Update the implementation of DiffuseGGX()

Assets/ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

         bsdfData.roughnessB = ClampRoughnessForAnalyticalLights(bsdfData.roughnessB);
 
         #ifdef LIT_USE_BSDF_PRE_LAMBDAV
-        Vis = V_SmithJointGGXAnisoLambdaV(  preLightData.TdotV, preLightData.BdotV, preLightData.NdotV, TdotL, BdotL, NdotL,
+        Vis = V_SmithJointGGXAnisoLambdaV(  preLightData.TdotV, preLightData.BdotV, NdotV, TdotL, BdotL, NdotL,
-        Vis = V_SmithJointGGXAniso( preLightData.TdotV, preLightData.BdotV, preLightData.NdotV, TdotL, BdotL, NdotL,
+        Vis = V_SmithJointGGXAniso( preLightData.TdotV, preLightData.BdotV, NdotV, TdotL, BdotL, NdotL,
                                     bsdfData.roughnessT, bsdfData.roughnessB);
         #endif
 
         bsdfData.roughness = ClampRoughnessForAnalyticalLights(bsdfData.roughness);
 
         #ifdef LIT_USE_BSDF_PRE_LAMBDAV
-        Vis = V_SmithJointGGX(NdotL, preLightData.NdotV, bsdfData.roughness, preLightData.ggxLambdaV);
+        Vis = V_SmithJointGGX(NdotL, NdotV, bsdfData.roughness, preLightData.ggxLambdaV);
-        Vis = V_SmithJointGGX(NdotL, preLightData.NdotV, bsdfData.roughness);
+        Vis = V_SmithJointGGX(NdotL, NdotV, bsdfData.roughness);
-    #ifdef LIT_DIFFUSE_LAMBERT_BRDF
-    float diffuseTerm = Lambert();
-    #else
-    float diffuseTerm = DisneyDiffuse(preLightData.NdotV, NdotL, LdotH, bsdfData.perceptualRoughness);
-    #endif
+
+#ifdef LIT_DIFFUSE_LAMBERT_BRDF
+    float  diffuseTerm = Lambert();
+#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);
+#endif
+
     diffuseLighting = bsdfData.diffuseColor * diffuseTerm;
 }
 

Assets/ScriptableRenderPipeline/ShaderLibrary/BSDF.hlsl

 }
 
 // Ref: Diffuse Lighting for GGX + Smith Microsurfaces, p. 113.
-float3 DiffuseGGXNoPI(float3 albedo, float NdotV, float NdotL, float NdotH, float LdotV, float roughness)
+float3 DiffuseGGXNoPI(float3 albedo, float NdotV, float NdotL, float NdotH, float LdotV, float perceptualRoughness)
-    float smooth    = transmitL * transmitV * 1.05;   // Normalize F_t over the hemisphere
-    float single    = lerp(smooth, rough, roughness); // Rescaled by PI
-    float multi     = roughness * (0.1159 * PI);      // Rescaled by PI
+    float smooth    = transmitL * transmitV * 1.05;             // Normalize F_t over the hemisphere
+    float single    = lerp(smooth, rough, perceptualRoughness); // Rescaled by PI
+    // This constant is picked s.t. setting perceptualRoughness, albedo and all angles 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
-    return single + albedo * multi;
+    return single + albedo * multiple;
-float3 DiffuseGGX(float3 albedo, float NdotV, float NdotL, float NdotH, float LdotV, float roughness)
+float3 DiffuseGGX(float3 albedo, float NdotV, float NdotL, float NdotH, float LdotV, float perceptualRoughness)
-    return INV_PI * DiffuseGGXNoPI(albedo, NdotV, NdotL, NdotH, LdotV, roughness);
+    // Note that we could save 2 cycles by inlining the multiplication by INV_PI.
+    return INV_PI * DiffuseGGXNoPI(albedo, NdotV, NdotL, NdotH, LdotV, perceptualRoughness);
 }
 
 #endif // UNITY_BSDF_INCLUDED