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LibGfx/ICC: Make mAB tags work for profiles that use PCSLAB

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ICC profiles work by transforming from the input color space
(one of many: RGB, CMYK, YUV, etc) to a "profile connection space" (PCS)
and then from there to the output color space.

However, there's not one but two possible profile connection spaces,
PCSXYZ and PCSLAB. The matrix/curve tags can only be used with PCSXYZ,
but the mAB, mBA, mft1, mft2 tags can be used with PCSLAB as well.

The PCSLAB encoding has L going from 0 to 100 and ab from -128 to 127,
instead of from 0 to 1. So they need to be scaled up at the end.

That's also the reason for the "mystery conversion factor": PCSXYZ
doesn't go from 0 to 1 either, but from 0 to 65535/32768, per ICC v4
6.3.4.2 General PCS encoding, Table 11 - PCSXYZ X, Y or Z encoding.

Between input and output are various curves (and the CLUT) that
have domain and range of 0..1. For these, the color has to be linearly
scaled to 0..1 before the curve and back to the actual range after
the curve. Doing that back-to-back is a no-op, so scaling back at
the very end is sufficient.
This commit is contained in:
Nico Weber 2023-12-03 21:03:56 -05:00 • committed by Sam Atkins
parent 64ffae9c55
commit fda9840f48
2 changed files with 29 additions and 17 deletions
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2
Userland/Libraries/LibGfx/ICC/Profile.cpp
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@ -1227,7 +1227,7 @@ ErrorOr<FloatVector3> Profile::to_pcs_a_to_b(TagData const& tag_data, ReadonlyBy
if (a_to_b.number_of_output_channels() != number_of_components_in_color_space(connection_space()))
return Error::from_string_literal("ICC::Profile::to_pcs_a_to_b: mAB output channel count does not match profile connection space size");
return a_to_b.evaluate(color);
return a_to_b.evaluate(connection_space(), color);
}
}
VERIFY_NOT_REACHED();