Review BRDF texture generation

Actually, that function should be redesigned...

1 files changed, 33 insertions, 40 deletions

diff --git a/examples/models/resources/shaders/brdf.fs b/examples/models/resources/shaders/brdf.fs
index 59ae384a..3e8777d2 100644
--- a/examples/models/resources/shaders/brdf.fs
+++ b/examples/models/resources/shaders/brdf.fs
@@ -1,12 +1,15 @@
 /*******************************************************************************************
 *
-*   rPBR [shader] - Bidirectional reflectance distribution function fragment shader
+*   BRDF LUT Generation - Bidirectional reflectance distribution function fragment shader
+*
+*   REF: https://github.com/HectorMF/BRDFGenerator
 *
 *   Copyright (c) 2017 Victor Fisac
 *
 **********************************************************************************************/
 
 #version 330
+
 #define         MAX_SAMPLES        1024u
 
 // Input vertex attributes (from vertex shader)
@@ -18,43 +21,30 @@ const float PI = 3.14159265359;
 // Output fragment color
 out vec4 finalColor;
 
-float DistributionGGX(vec3 N, vec3 H, float roughness);
-float RadicalInverse_VdC(uint bits);
 vec2 Hammersley(uint i, uint N);
-vec3 ImportanceSampleGGX(vec2 Xi, vec3 N, float roughness);
+float RadicalInverseVdC(uint bits);
 float GeometrySchlickGGX(float NdotV, float roughness);
 float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness);
+vec3 ImportanceSampleGGX(vec2 Xi, vec3 N, float roughness);
 vec2 IntegrateBRDF(float NdotV, float roughness);
 
-float DistributionGGX(vec3 N, vec3 H, float roughness)
-{
-    float a = roughness*roughness;
-    float a2 = a*a;
-    float NdotH = max(dot(N, H), 0.0);
-    float NdotH2 = NdotH*NdotH;
-
-    float nom   = a2;
-    float denom = (NdotH2*(a2 - 1.0) + 1.0);
-    denom = PI*denom*denom;
-
-    return nom/denom;
-}
-
-float RadicalInverse_VdC(uint bits)
+float RadicalInverseVdC(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; // / 0x100000000
+    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; // / 0x100000000
 }
 
+// Compute Hammersley coordinates
 vec2 Hammersley(uint i, uint N)
 {
-	return vec2(float(i)/float(N), RadicalInverse_VdC(i));
+	return vec2(float(i)/float(N), RadicalInverseVdC(i));
 }
 
+// Integrate number of importance samples for (roughness and NoV)
 vec3 ImportanceSampleGGX(vec2 Xi, vec3 N, float roughness)
 {
 	float a = roughness*roughness;
@@ -85,6 +75,7 @@ float GeometrySchlickGGX(float NdotV, float roughness)
     return nom/denom;
 }
 
+// Compute the geometry term for the BRDF given roughness squared, NoV, NoL
 float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
 {
     float NdotV = max(dot(N, V), 0.0);
@@ -97,29 +88,31 @@ float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
 
 vec2 IntegrateBRDF(float NdotV, float roughness)
 {
-    vec3 V = vec3(sqrt(1.0 - NdotV*NdotV), 0.0, NdotV);
     float A = 0.0;
-    float B = 0.0; 
+    float B = 0.0;
+    vec3 V = vec3(sqrt(1.0 - NdotV*NdotV), 0.0, NdotV);
     vec3 N = vec3(0.0, 0.0, 1.0);
 
-    for(uint i = 0u; i < MAX_SAMPLES; i++)
+    for (int i = 0; i < MAX_SAMPLES; i++)
     {
         // Generate a sample vector that's biased towards the preferred alignment direction (importance sampling)
-        vec2 Xi = Hammersley(i, MAX_SAMPLES);
-        vec3 H = ImportanceSampleGGX(Xi, N, roughness);
-        vec3 L = normalize(2.0*dot(V, H)*H - V);
-        float NdotL = max(L.z, 0.0);
-        float NdotH = max(H.z, 0.0);
-        float VdotH = max(dot(V, H), 0.0);
+        
+        vec2 Xi = Hammersley(i, MAX_SAMPLES);       // Compute a Hammersely coordinate
+        vec3 H = ImportanceSampleGGX(Xi, N, roughness); // Integrate number of importance samples for (roughness and NoV)
+        vec3 L = normalize(2.0*dot(V, H)*H - V);    // Compute reflection vector L
+        
+        float NdotL = max(L.z, 0.0);                // Compute normal dot light
+        float NdotH = max(H.z, 0.0);                // Compute normal dot half
+        float VdotH = max(dot(V, H), 0.0);          // Compute view dot half
 
         if (NdotL > 0.0)
         {
-            float G = GeometrySmith(N, V, L, roughness);
-            float G_Vis = (G*VdotH)/(NdotH*NdotV);
-            float Fc = pow(1.0 - VdotH, 5.0);
+            float G = GeometrySmith(N, V, L, roughness);    // Compute the geometry term for the BRDF given roughness squared, NoV, NoL
+            float GVis = (G*VdotH)/(NdotH*NdotV);   // Compute the visibility term given G, VoH, NoH, NoV, NoL
+            float Fc = pow(1.0 - VdotH, 5.0);       // Compute the fresnel term given VoH
 
-            A += (1.0 - Fc)*G_Vis;
-            B += Fc*G_Vis;
+            A += (1.0 - Fc)*GVis;                   // Sum the result given fresnel, geometry, visibility
+            B += Fc*GVis;
         }
     }