Add temporal integration support to the Disney SSS

ScriptableRenderPipeline/HDRenderPipeline/Camera/HDCamera.cs

         public Plane[] frustumPlanes;
         public Vector4[] frustumPlaneEquations;
         public Camera camera;
+        public uint taaFrameIndex;
         public PostProcessRenderContext postprocessRenderContext;
 
         public Matrix4x4 viewProjMatrix
                 }
 
                 isFirstFrame = false;
+
+                const uint taaFrameCount = 8;
+                taaFrameIndex = taaEnabled ? (uint)Time.renderedFrameCount % taaFrameCount : 0; 
+            }
+            else
+            {
+                // Warning: in the Game View, outside of the Play Mode, the counter gets stuck on a random frame.
+                // In this case, reset the frame index to 0.
+                taaFrameIndex = 0;
             }
 
             viewMatrix = gpuView;
             cmd.SetGlobalVector(HDShaderIDs._ScreenSize, screenSize);
             cmd.SetGlobalMatrix(HDShaderIDs._PrevViewProjMatrix, prevViewProjMatrix);
             cmd.SetGlobalVectorArray(HDShaderIDs._FrustumPlanes, frustumPlaneEquations);
+            cmd.SetGlobalInt(HDShaderIDs._TaaFrameIndex, (int)taaFrameIndex);
         }
 
         // Does not modify global settings. Used for shadows, low res. rendering, etc.
             material.SetVector(HDShaderIDs._ScreenSize, screenSize);
             material.SetMatrix(HDShaderIDs._PrevViewProjMatrix, prevViewProjMatrix);
             material.SetVectorArray(HDShaderIDs._FrustumPlanes, frustumPlaneEquations);
+            material.SetInt(HDShaderIDs._TaaFrameIndex, (int)taaFrameIndex);
         }
 
         // TODO: We should set all the value below globally and not let it under the control of Unity,

ScriptableRenderPipeline/HDRenderPipeline/HDStringConstants.cs

         public static readonly int _ScreenSize = Shader.PropertyToID("_ScreenSize");
         public static readonly int _PrevViewProjMatrix = Shader.PropertyToID("_PrevViewProjMatrix");
         public static readonly int _FrustumPlanes = Shader.PropertyToID("_FrustumPlanes");
+        public static readonly int _TaaFrameIndex = Shader.PropertyToID("_TaaFrameIndex");
 
         public static readonly int _DepthTexture                   = Shader.PropertyToID("_DepthTexture");
         public static readonly int _CameraColorTexture             = Shader.PropertyToID("_CameraColorTexture");

ScriptableRenderPipeline/HDRenderPipeline/Material/SubsurfaceScattering/SubsurfaceScattering.compute

 // Tweak parameters.
 #define SSS_BILATERAL_FILTER  1
 #define SSS_USE_LDS_CACHE     1
+#define SSS_TAA_INTEGRATION   1 // Smoother results at the cost of a tiny amount of flickering in under-sampled areas
 #define SSS_ENABLE_NEAR_FIELD 0 // Greatly increases the number of samples. Comes at a high cost.
 #define SSS_SAMPLE_TEST_HTILE 0 // Potential optimization. YMMV.
 #define SSS_USE_TANGENT_PLANE 0 // Improves the accuracy of the approximation(0 -> 1st order). High cost. Does not work with back-facing normals.
 
 #include "ShaderLibrary/Common.hlsl"
 #include "ShaderLibrary/Packing.hlsl"
-#include "ShaderLibrary/Sampling/Fibonacci.hlsl"
+#include "ShaderLibrary/Sampling/Sampling.hlsl"
 #include "ShaderLibrary/SpaceFillingCurves.hlsl"
 #include "../../ShaderVariables.hlsl"
 #include "../../Lighting/LightDefinition.cs.hlsl"
     float  r   = _FilterKernels[profileID][i][iR];
     // The relative sample position is known at the compile time.
     float  phi = SampleDiskFibonacci(i, n).y;
+#if (SSS_TAA_INTEGRATION != 0)
+    // Note that we repeat the pattern twice during the TAA cycle to reduce flickering.
+    phi += VanDerCorputBase2(_TaaFrameIndex % 4) * TWO_PI;
+#endif
     float2 vec = r * float2(cos(phi), sin(phi));
 
     // Compute the screen-space position and the squared distance (in mm) in the image plane.

ScriptableRenderPipeline/HDRenderPipeline/ShaderVariables.hlsl

     int unity_StereoEyeIndex;
 #endif
 
-    float4 unity_ShadowColor;
+    float3 unity_ShadowColor;
+    uint _TaaFrameIndex;      // [0, 7]
 CBUFFER_END
 
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