+#version 330
+
+in vec2 texCoords;
+out vec4 fragColor;
+
+const float PI = 3.14159265359;
+
+//TODO: Put this in a separate shader program
+float radicalInverse(uint bits) {
+ bits = (bits << 16u) | (bits >> 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);
+ return float(bits) * 2.3283064365386963e-10;
+}
+
+vec2 hammersley(uint i, uint N) {
+ return vec2(float(i) / float(N), radicalInverse(i));
+}
+
+float geometrySchlickGGX(float NdotV, float roughness) {
+ float a = roughness * roughness;
+ float k = (a * a) / 2.f;
+
+ return NdotV / (NdotV * (1.f - k) + k);
+}
+
+float geometrySmith(vec3 N, vec3 V, vec3 L, float roughness) {
+ float ggx1 = geometrySchlickGGX(max(dot(N, L), 0.f), roughness);
+ float ggx2 = geometrySchlickGGX(max(dot(N, V), 0.f), roughness);
+ return ggx1 * ggx2;
+}
+
+vec3 importanceSampleGGX(vec2 Xi, vec3 N, float roughness) {
+ float a = roughness * roughness;
+
+ float phi = 2.f * PI * Xi.x;
+ float cosTheta = sqrt((1.f - Xi.y) / (1.f + (a * a - 1.f) * Xi.y));
+ float sinTheta = sqrt(1.f - cosTheta * cosTheta);
+
+ // spherical -> cartesian
+ vec3 H = vec3(cos(phi) * sinTheta, sin(phi) * sinTheta, cosTheta);
+
+ vec3 up = abs(N.z) < 0.999 ? vec3(0, 0, 1) : vec3(1, 0, 0);
+ vec3 tangent = normalize(cross(up, N));
+ vec3 bitangent = cross(N, tangent);
+
+ vec3 sampleVec = tangent * H.x + bitangent * H.y + N * H.z;
+ return normalize(sampleVec);
+}
+
+vec2 integrateBRDF(float NdotV, float roughness) {
+ vec3 V = vec3(sqrt(1.f - NdotV * NdotV), 0.f, NdotV);
+
+ float A = 0, B = 0;
+
+ vec3 N = vec3(0, 0, 1);
+
+ const uint sampleCount = 1024u;
+ for (uint i = 0u; i < sampleCount; i++) {
+ vec2 Xi = hammersley(i, sampleCount);
+ vec3 H = importanceSampleGGX(Xi, N, roughness);
+ vec3 L = normalize(2.f * dot(V, H) * H - V);
+
+ float NdotL = max(L.z, 0.f);
+ float NdotH = max(H.z, 0.f);
+ float VdotH = max(dot(V, H), 0.f);
+
+ if (NdotL > 0) {
+ float G = geometrySmith(N, V, L, roughness);
+ float Gvis = (G * VdotH) / (NdotH * NdotV);
+ float Fc = pow(1.f - VdotH, 5.f);
+
+ A += (1.f - Fc) * Gvis;
+ B += Fc * Gvis;
+ }
+ }
+
+ A /= float(sampleCount);
+ B /= float(sampleCount);
+
+ return vec2(A, B);
+}
+
+void main() {
+ fragColor = vec4(integrateBRDF(texCoords.x, texCoords.y), 0, 0);
+}