Fix transparent/opaque motion vector inconsistencies by allowing camera-only transparent motion vectors

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -652,6 +652,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fix an issue with the color intensity of emissive for performance rtgi
 - Fixed issue with rendering being mostly broken when target platform disables VR. 
 - Workaround an issue caused by GetKernelThreadGroupSizes  failing to retrieve correct group size. 
+- Fix inconsistencies with transparent motion vectors and opaque by allowing camera only transparent motion vectors.
 
 ### Changed
 - Improve MIP selection for decals on Transparents

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Camera/HDCamera.cs

@@ -664,6 +664,9 @@ unsafe internal void UpdateShaderVariablesGlobalCB(ref ShaderVariablesGlobal cb,
 
             float exposureMultiplierForProbes = 1.0f / Mathf.Max(probeRangeCompressionFactor, 1e-6f);
             cb._ProbeExposureScale  = exposureMultiplierForProbes;
+
+            cb._TransparentCameraOnlyMotionVectors = (frameSettings.IsEnabled(FrameSettingsField.MotionVectors) &&
+                                                      !frameSettings.IsEnabled(FrameSettingsField.TransparentsWriteMotionVector)) ? 1 : 0;
         }
 
 

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipeline.cs

@@ -3069,6 +3069,7 @@ out ScriptableCullingParameters cullingParams
             if (camera.cameraType != CameraType.Game)
             {
                 currentFrameSettings.SetEnabled(FrameSettingsField.ObjectMotionVectors, false);
+                currentFrameSettings.SetEnabled(FrameSettingsField.TransparentsWriteMotionVector, false);
             }
 
             hdCamera = HDCamera.GetOrCreate(camera, xrPass.multipassId);
@@ -4013,7 +4014,7 @@ void RenderForwardOpaque(CullingResults cullResults, HDCamera hdCamera, Scriptab
 
         static bool NeedMotionVectorForTransparent(FrameSettings frameSettings)
         {
-            return frameSettings.IsEnabled(FrameSettingsField.MotionVectors) && frameSettings.IsEnabled(FrameSettingsField.TransparentsWriteMotionVector) && frameSettings.IsEnabled(FrameSettingsField.ObjectMotionVectors);
+            return frameSettings.IsEnabled(FrameSettingsField.MotionVectors);
         }
 
         RendererListDesc PrepareForwardTransparentRendererList(CullingResults cullResults, HDCamera hdCamera, bool preRefraction)

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/ShaderPass/MotionVectorVertexShaderCommon.hlsl

@@ -133,6 +133,13 @@ PackedVaryingsType MotionVectorVS(inout VaryingsType varyingsType, AttributesMes
         ApplyVertexModification(inputMesh, normalWS, previousPositionRWS, _LastTimeParameters.xyz);
 #endif
 
+#ifdef _WRITE_TRANSPARENT_MOTION_VECTOR
+        if (_TransparentCameraOnlyMotionVectors > 0)
+        {
+            previousPositionRWS = varyingsType.vmesh.positionRWS.xyz;
+        }
+#endif
+
         varyingsType.vpass.previousPositionCS = mul(UNITY_MATRIX_PREV_VP, float4(previousPositionRWS, 1.0));
     }
 

com.unity.render-pipelines.high-definition/Runtime/ShaderLibrary/ShaderVariablesGlobal.cs

@@ -252,5 +252,10 @@ unsafe struct ShaderVariablesGlobal
 
         [HLSLArray(7, typeof(Vector4))]
         public fixed float _ProbeVolumeAmbientProbeFallbackPackedCoeffs[7 * 4]; // 3 bands of SH, packed for storing global ambient probe lighting as fallback to probe volumes.
+
+        public int      _TransparentCameraOnlyMotionVectors;
+        public float    _Pad8;
+        public float    _Pad9;
+        public float    _Pad10;
     }
 }

com.unity.render-pipelines.high-definition/Runtime/ShaderLibrary/ShaderVariablesGlobal.cs.hlsl

@@ -142,6 +142,10 @@ GLOBAL_CBUFFER_START(ShaderVariablesGlobal, b0)
     float _ProbeVolumeBilateralFilterWeightMin;
     float _ProbeVolumeBilateralFilterWeight;
     float4 _ProbeVolumeAmbientProbeFallbackPackedCoeffs[7];
+    int _TransparentCameraOnlyMotionVectors;
+    float _Pad8;
+    float _Pad9;
+    float _Pad10;
 CBUFFER_END