[Lcms-user] First experiments with LCMS2

[Maurice L. <mau...@gm...>]

Hi,



I'm new to LittleCMS2 and I'm impressed. But my first test did reveal two
issues.



I used linkicc to create a CMYK input profile as follows:



linkicc -o DevLink2PSOcoated_v3.icc -t1 -n17 -x -r2.4 DevLink.icc
PSOcoated_v3.icc



The DevLink2PSOcoated_v3.icc profile is a CMYK to CMYK ICC version-2
DeviceLink profile (which maps CMYK (0, 0, 0, 0) to CMYK (0, 0, 0, 0)) and
the PSOcoated_v3.icc profile is a CMYK to Lab ICC version-2 Printer profile
(Fogra-51).



When looking at the resulting input profile in ICC Profile Inspector I see
that the resulting profile is a version-2.4 profile (as requested) with
linear (out = in) input and output channels (consisting of 2 points each, 0
and 65535), which is fine.

But the value of the first CLUT entry (for CMYK = (0, 0, 0, 0)) is equal to
(65535, 32896, 32896), which is the version-4 encoding for Lab (100, 0, 0).

It should have been (65280, 32768, 32768) which is the version-2 encoding
for Lab = (100, 0, 0).



The reason for this is that FixWhiteMisalignment() in cmsopt.c "fixes" the
first value in the CLUT from its correct version-2 encoded Lab value to the
version-4 encoded Lab value, which it got from _cmsEndPointsBySpace() for
the output Lab color space.

This is clearly not the intended behavior.



Another issue with the resulting input profile is that the
mediaWhitePointTag is set to the D50 white point instead of to the white
point of the PSOcoated_v3.icc profile.
This is because cmsCreateExtendedTransform() sets the EntryWhitePoint of
the transform to the white point of the first profile in the chain of
profiles (but since a DeviceLink profile does't have a white point tag, it
is set to the D50 white point).

Then, since we are creating an input profile, cmsTransform2DeviceLink()
sets the mediaWhitePointTag of the profile to the EntryWhitePoint.



I understand that, in general, choosing the correct mediaWhitePoint for the
resulting input profile is non-trivial. But it would be nice if the simpler
cases, like the one presented here, could be handled correctly.



With kind regards,

Maurice