Pekka Paalanen 556272bae9 color-lcms: change stock sRGB to true power-2.2 ...

The sRGB expected display behavior uses the pure power-law with exponent
2.2, not the two-piece sRGB transfer function.
cmsCreate_sRGBProfileTHR() used the two-piece TF, now we use the proper
display TF.

This is particularly meaningful when implicit sRGB content is converted
to HDR formats, in order to maintain the stimuli reproduction near zero.

cmlcms_send_image_desc_info() is already sending this, it doesn't need
fixing.

Changing the curve also changes the error tolerances. The change is
theoretically a no-op, but the curve and its inverse and temporary
rounding add error. The new curve is more prone to error, so it is not
surprising we need to raise the tolerance. The color transformation does
end up as power-2.2 analytical form and I do not think it is ever
lowered to a LUT in alpha-blending test, so there is no obvious fix
improving the accuracy. The worst case point in alpha-blending still
occurs at the very same point as before.

The test reference images are updated for the same reason, they would
fail otherwise.

Both alpha-blending and color-icc-output contain the same sRGB-optical
sub-test, hence the same error tolerance.

It is surprising to have to increase the ICC roundtrip error tolerance
in color-icc-output test, given that the curves are passed as parametric
to LittleCMS, and adobeRGB case works with the old tolerance even. I did
not investigate further.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

2025-06-03 15:47:20 +03:00

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