Re: ProfileMaker yellows turn green
Re: ProfileMaker yellows turn green
- Subject: Re: ProfileMaker yellows turn green
- From: Graeme Gill <email@hidden>
- Date: Mon, 10 May 2004 13:33:47 +1000
- Organization: Color Technology Solutions
Laura Rimbaud wrote:
Hello. I have been a reader of this list for some time
and am grateful for all the help and knowledge the
writers have shared. I have a question about
ProfileMaker Pro 4.15 and RGB profiles. On continuous
tone printers (our Kodak 8660 and 6800 models)
ProfileMaker profiles tend to turn saturated yellows
green.
Sorry I can't help you with any specific suggestions on
how to get the right result out of ProfileMaker, but technically
the explanation is probably one or a combination of the following:
1) Use of a colorspace such as L*a*b* for gamut mapping/clipping, which is
known to have a mismatch between lines of constant a*/b* ratio and human
perceived constant hue. This is especially obvious in the blue region,
but may have an effect elsewhere as well, such as the red or yellow.
2) Gamut mapping being done with a simple constant hue angle type of algorithm.
The yellow "cusps" of most devices in a perceptual colorspace are generally
very sharp and narrow, so any gamut mapping algorithms that operates on the
basis of keeping the source hue angle constant, and particularly if it has a simple
geometric approach to altering saturation, has little chance of intersecting
the destination yellow cusp at its tip (ie., where it is pure yellow in the
output device). A gamut mapping/clipping algorithm that has some weighting
towards the nearest point on the destination gamut is necessary to address
this problem.
3) The gamut mapping/clipping space not allowing for the Helmholtz-Kohlraush
effect, which models the effect whereby highly saturated colors appear
lighter than less saturated colors.
4) Standard ICC workflows are based on the "smart profile, dumb linker" model,
where things like gamut mapping are encoded at profile creation time, and
stored in the B2A tables of the destination profile. Since the destination
profile is usually created without any knowledge of the source gamut, most
packages (I assume) are forced to use a generic gamut mapping. This means
that even if points 1), 2) & 3) above were taken into consideration, there
is practically no chance that yellow will be mapped the way you would like,
since the destination profile has no knowledge of the source yellow it is
meant to be mapping from.
Is there
a technique that works to edit the profile or prevent
this problem? It looks like the RGB equivalent of the
difficulty in getting pure yellows in CMYK profiles. I
can tweak the profiles for our Roland to compensate
but I have not had success with RGB profiles.
If you send me a source and destination ICC profile, I would be
interested in reconstructing the destination profile (or better yet,
creating a device link profile) using my toolset, to see if it
addresses this particular issue.
Graeme Gill.
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