2004.12.16 17:52 "[Tiff] CIELAB/ITULAB", by

2004.12.16 17:52 "[Tiff] CIELAB/ITULAB", by


I am at a loss. I need to convert CFF (color fax) files from a CAPI Fax software to TIFF files. According to ITU T.4, Annex E, and T.42, the pages are coded in JPEG / CIE L*a*b* with an APP1 marker "G3FAX\0". I set Compression=7 (JPEG) and write (almost) all the JPEG data (tables + entropy data) in one strip.

If I set PhotometricInterpretation=8 (CIELAB) then the resulting TIFF image will be displayed (by most polular viewers) with correct shapes but in wrong colors.

So I extended the libtiff by the Decode tag and tried PhotometricInterpretation=10 (ITULAB) with the default values for Decode (taken from http://www.awaresystems.be/imaging/tiff/tifftags/decode.html). Unsurprisingly, some of the viewers refused the TIFF file but the rest showed the same wrong colors.

What can be going wrong? And why the hell the Decode tag is of type RATIONAL (unsigned32/unsigned32) while the values can be negative?

Thank 4 all answers