Re: [Lcms-user] Out of Gamut Alarm Question

[Marti <ma...@li...>]

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" <dcp...@ch...>
To: <lcm...@li...>
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
dcp...@ch...

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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