[Lcms-user] Creating equivalent matrix-shaper and 3D LUT profiles, problem with BPC

[Pekka P. <pek...@ha...>]

Hi,

I'm working on a Wayland compositor (Weston) to integrate LittleCMS in
it. I use LittleCMS to come up with the color transformation from ICC
profiles, then extract and optimize the cmsPipeline, and implement it
in OpenGL ES 3.0 shaders.

I have some difficulty in generating ICC profiles in Weston's test
suite. I want the test suite to exercise all code paths to ensure the
color operations are implemented correctly. I have separate code paths
for matrices+curves and 3D LUTs. Which code path gets chosen depends on
what processing blocks (cmsStage) an ICC file results in. Therefore, I
would like to craft identically behaving ICC files that hit the
different code paths (the goal).

Crafting matrix-shaper profiles is easy. Crafting an equivalent profile
that internally has a 3D LUT (DToB/BToD) is the problem. I can craft
such an ICC profile, but because I need to test with the perceptual
rendering intent, I should also include BPC between the zeros black
point of a matrix-shaper and the non-zero black point of the v4
perceptual PCS. Otherwise the assumption that a perceptual v4 CLUT
profile has the ICCv4 defined perceptual black point does not hold, and
the tests fail.

Is my goal reasonable to begin with, or should I just accept that
exercising the different Weston GL ES code paths by means of different
ICC profiles cannot lead to identical results and I need to define my
expected test results separately for each case?

If my goal is reasonable, then what would be a good way to add the
required BPC? Can I make LittleCMS compute it for me, or should I just
copy the BPC algorithm from LittleCMS?

I've tried creating the DToB/BToD test profile 3D LUT by creating a
chain of the usual matrix-shaper profile and the abstract XYZ profile
and sampling that, but now I'm convinced that I cannot get BPC applied
that way, not with the XYZ profile at least.


Thanks,
pq