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- /*
- * Copyright (C) 2021 Christopher J. Howard
- *
- * This file is part of Antkeeper source code.
- *
- * Antkeeper source code is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * Antkeeper source code is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with Antkeeper source code. If not, see <http://www.gnu.org/licenses/>.
- */
-
- #include "blackbody.hpp"
- #include <algorithm>
-
- namespace ast
- {
-
- /// Transforms colors from CIE XYZ to linear RGB
- static constexpr double3x3 xyz_to_rgb =
- {
- 3.2404542, -0.9692660, 0.0556434,
- -1.5371385, 1.8760108, -0.2040259,
- -0.4985314, 0.0415560, 1.0572252
- };
-
- double3 blackbody(double t)
- {
- // Approximate the Planckian locus in CIE 1960 UCS color space (Krystek's algorithm)
- double tt = t * t;
- double u = (0.860117757 + 1.54118254e-4 * t + 1.28641212e-7 * tt) / (1.0 + 8.42420235e-4 * t + 7.08145163e-7 * tt);
- double v = (0.317398726 + 4.22806245e-5 * t + 4.20481691e-8 * tt) / (1.0 - 2.89741816e-5 * t + 1.61456053e-7 * tt);
-
- // CIE 1960 UCS -> CIE xyY, Y = 1
- double2 xyy = double2{3.0 * u, 2.0 * v} / (2.0 * u - 8.0 * v + 4.0);
-
- // CIE xyY -> CIE XYZ
- double3 xyz = {xyy.x / xyy.y, 1.0, (1.0 - xyy.x - xyy.y) / xyy.y};
-
- // CIE XYZ -> linear RGB
- double3 rgb = xyz_to_rgb * xyz;
-
- // Normalize RGB to preserve chromaticity
- return rgb / std::max<double>(rgb.x, std::max<double>(rgb.y, rgb.z));
- }
-
- } // namespace ast
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