[argyllcms] Re: Gamut mapping problems in 1.1.0

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 13 Nov 2009 03:03:32 +1100

Idea Digital Imaging wrote:

However, my question about tiffgamut regarded using it to map the original RGB image prior to building the RGB to CMYK device link. Having thought about it the answer is that it makes no difference using -ir, -ip or -is when the source is a matrix profile.

Yes. It only makes a difference for CLUT based profiles that
have different A2B tables. Note though that collink always
uses the colorimetric device behavior to create the gamut
mapping, so to match, tiffgamut should also use the colorimetric
table (default).

I tried adjusting the knee setting to 0.5 but it made big changes to the lightness of saturated colours and very little difference to the skin tones. I assume that the "knee" is a point where the gamut compression stops/starts being linear and that changing it in isolation isn't likely to achieve all I want?

That's not what I would expect. The gamknf controls how much the gamut
surface compression/expansion penetrates the volume. A value of 0
gives almost uniform compression/expansion from the surface
to the center of the volume. A value of 1.0 will almost completely
preserve the source gamut that falls within the destination volume,
and clip colors outside that to the destination surface.

Unfortunately I can only explain this sort of thing in retouching terms, but what I was hoping was that somewhere like gammap.c (or better still somewhere that allows you to change the values after compiling) there would be a user adjustable method of compensating for the problems of getting big images into small spaces, like:

1. Boosting mid-tone contrast

The handling of lightness compression is unchanged from
the previous version. It uses knee compression to maintain
contrast ratio when the dynamic range is reduced.

2. Increasing (or maintaining) chroma in light to mid tones

This is only possible for colors within the destination gamut.
Any light to mid tone values outside the destination gamut
have to be reduced in gamut and/or changed in chroma.

3. Shadow and Highlight compression adjustments/overrides

This can be altered (see grey axis controls), but I thought
that this is reasonably set at the moment, and is unchanged from

I wonder what Graeme's thoughts are on expanding the "creative" use of gamut mapping? It seems a lot more useful to me than the ICC v4 workflow which assumes that we all want to get back to some "original scene" and will have to be totally reliant on profile manufacturers to do it.

It is something I'm looking into, but there are usability/flexibility
tradeoffs. Ideally one would get interactive feedback when adjusting
such controls.

I don't think I can address your particular issues until I get back
home and can evaluate things under more controlled conditions.

Graeme Gill.

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