. Photometric Interpretation 10 is ITULAB.

tiffinfo doesn't display the contents of the colormap. However,

comparing the 3 vectors libtiff returns with a hexdump of the file,

libtiff is presenting the color-map in the same order it is stored in

the file. This is correct, according the the TIFF V6.0 spec.