2005.09.09 14:58 "Re: [Tiff] creating JPEG file", by Bob Friesenhahn

unsigned integer data, unsigned integer data

JPEGTables (289 UNDEFINED): 255, 216, 255, 219, 0, 67, 0, 8, 6, 6,... YCbCrSubSampling (2 SHORT): 2, 2

I've seen YCbCrSubSampling used with YCbCr, of course, and with ITULAB,
and, logically extending it, it could also be used with any other color
space that has a single lightness/brightness channel followed by chroma
channels. But it is not clear to me how it should apply to CMYK. I don't
think this combination makes sense.

That does seem incorrect. As far as I know, this part was done automatically by libtiff (GraphicsMagick did not specifically request subsampling) so it may be that libtiff is doing something wrong with CMYK JPEG compression.

GraphicsMagick is doing something a bit odd though since it uses

   (void) TIFFSetField(tiff,TIFFTAG_JPEGCOLORMODE,JPEGCOLORMODE_RGB);

The only other choice appears to be JPEGCOLORMODE_RAW. Maybe JPEGCOLORMODE_RAW should be used for CMYK?

Bob
======================================
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/