[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 08 Jul 2014 10:54:31 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:


> Well that has finally clarified that point for me!  So with Perceptual the
> whole source gamut is squashed down to the destination gamut, even if all
> the colors are within the destination gamut.

Well, it depends on how you do it. By default the source gamut will
be assumed to be that of the encoding space. But you can override
this with something more precise. ie. "colprof -g src.gam"
or "collink -G src.gam" etc.

> This means that in a
> Perceptual mapping from ProPhoto to print, colors will be compressed
> resulting in desaturation of the image, particularly of the more saturated
> colors nearer the print gamut boundary.


> So the following strategy might make sense for a Relative intent conversion:
> - Going from ProPhoto to print, make sure the colors are more or less within
> the destination space to avoid too much clipping.

Yes, one approach is to manually gamut map in some way, and then use
a colorimetric conversion. In some cases a small amount of clipping provides
the "look" people are after by maintaining saturation, rather than
compressing to maintain the space for colors that may not be subjectively
so important. (This is a crude way of determining the "knee" in the
compression curve.)

> And the following for Perceptual:
> - Do a Relative conversion from ProPhoto to AdobeRGB (making sure the colors
> are more or less within the AdobeRGB space before the conversion to avoid
> too much clipping).
> - Do a Perceptual mapping from AdobeRGB to print.

Yes, that's one way. Another is to feed in a smaller gamut
as the source into colorpof/collink -g. The smaller
gamut could be from a colorspace (iccgamut) or from the images themselves 

> What would be nice would be to be able to make the smaller intermediate
> working color space using tiffgamut/colprof (from a range of typical
> images), but I don't see how that could be done.

I don't see why you would want to use the 2 step process, when
a 1 step with a smaller source gamut specified is more efficient.

Graeme Gill.

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