[argyllcms] Re: How handle Lab tiffs?
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sun, 26 Oct 2008 17:55:34 +1100
Alastair M. Robinson wrote:
The one scenario where you might want to do this is if you're using
tiffgamut to compute a given image's gamut with a view to making a
devicelink specifically for that image, and that image happens to be in
CIELab space.
That, I think, is the scenario Nikolay had in mind.
Ok, yes you're right. I thought it might just work, but there
are several problems currently. The problems hint at some conceptual
issues as well. Problems that I notice are:
The image gamut has no white and black point. I can simply assume
in tiffgamut that an Lab image has white and black points of 100,0,0 and 0,0,0,
but there is a trap waiting if someone converts from a device
space straight to Lab and doesn't ensure that the conversion
also expands and maps the device space white and black to
100,0,0 and 0,0,0
There is a bug in tiffgamut that means that selecting CIECAM02
output for compatibility with collink is being ignored. The resulting
gamut will be in L*a*b* space.
The gamut mapping code takes the number of guide points from
the source gamut surface, and this depends on the surface complexity
(number of facets) and shape. For the Lab space source, the surface is
a cube with only 8 vertices so the number of guide points, or for Jab
it is stupidly large with facets in places that aren't relevant to the
image gamut.
It should be possible to fix these various problems for the next release.
Graeme Gill.
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