[argyllcms] Re: Confused with PS, softproofing and monitor profile intents

  • From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 06 Nov 2010 14:14:53 +0100

Vallo Kallaste wrote:

Firstly i'm amused by the fact that Xrite profiling software which
comes with Colormunki, creates RGB-TRC (gamma) based profile with
Perceptual intent. As far as I understand it's nonsense, because
this is what cLUT profiles are for.

Do you mean the the Rendering Intent Flag in the profile header?

"Note that this flag might not have any meaning until the profile is used in some context, e.g. in a DeviceLink or an embedded source profile." (Spec. ICC.1:1998-09)

As long as there are no CLUTs in the profile, it doesn't matter whether it is set to relative or perceptual: the CMM will fall back on the TRC/matrix model in any case. Only absolute colorimetric might make a difference in an application that makes use of the Rendering Intent Flag, but such applications are really rare.

Next I tried to think what
happens when softproofing for printing in Photoshop and got stuck.
Can somebody enlighten me what happens behind the scenes in the PS
when one softproofs for specific printer-paper profile?
First case would be with matrix monitor profile, with only rel.
colorimetric intent.
Second case would be with cLUT monitor profile, containing also
perceptual intent. Source images are in ProPhotoRGB space and the
cLUT monitor profile has been made with 'colprof -S ProPhoto.icc
...'. Printer profile is manufacturer canned one, not that it
How are all the profiles and intents interacting? Is PS creating
device link profiles on the fly, considering also source image

Photoshop (or the CMM) creates a device link profile on the fly, but a very simple one: there is no dynamic gamut mapping.

The softproof link always works like this

image -> printer -> monitor

The image -> printer transformation is affected by the rendering intent and black point compensation settings you set for the softproof

and the printer -> monitor transformation is affected by the "simulate paper" and "simulate black" settings:

- simulate paper and black means absolute colorimetric
- simulate only black means relative colorimetric without BPC
- simulate neither paper nor black means relative colorimetric with BPC

It doesn't matter whether your monitor profile contains a real perceptual intent or not: Photoshop uses the colorimetric intents for softproofs in all cases (unless you hack the tag table of the monitor profile ;-)

When you just display an image in Photoshop (without softproofing) the transformation image -> monitor is always rel with BPC.

Perceptual intents in monitor profiles are therefore not of much use in Photoshop. The only way to use them are expicit manual tranformations (e.g. by converting an image to monitor space with RI perceptual)


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