Implement ImportanceSampleGGXDirIsotropic()

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

     L = 2.0 * dot(V, H) * H - V;
 }
 
+
+// Special case where N == V.
+void ImportanceSampleGGXDirIsotropic(float2 u,
+                                     float3 V,
+                                     float3 N,
+                                     float3 tangentX,
+                                     float3 tangentY,
+                                     float roughness,
+                                     out float3 L,
+                                     out float  NdotH)
+{
+    // GGX NDF sampling
+    float cosThetaH = sqrt((1.0 - u.x) / (1.0 + (roughness * roughness - 1.0) * u.x));
+    float sinThetaH = sqrt(saturate(1.0 - cosThetaH * cosThetaH));
+    float phiH      = TWO_PI * u.y;
+
+    // Transform from spherical into Cartesian.
+    float3 localH = float3(sinThetaH * cos(phiH), sinThetaH * sin(phiH), cosThetaH);
+
+    // localN == localV == float3(0.0, 0.0, 1.0).
+    NdotH = localH.z;
+
+    // Compute { L = reflect(-localV, H) }.
+    float VdotH = NdotH;
+    L = float3(0.0, 0.0, -1.0) + 2.0 * VdotH * localH;
+
+    // Local to world
+    L = tangentX * L.x + tangentY * L.y + N * L.z;
+}
+
 // ref: http://blog.selfshadow.com/publications/s2012-shading-course/burley/s2012_pbs_disney_brdf_notes_v3.pdf p26
 void ImportanceSampleAnisoGGXDir(   float2 u,
                                     float3 V,
                     uint sampleCount,      // Matches the size of the precomputed Fibonacci point set
                     bool prefilter = true) // static bool
 {
-    float3 acc          = float3(0.0, 0.0, 0.0);
-    float  accWeight    = 0;
+    float3 acc       = float3(0.0, 0.0, 0.0);
+    float  accWeight = 0;
 
     float2 randNum  = InitRandom(V.xy * 0.5 + 0.5);
 
     for (uint i = 0; i < sampleCount; ++i)
     {
-        float2 u    = k_Fibonacci2dSeq55[i];
-        u           = frac(u + randNum);
+        float2 u = k_Fibonacci2dSeq55[i];
+        u        = frac(u + randNum);
-        float3 H;
-        ImportanceSampleGGXDir(u, V, N, tangentX, tangentY, roughness, H, L);
+        float  NdotH;
+        ImportanceSampleGGXDirIsotropic(u, V, N, tangentX, tangentY, roughness, L, NdotH);
-        float NdotL = saturate(dot(N,L));
+        float NdotL = saturate(dot(N, L));
 
         float mipLevel;
 
             // - OmegaS : Solid angle associated to a sample
             // - OmegaP : Solid angle associated to a pixel of the cubemap
 
-            float NdotH     = saturate(dot(N, H));
             float pdf       = D_GGX(NdotH, roughness) * 0.25;
             float omegaS    = 1.0 / (sampleCount * pdf);                      // Solid angle associated with the sample
             // invOmegaP is precomputed on CPU and provide as a parameter of the function