Improve support of transmission for Disney Diffuse

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

 #define LTC_LUT_OFFSET (0.5 * rcp(LTC_LUT_SIZE))
 
 // Subsurface scattering constant
-#define SSS_WRAP_ANGLE (PI/12)              // Used for wrap lighting
+#define SSS_WRAP_ANGLE (PI/12)              // 15 degrees
 #define SSS_WRAP_LIGHT cos(PI/2 - SSS_WRAP_ANGLE)
 
 CBUFFER_START(UnitySSSParameters)
 #elif LIT_DIFFUSE_GGX_BRDF
     float3 diffuseTerm = DiffuseGGX(bsdfData.diffuseColor, NdotV, NdotL, NdotH, LdotV, bsdfData.roughness);
 #else
-    float  diffuseTerm = DisneyDiffuse(NdotV, NdotL, LdotV, bsdfData.perceptualRoughness);
+    // A note on subsurface scattering.
+    // The correct way to handle SSS is to transmit light inside the surface, perform SSS,
+    // and then transmit it outside towards the viewer.
+    // Transmit(X) = F_Transm_Schlick(F0, F90, NdotX), where F0 = 0, F90 = 1.
+    // Therefore, the diffuse BSDF should be decomposed as follows:
+    // f_d = A / Pi * F_Transm_Schlick(0, 1, NdotL) * F_Transm_Schlick(0, 1, NdotV) + f_d_reflection,
+    // with F_Transm_Schlick(0, 1, NdotV) applied after the SSS pass.
+    // The alternative (artistic) formulation of Disney is to set F90 = 0.5:
+    // f_d = A / Pi * F_Transm_Schlick(0, 0.5, NdotL) * F_Transm_Schlick(0, 0.5, NdotV) + f_retro_reflection.
+    // That way, darkening at grading angles is reduced to 0.5.
+    // In practice, applying F_Transm_Schlick(F0, F90, NdotV) after the SSS pass is expensive,
+    // as it forces us to read the normal buffer at the end of the SSS pass.
+    // Separating f_retro_reflection also has a small cost (mostly due to energy compensation
+    // for multi-bounce GGX), and the visual difference is negligible.
+    // Therefore, we choose not to separate diffuse lighting into reflected and transmitted.
+    float diffuseTerm = DisneyDiffuse(NdotV, NdotL, LdotV, bsdfData.perceptualRoughness);
 #endif
 
     // We don't multiply by 'bsdfData.diffuseColor' here. It's done only once in PostEvaluateBSDF().
 
     [branch] if (bsdfData.enableTransmission)
     {
-        // We apply wrapped lighting instead of the regular Lambertian diffuse
-        // to compensate for approximations within EvaluateTransmission().
-        float illuminance = Lambert() * ComputeWrappedDiffuseLighting(-NdotL, SSS_WRAP_LIGHT);
+        // Apply wrapped lighting to better handle thin objects (cards) at grazing angles.
+        float wrappedNdotL = ComputeWrappedDiffuseLighting(-NdotL, SSS_WRAP_LIGHT);
+
+    #ifdef LIT_DIFFUSE_LAMBERT_BRDF
+        illuminance  = Lambert() * wrappedNdotL;
+    #else
+        float tNdotL = saturate(-NdotL);
+        float  NdotV = max(preLightData.NdotV, MIN_N_DOT_V);
+        illuminance  = INV_PI * F_Transm_Schlick(0, 0.5, NdotV) * F_Transm_Schlick(0, 0.5, tNdotL) * wrappedNdotL;
+    #endif
 
         // We use diffuse lighting for accumulation since it is going to be blurred during the SSS pass.
         // We don't multiply by 'bsdfData.diffuseColor' here. It's done only once in PostEvaluateBSDF().
 
     [branch] if (bsdfData.enableTransmission)
     {
-        // We apply wrapped lighting instead of the regular Lambertian diffuse
-        // to compensate for approximations within EvaluateTransmission().
-        float illuminance = Lambert() * ComputeWrappedDiffuseLighting(-NdotL, SSS_WRAP_LIGHT);
+        // Apply wrapped lighting to better handle thin objects (cards) at grazing angles.
+        float wrappedNdotL = ComputeWrappedDiffuseLighting(-NdotL, SSS_WRAP_LIGHT);
+
+    #ifdef LIT_DIFFUSE_LAMBERT_BRDF
+        illuminance  = Lambert() * wrappedNdotL;
+    #else
+        float tNdotL = saturate(-NdotL);
+        float  NdotV = max(preLightData.NdotV, MIN_N_DOT_V);
+        illuminance  = INV_PI * F_Transm_Schlick(0, 0.5, NdotV) * F_Transm_Schlick(0, 0.5, tNdotL) * wrappedNdotL;
+    #endif
 
         // We use diffuse lighting for accumulation since it is going to be blurred during the SSS pass.
         // We don't multiply by 'bsdfData.diffuseColor' here. It's done only once in PostEvaluateBSDF().