Re: [Lcms-user] Unexpected gray to RGB result

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

Hello,

I was puzzled because I remember I added an algorithm to detect those bad
profiles and hot-fix them. Upon reviewing the code, the algorithm was there
but it failed on your profile!

So, you hit the rare case of gray profile + wrong V4 encoding + apparent
black point higher than L* > 50 + inverted curves.  All that was confusing
the black point compensation part that was supposed to fix the issue.

Thank you very much to help in finding a very elusive bug. 

Now it is fixed by
https://github.com/mm2/Little-CMS/commit/caab4c07e60022a0f776b543eaa30785e2b
b42ed

Best regards
Marti Maria
The LittleCMS Project
https://www.littlecms.com

-----Original Message-----
From: Ralf Junker <ral...@gm...> 
Sent: Sunday, November 27, 2022 9:41 AM
To: lcm...@li...
Subject: Re: [Lcms-user] Unexpected gray to RGB result

Thanks for the fast answer!

> The embedded profile is wrong. It claims to be V4 and perceptual, but 
> fails to scale on perceptual black.

Here is the C code that generated the profile:

int main()
{
   const cmsCIExyY *whitepoint = cmsD50_xyY();
   int i;
   cmsUInt16Number values[256];
   cmsToneCurve *tonecurve;
   cmsHPROFILE profile;

   for(i = 0;  i < 256; i++) {
     values[i] = (255 - i) * 65535 / 255;
   }

   tonecurve = cmsBuildTabulatedToneCurve16 (0, 256, values);
   profile = cmsCreateGrayProfile(whitepoint, tonecurve);

   cmsSaveProfileToFile(profile, "gray-inverted.icc"); }

The goal is a profile which swaps blacks and whites, dark grays and white
grays, to create an inverted, negative grayscale.

The profile's purpose is testing image rendering software with color
correction using lcms2. I found WhackedRGB.icc for color images, but nothing
for grayscale.

> Other CMM may choose to reject the profile, lcms uses it, despite it 
> is wrong.

My bad, I take your point. Please note that the profile somehow works with
Chromium, Firefox and Gimp which all show whites on the left and blacks on
the right of the image. And they all show it identically.

Viewers which do not apply the profile show blacks on the left and whites on
the right.

> See https://color.org/specification/ICC.1-2022-05.pdf
>
> "6.3.4.3 PCS encodings for white and black"

Unfortunately, I am unable translate this technical specification into
lcms2 code. Could you suggest a function or a chapter in the documentation?
I found nothing obviously related, I'm afraid.

Grateful,

Ralf


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