[Lcms-user] Gamut diagram

[Lukas S. <som...@gm...>]

Hello.

In the last months, I've started to develop a colour picker widget based on
the LCh model: https://github.com/sommerluk/perceptualcolor

It displays a given colour space gamut (sRGB or something other) within LCh
diagrams, to provide a perceptually uniform colour picker with a nice UI.
It's under MIT licence.

However, I'm new to colour management and don't have much experience with
it. It would be great if I could get some help here.

1.)
==========

What is the best way to know the gamut of a given colour space? What I try
to get is this:
https://raw.githubusercontent.com/sommerluk/perceptualcolor/master/other/expected.png

The colour blob in the middle is a cut through the LCh model at a given
lightness (the gamut is the sRGB gamut here, but it should work also with
other gamuts). The code works like this:
– Create a buffer with a Lab image
– Transform it with LittleCMS like this:
    cmsHPROFILE labProfileHandle = cmsCreateLab4Profile(NULL);
    cmsHPROFILE rgbProfileHandle = cmsCreate_sRGBProfile();
    m_transformLabToRgbHandle = cmsCreateTransform(
        labProfileHandle,             // input profile handle
        TYPE_Lab_DBL,                 // input buffer format
        rgbProfileHandle,             // output profile handle
        TYPE_RGB_DBL,                 // output buffer format
        INTENT_ABSOLUTE_COLORIMETRIC, // rendering intent
        0                             // flags
    );
– Test for each pixel if one of R, G or B component is outside the range
0–1. If so, treat is as transparent. If not, paint it actually on the
screen.

This approach works. However, there are also problems.

a) If I understand correctly the paper “Unbounded Color Engines” than
LittleCMS might switch to bounded mode if the profile does not support
unbounded mode. So comparing the output channels to the range 0–1 would be
useless, and the approach might not be reliably.

b) It is slow, and does not provide proofing.

So I tried something different:
    cmsHTRANSFORM xform = cmsCreateProofingTransform(
        labProfileHandle,                           // Input profile handle
        TYPE_Lab_FLT,                               // InputFormat
        rgbProfileHandle,                           // Output
        TYPE_BGRA_8,                                // OutputFormat
        rgbProfileHandle,                           // Proofing
        INTENT_ABSOLUTE_COLORIMETRIC,               // Intent
        INTENT_ABSOLUTE_COLORIMETRIC,               // ProofingIntent
        (cmsFLAGS_SOFTPROOFING|cmsFLAGS_GAMUTCHECK) // dwFlags
    );
It relies directly on LittleCMS for the out-of-gamut detection. It seems
that out-of-gamut colours are automatically transparent. The problem is
that the out-of-gamut detection seems to be far less exact than the other
method. Therefore, the gamut image has a very strange shape:
https://raw.githubusercontent.com/sommerluk/perceptualcolor/master/other/actual.png

What would be the recommended approach here?

2.)
==========
I've used INTENT_ABSOLUTE_COLORIMETRIC as rendering intent. It this the
correct choice for this use case?

3.)
==========
The build-in Lab profile has D50, but the build-in sRGB has D65. Does this
matter? Or can I safely ignore that?

Best regards

Lukas Sommer