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

[Adrian Knagg-B. <aje...@gm...>]

Thank you for the explanation. Yes, that's great, I will create a transform
from profile to XYZ, use it to transform each RGB primary using absolute
colorimetric, and then convert to xyY straightforwardly.

Thanks again,

Adrian.

On Mon, 29 Jan 2024, 22:02 , <mar...@li...> wrote:

> 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.
>
>
>