Marti Maria wrote:
Hi,
Gamut warning is just an approximation. ICC has a specific tag for doing that, but
unfortunately, most profiles doesn't have such tag or worst, the information inside the tag is plainly wrong. So, lcms is computing the gamut "on fly" using the rel.colorimetric
tables. This works fine for some profiles, and fails miserably for others. lcms algorithm
Does LCMS always compute the gamut on the fly, ie ignore the gamut tag when it is there?
Does it work reasonably well with LCMS Profiler's profiles?
Does it work better for certain types of profiles (CLUT / matrix shaper)?
You say LCMS uses the rel.colorimetric CLUT (?) tables for the gamut warning. What do you do in case of matrix shaper profiles?
is sophisticated enough to let some movement in order to infer the correct gamut but if the profile has some kind of gamut remapping embedded in the rel. colorimetric intent (which shouldn't) the method doesn't work.
For the following question I must apologize as it is a result of my lacking in-depth knowledge of ICC profiling:
How would a profile have gamut remapping embedded?
My reason for doing the gamut check is as follows: The images have to, like it or not, be delivered in Adobe1998 to most stock libraries. This is a very small gamut and the clipping is evident in the histograms as spikes in the dark or light end, often just in one channel. Some stock libraries reject images with evident clipping. As you see, for a lot of photographers the profiles are not just a matter of faithful colour rendering, in fact, this issue is probably more important.
I have experimented with a highly saturated image (rich yellows) and I found that in this particular case LCMS' matrix shaper profile did a great job. There was no gamut warning. When converting to A98 using LCMS engine, there was still a little clipping. But when using ICM or ACE there was none. Therefore I thought that the gamut warning is at least a good indicator which profile to choose.
The only other way round it I could see was desaturating the image prior to tagging it with the scanner profile.
Best Regards, Tarlika
----- Original Message ----- From: "T E Schmitz" <[EMAIL PROTECTED]>
To: "lms mailing list" <[EMAIL PROTECTED]>; "Marti Maria" <[EMAIL PROTECTED]>
Sent: Sunday, February 22, 2004 10:31 AM
Subject: [Lcms-user] Gamut Warning
Hello Marti,
How reliable is the gamut warning? Is it based on the same algorithm as the actual conversion?
I am using the gamut warning to make a decision as to which scanner profile to choose. There certainly seems to be a correlation between the extent of gamut warning and, after the conversion, the clipping visible in the histograms.
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