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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
Notes: sideshowbarker 2024-07-16 22:24:48 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/f3dbfb85d9
Pull-request: https://github.com/SerenityOS/serenity/pull/18578
3 changed files with 110 additions and 0 deletions
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50
Tests/LibGfx/TestICCProfile.cpp
View file

@ -153,3 +153,53 @@ TEST_CASE(to_pcs)
float f192 = sRGB_curve.evaluate(192 / 255.f);
EXPECT_APPROXIMATE_VECTOR3(xyz_from_sRGB(64, 128, 192), r_xyz * f64 + g_xyz * f128 + b_xyz * f192);
}
TEST_CASE(to_lab)
{
auto sRGB = MUST(Gfx::ICC::sRGB());
auto lab_from_sRGB = [sRGB](u8 r, u8 g, u8 b) {
u8 rgb[3] = { r, g, b };
return MUST(sRGB->to_lab(rgb));
};
// The `expected` numbers are from https://colorjs.io/notebook/ for this snippet of code:
// new Color("srgb", [0, 0, 0]).lab.toString();
//
// new Color("srgb", [1, 0, 0]).lab.toString();
// new Color("srgb", [0, 1, 0]).lab.toString();
// new Color("srgb", [0, 0, 1]).lab.toString();
//
// new Color("srgb", [1, 1, 0]).lab.toString();
// new Color("srgb", [1, 0, 1]).lab.toString();
// new Color("srgb", [0, 1, 1]).lab.toString();
//
// new Color("srgb", [1, 1, 1]).lab.toString();
Gfx::ICC::Profile::CIELAB expected[] = {
{ 0, 0, 0 },
{ 54.29054294696968, 80.80492033462421, 69.89098825896275 },
{ 87.81853633115202, -79.27108223854806, 80.99459785152247 },
{ 29.56829715344471, 68.28740665215547, -112.02971798617645 },
{ 97.60701009682253, -15.749846639252663, 93.39361164266089 },
{ 60.16894098715946, 93.53959546199253, -60.50080231921204 },
{ 90.66601315791455, -50.65651077286893, -14.961666625736525 },
{ 100.00000139649632, -0.000007807961277528364, 0.000006766250648659877 },
};
// We're off by more than the default EXPECT_APPROXIMATE() error, so use EXPECT_APPROXIMATE_WITH_ERROR().
// The difference is not too bad: ranges for L*, a*, b* are [0, 100], [-125, 125], [-125, 125],
// so this is an error of considerably less than 0.1 for u8 channels.
#define EXPECT_APPROXIMATE_LAB(l1, l2) \
EXPECT_APPROXIMATE_WITH_ERROR((l1).L, (l2).L, 0.01); \
EXPECT_APPROXIMATE_WITH_ERROR((l1).a, (l2).a, 0.03); \
EXPECT_APPROXIMATE_WITH_ERROR((l1).b, (l2).b, 0.02);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(0, 0, 0), expected[0]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(255, 0, 0), expected[1]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(0, 255, 0), expected[2]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(0, 0, 255), expected[3]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(255, 255, 0), expected[4]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(255, 0, 255), expected[5]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(0, 255, 255), expected[6]);
EXPECT_APPROXIMATE_LAB(lab_from_sRGB(255, 255, 255), expected[7]);
}

53
Userland/Libraries/LibGfx/ICC/Profile.cpp
View file

@ -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); }

7
Userland/Libraries/LibGfx/ICC/Profile.h
View file

@ -266,6 +266,13 @@ public:
// Call connection_space() to find out the space the result is in.
ErrorOr<FloatVector3> to_pcs(ReadonlyBytes);
struct CIELAB {
float L; // L*
float a; // a*
float b; // b*
};
ErrorOr<CIELAB> to_lab(ReadonlyBytes);
// Only call these if you know that this is an RGB matrix-based profile.
XYZ const& red_matrix_column() const;
XYZ const& green_matrix_column() const;