Merge pull request #460 from EvgeniiG/Unity-2017.3

Add support for [-1, 1] anisotropy

ScriptableRenderPipeline/Core/ShaderLibrary/CommonMaterial.hlsl

 // Convert anisotropic ratio (0->no isotropic; 1->full anisotropy in tangent direction) to roughness
 void ConvertAnisotropyToRoughness(float roughness, float anisotropy, out float roughnessT, out float roughnessB)
 {
-    // (0 <= anisotropy <= 1), therefore (0 <= anisoAspect <= 1)
-    // The 0.9 factor limits the aspect ratio to 10:1.
-    float anisoAspect = sqrt(1.0 - 0.9 * anisotropy);
-
-    roughnessT = roughness / anisoAspect; // Distort along tangent (rougher)
-    roughnessB = roughness * anisoAspect; // Straighten along bitangent (smoother)
+    // Use the parametrization of Sony Imageworks.
+    // Ref: Revisiting Physically Based Shading at Imageworks, p. 15.
+    roughnessT = roughness * (1 + anisotropy);
+    roughnessB = roughness * (1 - anisotropy);
 }
 
 // Ref: Donald Revie - Implementing Fur Using Deferred Shading (GPU Pro 2)
     return cross(B, grainDir);
 }
 
-// Fake anisotropic by distorting the normal.
+// Fake anisotropy by distorting the normal (non-negative anisotropy values only).
 // The grain direction (e.g. hair or brush direction) is assumed to be orthogonal to N.
 // Anisotropic ratio (0->no isotropic; 1->full anisotropy in tangent direction)
 float3 GetAnisotropicModifiedNormal(float3 grainDir, float3 N, float3 V, float anisotropy)

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

         // To have more precision encode the sign of xy in a separate uint
         uint octTangentSign = (octTangentWS.x < 0.0 ? 1 : 0) | (octTangentWS.y < 0.0 ? 2 : 0);
 
-        outGBuffer2 = float4(abs(octTangentWS), surfaceData.anisotropy, PackFloatInt8bit(surfaceData.metallic, octTangentSign, 4.0));
+        outGBuffer2 = float4(abs(octTangentWS), surfaceData.anisotropy * 0.5 + 0.5, PackFloatInt8bit(surfaceData.metallic, octTangentSign, 4.0));
     }
     else if (surfaceData.materialId == MATERIALID_LIT_CLEAR_COAT)
     {
         inGBuffer2.r = (octTangentSign & 1) ? -inGBuffer2.r : inGBuffer2.r;
         inGBuffer2.g = (octTangentSign & 2) ? -inGBuffer2.g : inGBuffer2.g;
         float3 tangentWS = UnpackNormalOctEncode(inGBuffer2.rg);
-        float anisotropy = inGBuffer2.b;
+        float anisotropy = inGBuffer2.b * 2 - 1;
+
         FillMaterialIdAnisoData(bsdfData.roughness, bsdfData.normalWS, tangentWS, anisotropy, bsdfData);
     }
     else if (bsdfData.materialId == MATERIALID_LIT_CLEAR_COAT && HasMaterialFeatureFlag(MATERIALFEATUREFLAGS_LIT_CLEAR_COAT))
         preLightData.TdotV = dot(bsdfData.tangentWS, V);
         preLightData.BdotV = dot(bsdfData.bitangentWS, V);
         preLightData.anisoGGXLambdaV = GetSmithJointGGXAnisoLambdaV(preLightData.TdotV, preLightData.BdotV, NdotV, bsdfData.roughnessT, bsdfData.roughnessB);
-        // Tangent = highlight stretch (anisotropy) direction. Bitangent = grain (brush) direction.
-        float3 anisoIblNormalWS = GetAnisotropicModifiedNormal(bsdfData.bitangentWS, iblNormalWS, V, bsdfData.anisotropy);
+        // For positive anisotropy values: tangent = highlight stretch (anisotropy) direction, bitangent = grain (brush) direction.
+        float3 grainDirWS = (bsdfData.anisotropy >= 0) ? bsdfData.bitangentWS : bsdfData.tangentWS;
+        float3 anisoIblNormalWS = GetAnisotropicModifiedNormal(grainDirWS, iblNormalWS, V, abs(bsdfData.anisotropy));
 
         // NOTE: If we follow the theory we should use the modified normal for the different calculation implying a normal (like NdotV) and use iblNormalWS
         // into function like GetSpecularDominantDir(). However modified normal is just a hack. The goal is just to stretch a cubemap, no accuracy here.

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.shader

 
         _TangentMap("TangentMap", 2D) = "bump" {}
         _TangentMapOS("TangentMapOS", 2D) = "white" {}
-        _Anisotropy("Anisotropy", Range(0.0, 1.0)) = 0
+        _Anisotropy("Anisotropy", Range(-1.0, 1.0)) = 0
         _AnisotropyMap("AnisotropyMap", 2D) = "white" {}
 
         _SubsurfaceProfile("Subsurface Profile", Int) = 0

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/LitTessellation.shader

 
         _TangentMap("TangentMap", 2D) = "bump" {}
         _TangentMapOS("TangentMapOS", 2D) = "white" {}
-        _Anisotropy("Anisotropy", Range(0.0, 1.0)) = 0
+        _Anisotropy("Anisotropy", Range(-1.0, 1.0)) = 0
         _AnisotropyMap("AnisotropyMap", 2D) = "white" {}
 
         _SubsurfaceProfile("Subsurface Profile", Int) = 0