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LibGfx/ICC: Improve XYZ coordinates of gray colors

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In XYZ space, gray doesn't have three equal values. Instead, it is
a line through the whitepoint.

(Multiplying by the whitepoint has the same effect as multiplying
the sRGB matrix with a (g, g, g) vector, since the numbers on
the matrix's rows add up to the whitepoint.)

Fixes the very slight red tint on all the figures in chapter 4
of the PDF 1.7 spec.
This commit is contained in:
Nico Weber 2023-12-30 23:42:47 -05:00 • committed by Andreas Kling
parent 726fa41d4a
commit bcb1e548f1
1 changed files with 2 additions and 1 deletions
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3
Userland/Libraries/LibGfx/ICC/Profile.cpp
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@ -1287,7 +1287,8 @@ ErrorOr<FloatVector3> Profile::to_pcs(ReadonlyBytes color) const
// ICC v4, F.2 grayTRCTag
// "connection = grayTRC[device]"
float gray = evaluate_curve(grayTRCTag, color[0] / 255.f);
return FloatVector3 { gray, gray, gray };
FloatVector3 white { pcs_illuminant().X, pcs_illuminant().Y, pcs_illuminant().Z };
return white * gray;
}
// FIXME: Per ICC v4, A.1 General, this should also handle HLS, HSV, YCbCr.