Re: [Lcms-user] Need help recovering the chromaticities that a profile was made with

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

Hello,

 

> is there something I'm not accounting for? And is there a more correct way to read the original chromaticities back out of a profile?

 

You are assuming the internals of ICC profiles works in a way they don’t. The cmsSigXXXColorantTag tags does NOT contain direct information on the primaries, numbers are “cooked” and it is certainly possible for those tags to be missing on some profiles.

 

You should avoid reading tags directly as much as possible. Instead, use an absolute colorimetric transform with no observer adaptation to get the primaries.  This would work no matter which profile type are you using.

 

Here an example by using the transicc tool, you can do the same creating a transform with absolute colorimetric intent making sure to set the observer adaptation state to 0 first. I am trying to recover sRGB red primary:

 

marti@I7:~$ transicc -t3 -d0 -i*sRGB -o*XYZ

LittleCMS ColorSpace conversion calculator - 5.1 [LittleCMS 2.16]

Copyright (c) 1998-2023 Marti Maria Saguer. See COPYING file for details.

 

Enter values, 'q' to quit

R? 255

G? 0

B? 0

 

X=41.2391 Y=21.2639 Z=1.9331

 

You convert this XYZ of (41.2391, 21.2639, 1.9331) to xyY, for example using this link: http://www.brucelindbloom.com/ColorCalculator.html

 

xyY = (0.6400, 0.3300, 21.2639) Which corresponds to the Rec709 chromaticity of red.

 

Hope that helps

 

Marti Maria

The LittleCMS Project

https://www.littlecms.com

 

 

 

 

From: Adrian Knagg-Baugh <aje...@gm...> 
Sent: Monday, January 29, 2024 10:34 PM
To: lcm...@li...
Subject: [Lcms-user] Need help recovering the chromaticities that a profile was made with

 

Hi,

 

I have extracted the chromaticity xy coordinates from a cmsHPROFILE as follows:


cmsCIEXYZ   *red;
cmsCIEXYZ   *green;
cmsCIEXYZ   *blue;
red   = cmsReadTag (profile, cmsSigRedColorantTag);
green = cmsReadTag (profile, cmsSigGreenColorantTag);
blue  = cmsReadTag (profile, cmsSigBlueColorantTag);

cmsXYZ2xyY(&redxyY, &red);
cmsXYZ2xyY(&greenxyY, &green);

cmsXYZ2xyY(&bluexyY, &blue);

 

However, if the profile I do this to is a sRGB profile made with chromaticities from the cmsCIExyYTRIPLE {{0.639998686, 0.330010138, 1.0}, {0.300003784, 0.600003357, 1.0}, {0.150002046, 0.059997204, 1.0}}, and the standard sRGB whitepoint cmsCIExyY d65_srgb_adobe_specs = {0.3127, 0.3290, 1.0}; I don't get back the same coordinates. Instead I get:

 

redxyY x: 0.648438, y: 0.330868, Y: 0.222488 
greenxyY x: 0.321176, y: 0.597877, Y: 0.716904 
bluexyY x: 0.155899, y: 0.066051, Y: 0.060608 

 

What's going on here: is there something I'm not accounting for? And is there a more correct way to read the original chromaticities back out of a profile?

 

Thanks,

 

Adrian.