[8.x.x Backport] Fix transparent/opaque motion vector inconsistencies by allowing camera-only transparent motion vectors

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

@@ -69,6 +69,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - The `CustomPassLoadCameraColor` and `CustomPassSampleCameraColor` functions now returns the correct color buffer when used in after post process instead of the color pyramid (which didn't had post processes).
 - Fixed for area light not updating baked light result when modifying with gizmo.
 - Fixed issue with white flash when enabling SSR.
+- Fix inconsistencies with transparent motion vectors and opaque by allowing camera only transparent motion vectors.
 
 ### Changed
 - Shadowmask and realtime reflection probe property are hide in Quality settings

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

@@ -662,6 +662,7 @@ internal void SetupGlobalParams(CommandBuffer cmd, int frameCount)
                 cmd.SetGlobalVectorArray(HDShaderIDs._XRPrevWorldSpaceCameraPos, m_XRPrevWorldSpaceCameraPos);
             }
 
+            cmd.SetGlobalInt(HDShaderIDs._TransparentCameraOnlyMotionVectors, (frameSettings.IsEnabled(FrameSettingsField.MotionVectors) && !frameSettings.IsEnabled(FrameSettingsField.TransparentsWriteMotionVector)) ? 1 : 0);
         }
 
         internal void AllocateAmbientOcclusionHistoryBuffer(float scaleFactor)

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

@@ -2728,6 +2728,7 @@ out ScriptableCullingParameters cullingParams
             if (camera.cameraType != CameraType.Game)
             {
                 currentFrameSettings.SetEnabled(FrameSettingsField.ObjectMotionVectors, false);
+                currentFrameSettings.SetEnabled(FrameSettingsField.TransparentsWriteMotionVector, false);
             }
 
             hdCamera = HDCamera.GetOrCreate(camera, xrPass.multipassId);
@@ -3596,7 +3597,7 @@ 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/HDStringConstants.cs

@@ -310,6 +310,8 @@ static class HDShaderIDs
         public static readonly int _TaaFrameInfo = Shader.PropertyToID("_TaaFrameInfo");
         public static readonly int _TaaJitterStrength = Shader.PropertyToID("_TaaJitterStrength");
 
+        public static readonly int _TransparentCameraOnlyMotionVectors = Shader.PropertyToID("_TransparentCameraOnlyMotionVectors ");
+
         public static readonly int _WorldSpaceCameraPos1 = Shader.PropertyToID("_WorldSpaceCameraPos1");
         public static readonly int _ViewMatrix1 = Shader.PropertyToID("_ViewMatrix1");
 

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/ShaderVariables.hlsl

@@ -283,6 +283,9 @@ CBUFFER_START(UnityGlobal)
 
     float4 _CoarseStencilBufferSize;
 
+    int _TransparentCameraOnlyMotionVectors;
+    float3 _Pad;
+
 CBUFFER_END
 
 // Custom generated by HDRP, not from Unity Engine (passed in via HDCamera)