[argyllcms] Re: Monitor profiles and their (black point?) "readability"
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 19 Jun 2008 01:21:46 +0200
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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