Remove the pre-generated mesh dependency from the GGX convolution shader

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/GGXConvolve.shader

             #include "../../ShaderLibrary/ImageBasedLighting.hlsl"
             #include "SkyManager.cs.hlsl"
 
+            TEXTURECUBE(_MainTex);
+            SAMPLERCUBE(sampler_MainTex);
+
+            TEXTURE2D_FLOAT(_GgxIblSamples);
+
+            #ifdef USE_MIS
+                TEXTURE2D(_MarginalRowDensities);
+                TEXTURE2D(_ConditionalDensities);
+            #endif
+
+            float _Level;
+            float _LastLevel;
+            float _InvOmegaP;
+            float4x4 _PixelCoordToViewDirWS; // Actually just 3x3, but Unity can only set 4x4
+
-                float3 positionCS : POSITION;
-                float3 eyeVector : NORMAL;
+                uint vertexID : SV_VertexID;
-                float3 eyeVector : TEXCOORD0;
-                // Unity renders upside down, so the clip space coordinates have to be flipped.
-                output.positionCS = float4(input.positionCS.x, -input.positionCS.y, UNITY_RAW_FAR_CLIP_VALUE, 1.0);
-                output.eyeVector = input.eyeVector;
+                output.positionCS = GetFullScreenTriangleVertexPosition(input.vertexID);
-            TEXTURECUBE(_MainTex);
-            SAMPLERCUBE(sampler_MainTex);
-
-            TEXTURE2D_FLOAT(_GgxIblSamples);
-
-            #ifdef USE_MIS
-                TEXTURE2D(_MarginalRowDensities);
-                TEXTURE2D(_ConditionalDensities);
-            #endif
-
-            float _Level;
-            float _LastLevel;
-            float _InvOmegaP;
-
-                // Vector interpolation is not magnitude-preserving.
-                float3 N = normalize(input.eyeVector);
+                // Points towards the camera
+                float3 viewDirWS = normalize(mul(float3(input.positionCS.xy, 1.0), (float3x3)_PixelCoordToViewDirWS));
+                // Reverse it to point into the scene
+                float3 N = -viewDirWS;
                 // Remove view-dependency from GGX, effectively making the BSDF isotropic.
                 float3 V = N;
 

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/RuntimeFilterIBL.cs

 
         void FilterCubemapCommon(CommandBuffer cmd,
             Texture source, RenderTexture target, int mipCount,
-            Mesh[] cubemapFaceMesh)
+            Matrix4x4[] worldToViewMatrices)
         {
             // Solid angle associated with a texel of the cubemap.
             float invOmegaP = (6.0f * source.width * source.width) / (4.0f * Mathf.PI);
                 string sampleName = String.Format("Filter Cubemap Mip {0}", mip);
                 cmd.BeginSample(sampleName);
 
-                MaterialPropertyBlock props = new MaterialPropertyBlock();
-                props.SetFloat("_Level", mip);
-
+                    Vector4   faceSize  = new Vector4(source.width >> mip, source.height >> mip, 1.0f / (source.width >> mip), 1.0f / (source.height >> mip));
+                    Matrix4x4 transform = SkyManager.ComputePixelCoordToWorldSpaceViewDirectionMatrix(0.5f * Mathf.PI, faceSize, worldToViewMatrices[face], true);
+
+                    MaterialPropertyBlock props = new MaterialPropertyBlock();
+                    props.SetFloat("_Level", mip);
+                    props.SetMatrix(HDShaderIDs._PixelCoordToViewDirWS, transform);
+
-
-                    cmd.DrawMesh(cubemapFaceMesh[face], Matrix4x4.identity, m_GgxConvolveMaterial, 0, 0, props);
+                    Utilities.DrawFullScreen(cmd, m_GgxConvolveMaterial, props);
                 }
                 cmd.EndSample(sampleName);
             }
         public void FilterCubemap(CommandBuffer cmd,
             Texture source, RenderTexture target, int mipCount,
-            Mesh[] cubemapFaceMesh)
+            Matrix4x4[] worldToViewMatrices)
-            FilterCubemapCommon(cmd, source, target, mipCount, cubemapFaceMesh);
+            FilterCubemapCommon(cmd, source, target, mipCount, worldToViewMatrices);
         }
 
         // Filters MIP map levels (other than 0) with GGX using multiple importance sampling.
-            Mesh[] cubemapFaceMesh)
+            Matrix4x4[] worldToViewMatrices)
         {
             // Bind the input cubemap.
             m_BuildProbabilityTablesCS.SetTexture(m_ConditionalDensitiesKernel, "envMap", source);
             m_GgxConvolveMaterial.SetTexture("_ConditionalDensities", conditionalCdf);
             m_GgxConvolveMaterial.SetTexture("_MarginalRowDensities", marginalRowCdf);
 
-            FilterCubemapCommon(cmd, source, target, mipCount, cubemapFaceMesh);
+            FilterCubemapCommon(cmd, source, target, mipCount, worldToViewMatrices);
         }
     }
 }

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/SkyManager.cs

         IBLFilterGGX            m_iblFilterGgx = null;
 
         Vector4                 m_CubemapScreenSize;
+        Matrix4x4[]             m_faceWorldToViewMatrixMatrices = new Matrix4x4[6];
         Matrix4x4[]             m_facePixelCoordToViewDirMatrices = new Matrix4x4[6];
         Matrix4x4[]             m_faceCameraInvViewProjectionMatrix = new Matrix4x4[6];
         Mesh[]                  m_CubemapFaceMesh = new Mesh[6];
                     Matrix4x4 worldToView = lookAt * Matrix4x4.Scale(new Vector3(1.0f, 1.0f, -1.0f)); // Need to scale -1.0 on Z to match what is being done in the camera.wolrdToCameraMatrix API. ...
                     Vector4   screenSize  = new Vector4((int)m_SkySettings.resolution, (int)m_SkySettings.resolution, 1.0f / (int)m_SkySettings.resolution, 1.0f / (int)m_SkySettings.resolution);
 
+                    m_faceWorldToViewMatrixMatrices[i]     = worldToView;
                     m_facePixelCoordToViewDirMatrices[i]   = ComputePixelCoordToWorldSpaceViewDirectionMatrix(0.5f * Mathf.PI, screenSize, worldToView, true);
                     m_faceCameraInvViewProjectionMatrix[i] = Utilities.GetViewProjectionMatrix(lookAt, cubeProj).inverse;
 
                 {
                     if (m_useMIS && m_iblFilterGgx.SupportMIS)
                     {
-                        m_iblFilterGgx.FilterCubemapMIS(cmd, input, target, mipCount, m_SkyboxConditionalCdfRT, m_SkyboxMarginalRowCdfRT, m_CubemapFaceMesh);
+                        m_iblFilterGgx.FilterCubemapMIS(cmd, input, target, mipCount, m_SkyboxConditionalCdfRT, m_SkyboxMarginalRowCdfRT, m_faceWorldToViewMatrixMatrices);
-                        m_iblFilterGgx.FilterCubemap(cmd, input, target, mipCount, m_CubemapFaceMesh);
+                        m_iblFilterGgx.FilterCubemap(cmd, input, target, mipCount, m_faceWorldToViewMatrixMatrices);
                     }
                 }
             }