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91 lines
3.0 KiB
C++
91 lines
3.0 KiB
C++
/*
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* Copyright (c) 2023, Nico Weber <thakis@chromium.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Format.h>
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#include <AK/Math.h>
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#include <LibGfx/DeltaE.h>
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#include <math.h>
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namespace Gfx {
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float DeltaE(CIELAB const& c1, CIELAB const& c2)
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{
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// https://en.wikipedia.org/wiki/Color_difference#CIEDE2000
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// http://zschuessler.github.io/DeltaE/learn/
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// https://www.hajim.rochester.edu/ece/sites/gsharma/ciede2000/ciede2000noteCRNA.pdf
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float delta_L_prime = c2.L - c1.L;
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float L_bar = (c1.L + c2.L) / 2;
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float C1 = hypotf(c1.a, c1.b);
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float C2 = hypotf(c2.a, c2.b);
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float C_bar = (C1 + C2) / 2;
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float G = 0.5f * (1 - sqrtf(powf(C_bar, 7) / (powf(C_bar, 7) + powf(25, 7))));
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float a1_prime = (1 + G) * c1.a;
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float a2_prime = (1 + G) * c2.a;
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float C1_prime = hypotf(a1_prime, c1.b);
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float C2_prime = hypotf(a2_prime, c2.b);
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float C_prime_bar = (C1_prime + C2_prime) / 2;
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float delta_C_prime = C2_prime - C1_prime;
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auto h_prime = [](float b, float a_prime) {
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if (b == 0 && a_prime == 0)
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return 0.f;
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float h_prime = atan2(b, a_prime);
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if (h_prime < 0)
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h_prime += 2 * static_cast<float>(M_PI);
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return AK::to_degrees(h_prime);
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};
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float h1_prime = h_prime(c1.b, a1_prime);
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float h2_prime = h_prime(c2.b, a2_prime);
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float delta_h_prime;
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if (C1_prime == 0 || C2_prime == 0)
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delta_h_prime = 0;
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else if (fabsf(h1_prime - h2_prime) <= 180.f)
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delta_h_prime = h2_prime - h1_prime;
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else if (h2_prime <= h1_prime)
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delta_h_prime = h2_prime - h1_prime + 360;
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else
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delta_h_prime = h2_prime - h1_prime - 360;
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auto sin_degrees = [](float x) { return sinf(AK::to_radians(x)); };
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auto cos_degrees = [](float x) { return cosf(AK::to_radians(x)); };
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float delta_H_prime = 2 * sqrtf(C1_prime * C2_prime) * sin_degrees(delta_h_prime / 2);
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float h_prime_bar;
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if (C1_prime == 0 || C2_prime == 0)
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h_prime_bar = h1_prime + h2_prime;
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else if (fabsf(h1_prime - h2_prime) <= 180.f)
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h_prime_bar = (h1_prime + h2_prime) / 2;
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else if (h1_prime + h2_prime < 360)
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h_prime_bar = (h1_prime + h2_prime + 360) / 2;
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else
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h_prime_bar = (h1_prime + h2_prime - 360) / 2;
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float T = 1 - 0.17f * cos_degrees(h_prime_bar - 30) + 0.24f * cos_degrees(2 * h_prime_bar) + 0.32f * cos_degrees(3 * h_prime_bar + 6) - 0.2f * cos_degrees(4 * h_prime_bar - 63);
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float S_L = 1 + 0.015f * powf(L_bar - 50, 2) / sqrtf(20 + powf(L_bar - 50, 2));
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float S_C = 1 + 0.045f * C_prime_bar;
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float S_H = 1 + 0.015f * C_prime_bar * T;
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float R_T = -2 * sqrtf(powf(C_prime_bar, 7) / (powf(C_prime_bar, 7) + powf(25, 7))) * sin_degrees(60 * exp(-powf((h_prime_bar - 275) / 25, 2)));
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// "kL, kC, and kH are usually unity."
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float k_L = 1, k_C = 1, k_H = 1;
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float L = delta_L_prime / (k_L * S_L);
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float C = delta_C_prime / (k_C * S_C);
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float H = delta_H_prime / (k_H * S_H);
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return sqrtf(powf(L, 2) + powf(C, 2) + powf(H, 2) + R_T * C * H);
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}
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}
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