[argyllcms] Re: Creating a CLUT sRGB profile for perceptual rendering intent

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sun, 11 Nov 2007 05:51:24 +1100


Mark wrote:

Hello list,

I have an idea, but I don't know exactly how to get there... I have some large gamut slides in ProPhoto that I want to convert to sRGB for web display. As we all know, since sRGB profiles are matrix based, CMS's will only do colorimetric renderings for this conversion - so I'll eventually have lots of clipped colors.

I've been thinking on how to overcome that; I guess it should be possible to create a CLUT output profile for sRGB, supporting perceptual rendering. So the idea is to use this CLUT profile for the gamut mapping from ProPhoto (or whatever) to sRGB and then swap the CLUT sRGB profile by a matrix sRGB profile in the resulting tiff/ jpeg/... so the file gets tagged as a "standard" sRGB in the end.

Could that work or is there something I am missing?

You can build a CLUT based sRGB profile by using fakeread to
fill in the patch values created by targen, and then
construct a profile in the usual manner. This is
probably the best course if you want to use standard
ICC aware tools to do the conversion.

If you were to use cctiff, then you could simply create
a device link using icclink -G, as this will create
the appropriate gamut mapping too. This will be a better
quality result.

A "gotcha" with a large gamut source color space, is that
by default the gamut mapping will assume that the source gamut
is the colorspace gamut. This may over compress the actual
gamut of the images.

You can solve this problem by creating an image gamut using
tiffgamut, either for each image, or for a composite made
from all the images you wan to convert (perhaps after scaling
them down). You would feed this into icclink with the -G flag,
or profile with the -g flag.

Note that the resulting link or profile is tuned for that image
or image set.

Graeme Gill.

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  • » [argyllcms] Re: Creating a CLUT sRGB profile for perceptual rendering intent