AWARE SYSTEMS
TIFF and LibTiff Mail List Archive

Thread

1993.12.08 18:07 "CMYK Tiff Question", by Robert Meisner DT
1993.12.08 21:25 "Re: CMYK Tiff Question", by Dan Seyb
1993.12.08 12:40 "Re: CMYK Tiff Question", by Richard Minner

1993.12.08 12:40 "Re: CMYK Tiff Question", by Richard Minner

The conversion is:

        R = 1 - C
        G = 1 - M
        B = 1 - Y

        if C == M == Y == 0,
                R = G = B = 1 - K

        The only place that might get tricky would be if the scanner
        does undercolor removal (C, M, or Y != 0 and K != 0), and that
        would be a smart scanner.

                   ^^^^^^^^^^^^^

I think you have that backwards: a scanner that does _not_ do any undercolor removal is a _dumb_ scanner. :-) Seriously, the case of C,M or Y != 0 and K != 0 is not unusual. The naive conversion is still pretty simple, but you have to clip the values onto [0,1]:

        R = 1 - (C + K)
        G = 1 - (M + K)
        B = 1 - (Y + K)

This is indeed _naive_, but close enough for government work; the images will be recognizable. To get serious is a MAJOR can of worms; there are people who do nothing but work on this stuff -- for years and years. Various color management systems (Kodak's seems to be doing very well) are appearing to deal with color space conversion issues, which are _hard_. You have to deal with precise charaterizations (device independent) of the spaces, gamut incompatibilities (some spaces can represent colors that other spaces cannot), blah blah blah.

Have fun!

--

Richard Minner  rtm@island.com  {uunet,sun,well}!island!rtm
Island Graphics Corporation  Sacramento, CA  (916) 736-1323