[argyllcms] Re: Gamut mapping problems in 1.1.0

  • From: Idea Digital Imaging <qcore@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 12 Nov 2009 15:21:47 +0000

On 10 Nov 2009, at 16:34, Klaus Karcher wrote:

Imagine this workflow:

RGBworkingspace -[ICC perceptual]-> ISOcoated_v2 -[tiffgamut/ collink]-> yourOutput

(a) tiffgamut -ir -pj will treat the image in the ISOcoated stage as the original (i.e it will return the gamut of the image in ISOcoated)

in contrast

(b) tiffgamut -ip -pj will give you a gamut closer to the RGB image (as it uses ISO coated's perceptual tables to "undo" the previous gamut mapping and "re-expand" the gamut as far as possible to the camera stage)

there are two ICC whitepapers covering this topic:
<http://www.color.org/ ICC_white_paper_2_Perceptual_rendering_use_cases.pdf>

They call option (a) "re-targeting" and option (b) "re-purposing" and suggest that every ICC v4 profile should use those mirror- symmetrical A2B/B2A tables (but there are good reasons why some of us disagree)

Thanks for the explanation Klaus. I've read Graeme's iccgamutmapping.html and can see where problems will arise with the ICC v4 workflow.

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.

The debate of re-target/re-purpose is interesting though. In my case I operate in the "ambiguous" category. The original images have been extensively retouched in an RGB working space and nobody would want to refer back to the original scene :)

This means that the separation for proof/print can be a mixture of re- purposing/re-targeting.

I'll have a go at playing with the knee value to see what difference it makes to the saturation in the skin tones.

me too :-)

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?

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

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

3. Shadow and Highlight compression adjustments/overrides

But there was just loads of complicated code that I couldn't understand :(

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.

Martin Orpen
Idea Digital Imaging Ltd

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