Re: [Lcms-user] Clarification about chromatic adaptation

[Marti M. <mar...@li...>]

Hi,

The chromatic adaptation matrix is just an informative field on what has been done on intents perceptual, relative colorimetric and saturation. It is not used when building a transform, the “true” chromatic adaptation is embedded in the colorant matrix or in the LUT.

Having said so, there is one situation where this is used. If you create a absolute colorimetric transform with observer not adapted (and only in this case!) the CMM “undoes” the chromatic adaptation of the profile using the CHAD matrix. It works to some extent, because many vendors put whatever in the CHAD and not disclose their “secret sauce”.  So in your sample, using -d0 you will get some differences. Remember in ICC 4 absolute colorimetric observer is fully adapted by default, therefore no differences on different illuminants.

Regards
Marti

> On 11 Mar 2022, at 11:37, Andrea Galligani <and...@ma...> wrote:
> 
> Hi to all,
> I realized an esperiment coping a standard profile and changing the chromatic adaptation matrix from identity to a
> Bradford D65->D50 one. I saved a D65 Lab profile using the code:
> 
> cmsHPROFILE                 hProfile   = cmsCreateLab2Profile( &D65illuminant );
> cmsSaveProfileToFile( hProfile, "LabD65.icc" );
> 
> Than I created a very simple CGATS with base pure CMYK tints and I used transicc to translate
> CMYK to Lab using this commands
> 
> transicc -iSNAP2007_D50.icc -o*Lab2 -t3 CMYK.txt CMYK_D50.txt
> 
> and
> 
> transicc -iSNAP2007_D65.icc -oLabD65.icc -t3 CMYK.txt CMYK_D65.txt
> 
> I expected different results from D50 and D65, similar to measures obtained
> measuring the printed patches under the different illuminants, but
> transicc produced almost identical results (Max DE00 0.0015).
> 
> Did I do somethink wrong?
> 
> Thanks
> Andrea
> 
> -- 
> 
> Andrea Galligani
> 
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