[argyllcms] ICCv4 Gamut Mapping [was: ArgyllCMS V0.70 Beta 7 test now available.]

Graeme Gill wrote:

I've written a bit more information about this topic at
<http://www.argyllcms.com/doc7/iccgamutmapping.html>


Thanks for this information, Graeme.
I'm surprised about your conclusion that "only one (non colorimetric) intent can really be supported [in ICCv4 profiles using the PRMG], that of saturation". Can you explain it a little bit more detailed?
As outlined in the ICC Technical Note 01-2003 <http://color.org/v2profiles_v4.pdf>, "the BToA perceptual rendering transform [...] is an UNDO of the AToB perceptual rendering transform [in an input profile and] the AToB perceptual rendering transform is an UNDO of the BToA perceptual rendering transform [in output and display profiles]"
For my comprehension, the A2B0 transform of an input profile should apply a CAM to compensate the different viewing conditions between the input space and the PCS followed by a gamut compression if necessary. The role of the PRMG is to mark the boundary beyond which clipping may occur. I see no need for gamut expansion in A2B0 transforms of input profiles unless the device has a known weakness in a certain range of colors.
The A2B0 transform of an output profile should map the intersection of its own gamut and the PRMG to the PRMG. In most cases this will result in an gamut expansion.
The B2A0 transform of an output profile should be the inverse of the A2B0 table, i.e. a gamut compression in most cases.
The only drawback I see is that wide gamut output devices will rarely tap their full gamut in perceptual transforms. 

Regards,
Klaus

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