Fix diffuse transmission

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

 
     [branch] if (bsdfData.enableTransmission)
     {
-        // Use the reversed normal from the front for the back of the object.
-        illuminance = F_Transm_Schlick(bsdfData.fresnel0.x, saturate(-NdotL));  // Transmission is only valid for dielectric
+        // Currently, we only model diffuse transmission. Specular transmission is not yet supported.
+        // We assume that the back side of the object is a uniformly illuminated infinite plane
+        // (we reuse the illumination) with the reversed normal of the current sample.
+        // We apply wrapped lighting instead of the regular Lambertian diffuse
+        // to compensate for these approximations.
+        illuminance = ComputeWrappedDiffuseLighting(NdotL, SSS_WRAP_LIGHT);
-        float3 backLight = (cookie.rgb * lightData.color) * (illuminance * lightData.diffuseScale);
+        float3 backLight = (cookie.rgb * lightData.color) * (Lambert() * illuminance * lightData.diffuseScale);
         // TODO: multiplication by 'diffuseColor' and 'transmittance' is the same for each light.
         float3 transmittedLight = backLight * (bsdfData.diffuseColor * bsdfData.transmittance);
 
 
     [branch] if (bsdfData.enableTransmission)
     {
-        // Use the reversed normal from the front for the back of the object.
-        illuminance = F_Transm_Schlick(bsdfData.fresnel0.x , saturate(-NdotL)) * attenuation;  // Transmission is only valid for dielectric
+        // Currently, we only model diffuse transmission. Specular transmission is not yet supported.
+        // We assume that the back side of the object is a uniformly illuminated infinite plane
+        // (we reuse the illumination) with the reversed normal of the current sample.
+        // We apply wrapped lighting instead of the regular Lambertian diffuse
+        // to compensate for these approximations.
+        illuminance = ComputeWrappedDiffuseLighting(NdotL, SSS_WRAP_LIGHT) * attenuation;
-        float3 backLight = (cookie.rgb * lightData.color) * (illuminance * lightData.diffuseScale);
+        float3 backLight = (cookie.rgb * lightData.color) * (Lambert() * illuminance * lightData.diffuseScale);
         // TODO: multiplication by 'diffuseColor' and 'transmittance' is the same for each light.
         float3 transmittedLight = backLight * (bsdfData.diffuseColor * bsdfData.transmittance);
 
 
     [branch] if (bsdfData.enableTransmission)
     {
-        // Use the reversed normal from the front for the back of the object.
-        illuminance = F_Transm_Schlick(bsdfData.fresnel0.x, saturate(-NdotL)) * clipFactor; // Transmission is only valid for dielectric
+        // Currently, we only model diffuse transmission. Specular transmission is not yet supported.
+        // We assume that the back side of the object is a uniformly illuminated infinite plane
+        // (we reuse the illumination) with the reversed normal of the current sample.
+        // We apply wrapped lighting instead of the regular Lambertian diffuse
+        // to compensate for these approximations.
+        illuminance = ComputeWrappedDiffuseLighting(NdotL, SSS_WRAP_LIGHT) * clipFactor;
-        float3 backLight = (cookie.rgb * lightData.color) * (illuminance * lightData.diffuseScale);
+        float3 backLight = (cookie.rgb * lightData.color) * (Lambert() * illuminance * lightData.diffuseScale);
         // TODO: multiplication by 'diffuseColor' and 'transmittance' is the same for each light.
         float3 transmittedLight = backLight * (bsdfData.diffuseColor * bsdfData.transmittance);
 

Assets/ScriptableRenderPipeline/ShaderLibrary/CommonMaterial.hlsl

     return 0.25 * (expOneThird + 3 * expOneThird * expOneThird * expOneThird) * volumeAlbedo;
 }
 
+// Ref: Steve McAuley - Energy-Conserving Wrapped Diffuse
+float ComputeWrappedDiffuseLighting(float NdotL, float w)
+{
+    return saturate((-NdotL + w) / ((1 + w) * (1 + w)));
+}
+
 // MACRO from Legacy Untiy
 // Transforms 2D UV by scale/bias property
 #define TRANSFORM_TEX(tex, name) ((tex.xy) * name##_ST.xy + name##_ST.zw)