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+/**
+ * @file class1/deferred/genbrdflut.glsl
+ *
+ * $LicenseInfo:firstyear=2022&license=viewerlgpl$
+ * Second Life Viewer Source Code
+ * Copyright (C) 2022, Linden Research, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation;
+ * version 2.1 of the License only.
+ *
+ * This library 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
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
+ * $/LicenseInfo$
+ */
+
+/* Taken from Sascha Willem's Vulkan GLTF refernce implementation
+MIT License
+
+Copyright (c) 2018 Sascha Willems
+
+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.
+*/
+
+/*[EXTRA_CODE_HERE]*/
+
+in vec2 vary_uv;
+
+out vec4 outColor;
+
+#define NUM_SAMPLES 1024u
+
+const float PI = 3.1415926536;
+
+// Based omn http://byteblacksmith.com/improvements-to-the-canonical-one-liner-glsl-rand-for-opengl-es-2-0/
+float random(vec2 co)
+{
+ float a = 12.9898;
+ float b = 78.233;
+ float c = 43758.5453;
+ float dt= dot(co.xy ,vec2(a,b));
+ float sn= mod(dt,3.14);
+ return fract(sin(sn) * c);
+}
+
+vec2 hammersley2d(uint i, uint N)
+{
+ // Radical inverse based on http://holger.dammertz.org/stuff/notes_HammersleyOnHemisphere.html
+ uint bits = (i << 16u) | (i >> 16u);
+ bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
+ bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
+ bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
+ bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
+ float rdi = float(bits) * 2.3283064365386963e-10;
+ return vec2(float(i) /float(N), rdi);
+}
+
+// Based on http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_slides.pdf
+vec3 importanceSample_GGX(vec2 Xi, float roughness, vec3 normal)
+{
+ // Maps a 2D point to a hemisphere with spread based on roughness
+ float alpha = roughness * roughness;
+ float phi = 2.0 * PI * Xi.x + random(normal.xz) * 0.1;
+ float cosTheta = sqrt((1.0 - Xi.y) / (1.0 + (alpha*alpha - 1.0) * Xi.y));
+ float sinTheta = sqrt(1.0 - cosTheta * cosTheta);
+ vec3 H = vec3(sinTheta * cos(phi), sinTheta * sin(phi), cosTheta);
+
+ // Tangent space
+ vec3 up = abs(normal.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
+ vec3 tangentX = normalize(cross(up, normal));
+ vec3 tangentY = normalize(cross(normal, tangentX));
+
+ // Convert to world Space
+ return normalize(tangentX * H.x + tangentY * H.y + normal * H.z);
+}
+
+// Geometric Shadowing function
+float G_SchlicksmithGGX(float dotNL, float dotNV, float roughness)
+{
+ float k = (roughness * roughness) / 2.0;
+ float GL = dotNL / (dotNL * (1.0 - k) + k);
+ float GV = dotNV / (dotNV * (1.0 - k) + k);
+ return GL * GV;
+}
+
+vec2 BRDF(float NoV, float roughness)
+{
+ // Normal always points along z-axis for the 2D lookup
+ const vec3 N = vec3(0.0, 0.0, 1.0);
+ vec3 V = vec3(sqrt(1.0 - NoV*NoV), 0.0, NoV);
+
+ vec2 LUT = vec2(0.0);
+ for(uint i = 0u; i < NUM_SAMPLES; i++) {
+ vec2 Xi = hammersley2d(i, NUM_SAMPLES);
+ vec3 H = importanceSample_GGX(Xi, roughness, N);
+ vec3 L = 2.0 * dot(V, H) * H - V;
+
+ float dotNL = max(dot(N, L), 0.0);
+ float dotNV = max(dot(N, V), 0.0);
+ float dotVH = max(dot(V, H), 0.0);
+ float dotNH = max(dot(H, N), 0.0);
+
+ if (dotNL > 0.0) {
+ float G = G_SchlicksmithGGX(dotNL, dotNV, roughness);
+ float G_Vis = (G * dotVH) / (dotNH * dotNV);
+ float Fc = pow(1.0 - dotVH, 5.0);
+ LUT += vec2((1.0 - Fc) * G_Vis, Fc * G_Vis);
+ }
+ }
+ return LUT / float(NUM_SAMPLES);
+}
+
+void main()
+{
+ outColor = vec4(BRDF(vary_uv.s, 1.0-vary_uv.t), 0.0, 1.0);
+}