[argyllcms] Re: Monitor profiles and their (black point?) "readability"

Marco N. wrote:
What I see is that with GMB profile all three software render the image correctly in the same way, with Argyll profile only PS renders the image correctly, the two other software clip the shadows and I don't know why.
As the Argyll profile has a media black point tag with Y > 0, everything below the black point will get clipped in colorimetric transforms without BPC. The GMB profile has no black point tag, therefore the CMM assumes XYZ 0/0/0 as black point. When yo convert RGB 0/0/0 from the Argyll to the GMB profile without BPC, you will get RGB 17/17/17, when you convert from GMB to Argyll, source values below 17/17/17 will get clipped.
Photoshop always uses Black Point Compensation for display transforms (see <http://color.org/AdobeBPC.pdf>) as long as you don't make a softproof. BPC lessens the clipping (in the example above, only RGB values below 6/6/6 get clipped with BPC) and has side effects on the gamut mapping. BPC is not part of the ICC Spec, therefore the behavior of Firefox 3, Safari & Co. is correct for the purpose of the Spec.
Kodak proposed to delete the mediaBlackPointTag and AFAIK this proposal is already approved by the ICC. Whether this decision makes sense is a different kettle of fish. 

Klaus Karcher

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