[argyllcms] Re: ICCv4 Gamut Mapping

  • From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 23 Oct 2007 03:24:14 +0200

Graeme Gill wrote:
I may not be following you correctly, but:

In my interpretation the PCS side of an input profile is a subset of the
PRMG

With "the PCS side of an input profile" I mean the source gamut after compensating the viewing conditions and applying the A2B gamut mapping.

The profile has to compress out of gamut source colors to fit within the PRMG to avoid clipping in the A2B direction, but it had not necessarily to expand any color on its hull to the PRMG if clipping were permitted in the opposite direction.

[...] I don't think
it changes anything, even if you do, because the source of
a link could be either an input profile or output profile
A2B.

 > and the PRMG is a subset of the PCS side of an output device:

A real device will (in general) have a gamut that may fall
inside and outside the PRMG.

I mean the gamut of the device after compensating the viewing conditions and applying the gamut mapping again.

but there may be
clipping in output to output transforms (for colors outside the PRMG)

In which case it's not an implementation of a perceptual transform.

 > If a output device is able to render colors outside the PRMG, they can
 > only be addressed in output to output transforms.

Definitely a problem then, since input devices often have very large
gamuts, and you would expect to be able to address a color that is within
both an input and output devices gamut through a perceptual link.

Acknowledged. That's a rub.

My explanation does not depend on any distinction between input and output
devices (there is no real distinction anyway, there are scenarios in which
both forward and backward transforms could be needed of any type of device), and was in fact made on the basis of all the device profiles being output devices.

So we can draw the same conclusion you already arrived at earlier in this thread:

You can only do a really good gamut
mapping if you know the source and the destination.

Your explanation moved me forward.

Thanks for your patience,
Klaus

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  • » [argyllcms] Re: ICCv4 Gamut Mapping