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+#ifndef __LYSENKO_INC
+#define __LYSENKO_INC
+
+/*
+The MIT License (MIT)
+
+Copyright (c) 2014 Mikola Lysenko
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+ */
+float Fd_OrenNayar(float roughness, float NoV, float NoL, float LoV) {
+  float s = LoV - NoL * NoV;
+  float t = lerp(1.0f, max(NoL, NoV), step(0.0f, s));
+  float sigma2 = roughness * roughness;
+  float A = 1.0f + sigma2 * rcp(sigma2 + 0.13f) + 0.5f / (sigma2 + 0.33f);
+  float B = 0.45f * sigma2 / (sigma2 + 0.09f);
+
+  return max(0.0f, NoL * (A + B * s / t));
+}
+
+#endif  // __LYSENKO_INC
+