Absolute vs Relative Colorimetry in the ICC Profiles?
Absolute vs Relative Colorimetry in the ICC Profiles?
- Subject: Absolute vs Relative Colorimetry in the ICC Profiles?
- From: Roger Breton <email@hidden>
- Date: Fri, 21 Sep 2001 10:54:18 -0400
Which colorimetry gets encoded in an ICC profile?
The 1998-09 ICC Spec on p.82 exhorts that "The PCS is based on Relative
Colorimetry". Fine. It is not clear that profiles have to incorporate
relative colorimetry as well. I am just trying to find out what to expect in
a profile A2Bx and B2Ax tags: relative or absolute colorimetry?
If, indeed, profiles are encoded "relatively" then this means that makers of
the ICC profile have to convert the absolute colorimetric measurements read
off the various characterization targets, by color measuring instruments, to
some relative form through a choice of Bradford or Von Kries methods, in
order to populate the LUTs (after interpolation)?
Otherwise, if profiles are encoded "absolutely" then this would mean that
the makers of the profile just go ahead and populate the LUTs using the same
absolute colorimetric data (massaged for interpolation) that was read off
the original characterization target? Logical?
If this second scenario turns out to be true, would it be then mean that, at
run-time, the CMM makes the absolute data it finds in the ICC profiles
"relative" by chromatically adapting to the paper white, in both source and
destination profiles, using Bradford or Von Kries, pulling the information
from the MediaWhitePoint Tag, I understand thus making both sides of the
equation L = 100, a=b=0, so that C=M=Y=K=0 and R=G=B=255?
Lastly, the ICC mandates on p.118 that the "coordinates of the output paper
be *adapted* to the PCS Illuminant"? Wouldn't that defeat the purpose of the
media white point data in the first place? Why wouldn't the Media White
Point be encoded in "absolute" terms?
Roger Breton