Merge pull request #550 from Unity-Technologies/Some-update-to-refraction

Some update to refraction

ScriptableRenderPipeline/Core/ShaderLibrary/CommonLighting.hlsl

 }
 
 // ior is a value between 1.0 and 2.5
-float ConvertIORToFresnel0(float ior)
+float IORToFresnel0(float ior)
 {
     return Sqr((ior - 1.0) / (ior + 1.0));
 }

ScriptableRenderPipeline/Core/ShaderLibrary/Refraction.hlsl

 
 struct RefractionModelResult
 {
-    float       opticalDepth;
+    float       distance;       // length of the transmission during refraction through the shape
     float3      positionWS;     // out ray position
     float3      rayWS;          // out ray direction
 };
     // Second refraction (tangent sphere out)
     float NoR1 = dot(normalWS, R1);
     // Optical depth within the sphere
-    float opticalDepth = -NoR1 * thickness;
+    float distance = -NoR1 * thickness;
-    float3 P1 = positionWS + R1 * opticalDepth;
+    float3 P1 = positionWS + R1 * distance;
     // Out normal
     float3 N1 = normalize(C - P1);
     // Out refracted ray
 
     RefractionModelResult result;
-    result.opticalDepth = opticalDepth;
+    result.distance = distance;
     result.positionWS = P1;
     result.rayWS = R2;
 
     float3 R = refract(-V, normalWS, 1.0 / ior);
 
     // Optical depth within the thin plane
-    float opticalDepth = thickness / dot(R, -normalWS);
+    float distance = thickness / dot(R, -normalWS);
-    result.opticalDepth = opticalDepth;
-    result.positionWS = positionWS + R * opticalDepth;
+    result.distance = distance;
+    result.positionWS = positionWS + R * distance;
     result.rayWS = -V;
 
     return result;

ScriptableRenderPipeline/Core/ShaderLibrary/VolumeRendering.hlsl

            HenyeyGreensteinPhasePartVarying(asymmetry, LdotD);
 }
 
+// Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
+float TransmittanceColorAtDistanceToAbsorption(float3 transmittanceColor, float atDistance)
+{
+    return -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+}
+
+
 #endif // UNITY_VOLUME_RENDERING_INCLUDED

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

     bsdfData.ior = ior;
 
     // IOR define the fresnel0 value, so update it also for consistency (and even if not physical we still need to take into account any metal mask)
-    bsdfData.fresnel0 = lerp(ConvertIORToFresnel0(ior).xxx, baseColor, metallic);
+    bsdfData.fresnel0 = lerp(IORToFresnel0(ior).xxx, baseColor, metallic);
-    // Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
-    bsdfData.absorptionCoefficient = -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
+    bsdfData.absorptionCoefficient = TransmittanceColorAtDistanceToAbsorption (transmittanceColor, atDistance);
     bsdfData.transmittanceMask = transmittanceMask;
     bsdfData.thickness = max(thickness, 0.0001);
 }
     // Refraction
     float3 transmissionRefractV;            // refracted view vector after exiting the shape
     float3 transmissionPositionWS;          // start of the refracted ray after exiting the shape
-    float transmissionOpticalDepth;         // length of the transmission during refraction through the shape
     float3 transmissionTransmittance;       // transmittance due to absorption
     float transmissionSSMipLevel;           // mip level of the screen space gaussian pyramid for rough refraction
 
 };
 
-// This is a refract - TODO: do we call original refract or this one, original maybe slightly emore expensive, to check
+// This is a refract - TODO: do we call original refract or this one, original maybe slightly more expensive, to check
 float3 ClearCoatTransform(float3 X, float3 N, float ieta)
 {
     float XdotN = saturate(dot(N, X));
     RefractionModelResult refraction = REFRACTION_MODEL(V, posInput, bsdfData);
     preLightData.transmissionRefractV = refraction.rayWS;
     preLightData.transmissionPositionWS = refraction.positionWS;
-    preLightData.transmissionOpticalDepth = refraction.opticalDepth;
-    preLightData.transmissionTransmittance = exp(-bsdfData.absorptionCoefficient * refraction.opticalDepth);
-    preLightData.transmissionSSMipLevel = PerceptualRoughnessToMipmapLevel(bsdfData.perceptualRoughness, uint(_GaussianPyramidColorMipSize.z));
+    preLightData.transmissionTransmittance = exp(-bsdfData.absorptionCoefficient * refraction.distance);
+    // Empirical remap to try to match a bit the refractio probe blurring for the fallback
+    preLightData.transmissionSSMipLevel = sqrt(bsdfData.perceptualRoughness) * uint(_GaussianPyramidColorMipSize.z);
-    preLightData.transmissionOpticalDepth = 0;
     preLightData.transmissionTransmittance = float3(1.0, 1.0, 1.0);
     preLightData.transmissionSSMipLevel = 0;
 #endif