Change the formulation of the FGD term

ScriptableRenderPipeline/Core/ShaderLibrary/ImageBasedLighting.hlsl

 
         if (NdotL > 0.0)
         {
-            // Integral is
-            //   1 / NumSample * \int[  L * fr * (N.L) / pdf ]  with pdf =  D(H) * (N.H) / (4 * (L.H)) and fr = F(H) * G(V, L) * D(H) / (4 * (N.L) * (N.V))
-            // This is split  in two part:
-            //   A) \int[ L * (N.L) ]
-            //   B) \int[ F(H) * 4 * (N.L) * V(V, L) * (L.H) / (N.H) ] with V(V, L) = G(V, L) / (4 * (N.L) * (N.V))
-            //      = \int[ F(H) * weightOverPdf ]
+            // Integral{BSDF * <N,L> dw} =
+            // Integral{(F0 + (1 - F0) * (1 - <V,H>)^5) * (BSDF / F) * <N,L> dw} =
+            // F0 * Integral{(BSDF / F) * <N,L> dw} +
+            // (1 - F0) * Integral{(1 - <V,H>)^5 * (BSDF / F) * <N,L> dw} =
+            // F0 * y + (1 - F0) * x = lerp(x, y, F0)
-            // Recombine at runtime with: ( f0 * weightOverPdf * (1 - Fc) + f90 * weightOverPdf * Fc ) with Fc =(1 - V.H)^5
-            float Fc            = pow(1.0 - VdotH, 5.0);
-            acc.x               += (1.0 - Fc) * weightOverPdf;
-            acc.y               += Fc * weightOverPdf;
+            acc.x += weightOverPdf;
+            acc.y += weightOverPdf * pow(1 - VdotH, 5);
         }
 
         // for Disney we still use a Cosine importance sampling, true Disney importance sampling imply a look up table

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 void GetPreIntegratedFGD(float NdotV, float perceptualRoughness, float3 fresnel0, out float3 specularFGD, out float diffuseFGD)
 {
     // Pre-integrate GGX FGD
-    //  _PreIntegratedFGD.x = Gv * (1 - Fc)  with Fc = (1 - H.L)^5
-    //  _PreIntegratedFGD.y = Gv * Fc
+    // Integral{BSDF * <N,L> dw} =
+    // Integral{(F0 + (1 - F0) * (1 - <V,H>)^5) * (BSDF / F) * <N,L> dw} =
+    // F0 * Integral{(BSDF / F) * <N,L> dw} +
+    // (1 - F0) * Integral{(1 - <V,H>)^5 * (BSDF / F) * <N,L> dw} =
+    // F0 * y + (1 - F0) * x = lerp(x, y, F0)
-    // _PreIntegratedFGD.z = DisneyDiffuse
+    // z = DisneyDiffuse
-    // f0 * Gv * (1 - Fc) + Gv * Fc
-    specularFGD = fresnel0 * preFGD.x + preFGD.y;
+    specularFGD = lerp(preFGD.xxx, preFGD.yyy, fresnel0);
 
 #ifdef LIT_DIFFUSE_LAMBERT_BRDF
     diffuseFGD = 1.0;