[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 05 Jul 2014 14:29:25 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:

> Logically (to me at any rate) I would think that there should NOT be a
> change in the AtoB direction (at least from a soft-proofing point of view),
> so it would seem to me that Argyll is doing the right thing and X-Rite is
> not.

Hi,
        it's not really right or wrong - the ICC profile format
allows for intent specific A2B tables, and the standardized V4 PRMG
proprietary V2 & V4 "intermediate gamut" approaches to gamut mapping
would make use of this. This approach has notable limitations, and I've
chose to make Argyll's gamut mapping more "pure". With Argyll's approach,
using intent specific A2B's doesn't make any sense.

> I have read in various places comments like: "Because Perceptual squashes
> and stretches in the BtoA direction (to get the source gamut to fit the
> destination gamut), it can to some extent be reversed in the AtoB
> direction". Misinformation?

True enough, but if the "squashing" is really "clipping", then you can't
un-clip, so there is no guarantee that such an A2B is the perfect inverse
of the corresponding B2A. One of the disadvantages of the intermediate
gamut approach is that it's likely to be less precise overall, because of
the double gamut mapping.

Graeme Gill.


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