Optimize IntegrateLD() by using D_GGX_Inverse()

Assets/ScriptableRenderLoop/ShaderLibrary/BSDF.hlsl

     return INV_PI * D_GGXNoPI(NdotH, roughness);
 }
 
+float D_GGX_Inverse(float NdotH, float roughness)
+{
+    roughness = max(roughness, UNITY_MIN_ROUGHNESS);
+
+    float a2 = roughness * roughness;
+    float f  = (NdotH * a2 - NdotH) * NdotH + 1.0;
+    float g  = (f * f) / a2;
+
+    return PI * g;
+}
+
 // Ref: http://jcgt.org/published/0003/02/03/paper.pdf
 float V_SmithJointGGX(float NdotL, float NdotV, float roughness)
 {

Assets/ScriptableRenderLoop/ShaderLibrary/ImageBasedLighting.hlsl

             // - OmegaS : Solid angle associated to a sample
             // - OmegaP : Solid angle associated to a pixel of the cubemap
 
-            float pdf       = D_GGX(NdotH, roughness) * 0.25;
-            float omegaS    = 1.0 / (sampleCount * pdf);                      // Solid angle associated with the sample
+            float invPdf    = D_GGX_Inverse(NdotH, roughness) * 4.0;
+            float omegaS    = rcp(sampleCount) * invPdf;                      // Solid angle associated with the sample
             // invOmegaP is precomputed on CPU and provide as a parameter of the function
             // float omegaP = FOUR_PI / (6.0f * cubemapWidth * cubemapWidth); // Solid angle associated with the pixel of the cubemap
             mipLevel        = 0.5 * log2(omegaS * invOmegaP) + 1.0;           // Clamp is not necessary as the hardware will do it