[argyllcms] Re: Gamut mapping problems in 1.1.0
- From: Idea Digital Imaging <qcore@xxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 12 Nov 2009 19:17:25 +0000
On 12 Nov 2009, at 16:03, Graeme Gill wrote:
Idea Digital Imaging wrote:
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.
I think the reason it made little difference to the skin is because
the skin was all in gamut in the first place and the problem seems to
be restricted to a relatively small area in my tests.
What is getting lost is the red "blush" in the light skin tones (on
the petals it was a hint of a warm glow). If I overlay a strip of the
separated image over the same file that has been converted with
relative intent and black point compensation in Photoshop (still
there using perceptual too) you can see that this delicate blush is
going neutral:
<http://inside.rgb2cmyk.com/images/cheek.tif>
And the more their is of that tone in the face the duller the skin
becomes.
Perhaps it's an issue with the profile itself -- ISO Coated v2 (300)
-- or with the viewing conditions etc which only appears when you try
gamut mapping? There are a few too many variables in this process.
I don't think I can address your particular issues until I get back
home and can evaluate things under more controlled conditions.
My needs and feature requests are niche so can appreciate that you've
got lots more pressing/interesting stuff to do.
Regards
--
Martin Orpen
Idea Digital Imaging Ltd
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