Optimize DisneyDiffuse()

ScriptableRenderPipeline/Core/ShaderLibrary/BSDF.hlsl

     return INV_PI;
 }
 
-float DisneyDiffuseNoPI(float NdotV, float NdotL, float LdotH, float perceptualRoughness)
+float DisneyDiffuseNoPI(float NdotV, float NdotL, float LdotV, float perceptualRoughness)
-    float fd90 = 0.5 + 2 * LdotH * LdotH * perceptualRoughness;
+    // (2 * LdotH * LdotH) = 1 + LdotV
+    // float fd90 = 0.5 + 2 * LdotH * LdotH * perceptualRoughness;
+    float fd90 = 0.5 + (perceptualRoughness + perceptualRoughness * LdotV);
-    float viewScatter = F_Schlick(1.0, fd90, NdotV);
+    float viewScatter  = F_Schlick(1.0, fd90, NdotV);
-float DisneyDiffuse(float NdotV, float NdotL, float LdotH, float perceptualRoughness)
+float DisneyDiffuse(float NdotV, float NdotL, float LdotV, float perceptualRoughness)
-    return INV_PI * DisneyDiffuseNoPI(NdotV, NdotL, LdotH, perceptualRoughness);
+    return INV_PI * DisneyDiffuseNoPI(NdotV, NdotL, LdotV, perceptualRoughness);
-    float facing    = 0.5 + 0.5 * LdotV;
+    float facing    = 0.5 + 0.5 * LdotV;                        // (LdotH)^2
-    // This constant is picked s.t. setting perceptualRoughness, albedo and all angles to 1
+    // This constant is picked s.t. setting perceptualRoughness, albedo and all cosines to 1
     // allows us to match the Lambertian and the Disney Diffuse models. Original value: 0.1159.
     float multiple  = perceptualRoughness * (0.079577 * PI);    // Rescaled by PI
 

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 #elif LIT_DIFFUSE_GGX_BRDF
     float3 diffuseTerm = DiffuseGGX(bsdfData.diffuseColor, NdotV, NdotL, NdotH, LdotV, bsdfData.perceptualRoughness);
 #else
-    float  diffuseTerm = DisneyDiffuse(NdotV, NdotL, LdotH, bsdfData.perceptualRoughness);
+    float  diffuseTerm = DisneyDiffuse(NdotV, NdotL, LdotV, bsdfData.perceptualRoughness);
 #endif
 
     diffuseLighting = bsdfData.diffuseColor * diffuseTerm;