[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 03 Jul 2014 16:46:04 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:

> I'm confused about what happens if we do a round-trip conversion from, say
> ProPhoto to Destination and back to ProPhoto using a Perceptual intent
> (say).
> What I would expect would be:
> 1. ProPhoto->PCS:       Relative (since the working space uses a
> matrix-based profile). Uses ProPhoto Profile.
> 2. PCS->Destination:    Perceptual. Uses Destination profile BtoA.
> 3. Destination->PCS:    Perceptual. Uses Destination profile AtoB.
> 4. PCS->ProPhoto:       Relative. Uses ProPhoto profile.
> If that was the case then I would expect to see a change at 3, which I do if
> I use a profile made using i1Profiler or using the canned paper profile.
> However I see no difference (or can measure no difference using an i1Pro)
> using an Argyll-generated profile. This is affecting the soft-proofing,
> which (I presume) uses a round-trip as above.

        I don't understand why you would expect to see a change at step 3.
You would expect to notice something at step 2, because a print destination
typically has a smaller gamut than ProPhoto, but (at least for ArgyllCMS 
a perceptual AtoB table is always the same as the colorimetric one - there is
no attempt to invert the perceptual BtoA, since this serves no purpose in
the context of how ArgyllCMS facilitates gamut mapping, and certainly
is not required of an ICCV2 profile.

ICCV4 profiles might be different, since they have the (optional) notion of
perceptual transform to/from a reference gamut, with all the limitations
that entails.

Graeme Gill.

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