💿🐜 Antkeeper source code https://antkeeper.com
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  1. /*
  2. * Copyright (C) 2021 Christopher J. Howard
  3. *
  4. * This file is part of Antkeeper source code.
  5. *
  6. * Antkeeper source code is free software: you can redistribute it and/or modify
  7. * it under the terms of the GNU General Public License as published by
  8. * the Free Software Foundation, either version 3 of the License, or
  9. * (at your option) any later version.
  10. *
  11. * Antkeeper source code is distributed in the hope that it will be useful,
  12. * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13. * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14. * GNU General Public License for more details.
  15. *
  16. * You should have received a copy of the GNU General Public License
  17. * along with Antkeeper source code. If not, see <http://www.gnu.org/licenses/>.
  18. */
  19. #ifndef ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP
  20. #define ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP
  21. #include "math/constants.hpp"
  22. #include <cmath>
  23. namespace physics {
  24. namespace light {
  25. /// Light-scattering phase functions.
  26. namespace phase {
  27. /**
  28. * Cornette-Shanks phase function.
  29. *
  30. * @param mu Cosine of the angle between the light and view directions.
  31. * @param g Asymmetry factor, on [-1, 1]. Positive values cause forward scattering, negative values cause back scattering.
  32. */
  33. template <class T>
  34. T cornette_shanks(T mu, T g);
  35. /**
  36. * HenyeyGreenstein phase function.
  37. *
  38. * @param mu Cosine of the angle between the light and view directions.
  39. * @param g Asymmetry factor, on `[-1, 1]`. Positive values cause forward scattering, negative values cause back scattering.
  40. *
  41. * @see http://www.pbr-book.org/3ed-2018/Volume_Scattering/Phase_Functions.html
  42. */
  43. template <class T>
  44. T henyey_greenstein(T mu, T g);
  45. /**
  46. * Isotropic phase function. Causes light to be scattered uniformly in all directions.
  47. */
  48. template <class T>
  49. constexpr T isotropic()
  50. {
  51. return T(1) / (T(4) * math::pi<T>);
  52. }
  53. /**
  54. * Rayleigh phase function.
  55. *
  56. * @param mu Cosine of the angle between the light and view directions.
  57. */
  58. template <class T>
  59. T rayleigh(T mu);
  60. template <class T>
  61. T cornette_shanks(T mu, T g)
  62. {
  63. static const T k = T(3) / (T(8) * math::pi<T>);
  64. const T gg = g * g;
  65. const T num = (T(1) - gg) * (T(1) + mu * mu);
  66. const T den = (T(2) + gg) * std::pow(T(1) + gg - T(2) * g * mu, T(1.5));
  67. return k * num / den;
  68. }
  69. template <class T>
  70. T henyey_greenstein(T mu, T g)
  71. {
  72. const T gg = g * g;
  73. return (T(1) - gg) / (T(4) * math::pi<T> * std::pow(T(1) + gg - T(2) * g * mu, T(1.5)));
  74. }
  75. template <class T>
  76. T rayleigh(T mu)
  77. {
  78. static const T k = T(3) / (T(16) * math::pi<T>);
  79. return k * (1.0 + mu * mu);
  80. }
  81. } // namespace phase
  82. } // namespace light
  83. } // namespace physics
  84. #endif // ANTKEEPER_PHYSICS_LIGHT_PHASE_HPP