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ICC: Add Profile::to_lab()

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This can be used to convert a profile-dependent color to the L*a*b*
color space.

(I'd like to use this to implement the DeltaE (CIE 2000) algorithm,
which is a metric for how similar two colors are perceived.
(And I'd like to use that to evaluate color conversion roundtrip
quality, once I've implemented full conversions.)
This commit is contained in:
Nico Weber 2023-04-29 15:16:22 -04:00 • committed by Jelle Raaijmakers
parent 5fc11a316f
commit f3dbfb85d9
3 changed files with 110 additions and 0 deletions
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53
Userland/Libraries/LibGfx/ICC/Profile.cpp
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@ -1470,6 +1470,59 @@ ErrorOr<FloatVector3> Profile::to_pcs(ReadonlyBytes color)
VERIFY_NOT_REACHED();
}
ErrorOr<Profile::CIELAB> Profile::to_lab(ReadonlyBytes color)
{
auto pcs = TRY(to_pcs(color));
if (connection_space() == ColorSpace::PCSLAB)
return CIELAB { pcs[0], pcs[1], pcs[2] };
if (connection_space() != ColorSpace::PCSXYZ) {
VERIFY(device_class() == DeviceClass::DeviceLink);
return Error::from_string_literal("ICC::Profile::to_lab: conversion for DeviceLink not implemented");
}
// 6.3.2.2 Translation between media-relative colorimetric data and ICC-absolute colorimetric data
// 6.3.2.3 Computation of PCSLAB
// 6.3.4 Colour space encodings for the PCS
// A.3 PCS encodings
auto f = [](float x) {
if (x > powf(6.f / 29.f, 3))
return cbrtf(x);
return x / (3 * powf(6.f / 29.f, 2)) + 4.f / 29.f;
};
// "X/Xn is replaced by Xr/Xi (or Xa/Xmw)"
// 6.3.2.2 Translation between media-relative colorimetric data and ICC-absolute colorimetric data
// "The translation from ICC-absolute colorimetric data to media-relative colorimetry data is given by Equations
// Xr = (Xi/Xmw) * Xa
// where
// Xr media-relative colorimetric data (i.e. PCSXYZ);
// Xa ICC-absolute colorimetric data (i.e. nCIEXYZ);
// Xmw nCIEXYZ values of the media white point as specified in the mediaWhitePointTag;
// Xi PCSXYZ values of the PCS white point defined in 6.3.4.3."
// 6.3.4.3 PCS encodings for white and black
// "Table 14 — Encodings of PCS white point: X 0,9642 Y 1,0000 Z 0,8249"
// That's identical to the values in 7.2.16 PCS illuminant field (Bytes 68 to 79).
// 9.2.36 mediaWhitePointTag
// "For displays, the values specified shall be those of the PCS illuminant as defined in 7.2.16."
// ...so for displays, this is all equivalent I think? It's maybe different for OutputDevice profiles?
float Xn = pcs_illuminant().X;
float Yn = pcs_illuminant().Y;
float Zn = pcs_illuminant().Z;
float x = pcs[0] / Xn;
float y = pcs[1] / Yn;
float z = pcs[2] / Zn;
float L = 116 * f(y) - 16;
float a = 500 * (f(x) - f(y));
float b = 200 * (f(y) - f(z));
return CIELAB { L, a, b };
}
XYZ const& Profile::red_matrix_column() const { return xyz_data(redMatrixColumnTag); }
XYZ const& Profile::green_matrix_column() const { return xyz_data(greenMatrixColumnTag); }
XYZ const& Profile::blue_matrix_column() const { return xyz_data(blueMatrixColumnTag); }