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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/>.
- */
-
- #ifndef ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP
- #define ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP
-
- #include "math/constants.hpp"
- #include <cmath>
-
- namespace physics {
- namespace light {
-
- /// Light-scattering phase functions.
- namespace phase {
-
- /**
- * Cornette-Shanks phase function.
- *
- * @param mu Cosine of the angle between the light and view directions.
- * @param g Asymmetry factor, on [-1, 1]. Positive values cause forward scattering, negative values cause back scattering.
- */
- template <class T>
- T cornette_shanks(T mu, T g);
-
- /**
- * Henyey–Greenstein phase function.
- *
- * @param mu Cosine of the angle between the light and view directions.
- * @param g Asymmetry factor, on `[-1, 1]`. Positive values cause forward scattering, negative values cause back scattering.
- *
- * @see http://www.pbr-book.org/3ed-2018/Volume_Scattering/Phase_Functions.html
- */
- template <class T>
- T henyey_greenstein(T mu, T g);
-
- /**
- * Isotropic phase function. Causes light to be scattered uniformly in all directions.
- */
- template <class T>
- constexpr T isotropic()
- {
- return T(1) / (T(4) * math::pi<T>);
- }
-
- /**
- * Rayleigh phase function.
- *
- * @param mu Cosine of the angle between the light and view directions.
- */
- template <class T>
- T rayleigh(T mu);
-
- template <class T>
- T cornette_shanks(T mu, T g)
- {
- static const T k = T(3) / (T(8) * math::pi<T>);
- const T gg = g * g;
- const T num = (T(1) - gg) * (T(1) + mu * mu);
- const T den = (T(2) + gg) * std::pow(T(1) + gg - T(2) * g * mu, T(1.5));
- return k * num / den;
- }
-
- template <class T>
- T henyey_greenstein(T mu, T g)
- {
- const T gg = g * g;
- return (T(1) - gg) / (T(4) * math::pi<T> * std::pow(T(1) + gg - T(2) * g * mu, T(1.5)));
- }
-
- template <class T>
- T rayleigh(T mu)
- {
- static const T k = T(3) / (T(16) * math::pi<T>);
- return k * (1.0 + mu * mu);
- }
-
- } // namespace phase
-
- } // namespace light
- } // namespace physics
-
- #endif // ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP
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