[argyllcms] Re: Custom Illuminant

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Thu, 3 Jul 2014 14:25:57 +0100

Hi Graeme,

I only expect things out of ignorance (or perhaps by observation, not by
understanding either the mechanisms involved or the color theory).

With a profile (either V2 or V4) from Canson, or one that I've made myself
with i1Profiler, there is a change from Destination to PCS which is in the
opposite direction to the PCS to Destination change. So I assume that with
these profiles both the BtoA and AtoB tables are Perceptual tables.  In
Argyll it is a Relative table as you say, so there is no change.

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.

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?

Robert

-----Original Message-----
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Graeme Gill
Sent: 03 July 2014 07:46
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Custom Illuminant

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.

Hi,
        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
profiles)
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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