Hi,
Sorry for the long delay. I've been very busy past week...
Regarding this particular profile, sounds weird, but this is how it handles colors. I assume you are using Adobe's SWOP.
Let's do some numbers. If I make a SWOP -> Lab transform with perceptual intent, I got:
icctrans -c0 -t0 -o*Lab -iUSWebCoatedSWOP.icc little cms ColorSpace conversion calculator - v1.8 Enter values, 'q' to quit C (0..100)? 100 M (0..100)? 100 Y (0..100)? 0 K (0..100)? 0
L*=25.3324 a*=25.2695 b*=-55.3320
Ok, So this is the Lab value of the color we want to check watever it is inside gamut hull. The algorithm lcms is using is as simple as move it back and forth across rel. colorimetric intent. If the profile changes the value, then color is out of gamut. Same values means inside gamut. This method is not perfect, but works quite well as a coarse approximation.
Let's now convert this Lab value across colorimetric intent to CMYK colorant.
icctrans -c0 -t1 -i*Lab -oUSWebCoatedSWOP.icc little cms ColorSpace conversion calculator - v1.8 Enter values, 'q' to quit L*? 25.3324 a*? 25.2695 b*? -55.3320 C=100.00 M=98.72 Y=8.84 K=5.62
The colorant values are not (100,100,0,0) but that doesn't matter, because same color can be obtained by different CMYK combinations. The important part would be that CMYK contone SHOULD give the same Lab if measured across AtoB1 table-But as you can see that is not the case:
F:\lcms>icctrans -c0 -t1 -o*Lab -iUSWebCoatedSWOP.icc little cms ColorSpace conversion calculator - v1.8 Enter values, 'q' to quit C (0..100)? 100 M (0..100)? 98.72 Y (0..100)? 8.84 K (0..100)? 5.62 L*=29.6247 a*=16.2070 b*=-41.4492
So, there is a huge shift from (25, 25, -55) to (30, 16, -41), hence the profile
handles this value as out of gamut.
Which is not so surprising at all, because (100, 100, 0, 0) is over gamut boundaries. The profile is doing some slight gamut remapping in the extreme to avoid gradient blocking. That is not so bad at all, images should not use such extreme contone values.
Regards Marti Maria.
----- Original Message ----- From: "David Purton" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Wednesday, February 16, 2005 1:53 PM
Subject: [Lcms-user] Out of Gamut Alarm Question
Hi,
Quick question on lcms handling of out of gamut checking.
I notice that when I proof a CMYK colour to RGB, using the same proofing profile as input CMYK profile, some colours are marked out of gamut. For example, C100 M100 Y0 K0 using SWOP as the CMYK profile and sRGB for the RGB, is marked as out of gamut. (perceptual intents)
Is this supposed to happen?
I would have thought that if my input profile and my proofing profile are the same, then all colours are by definition in gamut.
It seems to especially crop up in dark colours.
(fwiw, CorelDRAW's colour management, does not mark this colour out of gamut under these circumstances)
cheers
dc
(Cc me if you think to, I'm not subscribed)
-- David Purton [EMAIL PROTECTED]
For the eyes of the LORD range throughout the earth to
strengthen those whose hearts are fully committed to him.
2 Chronicles 16:9a
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