Homegenize code of motion vector + fix discrepency between object and camera motion blur

com.unity.render-pipelines.high-definition/HDRP/Debug/DebugFullScreen.shader

             #include "HDRP/Material/Lit/Lit.cs.hlsl"
             #include "../ShaderVariables.hlsl"
             #include "../Debug/DebugDisplay.cs.hlsl"
+            #include "../Material/Builtin/BuiltinData.hlsl"
 
             CBUFFER_START (UnityDebug)
             float _FullScreenDebugMode;
                 return d;
             }
 
+            // return motion vector in NDC space [0..1]
-                return SAMPLE_TEXTURE2D(_DebugFullScreenTexture, s_point_clamp_sampler, coords).xy;
+                float2 velocityNDC;
+                DecodeVelocity(SAMPLE_TEXTURE2D(_DebugFullScreenTexture, s_point_clamp_sampler, coords), velocityNDC);
+
+                return velocityNDC;
             }
             // end motion vector utilties
 

com.unity.render-pipelines.high-definition/HDRP/Material/Builtin/BuiltinData.hlsl

     shadowMask = inBuffer;
 }
 
+// TODO: CAUTION: current DecodeVelocity is not used in motion vector / TAA pass as it come from Postprocess stack
+// This will be fix when postprocess will be integrated into HD, but it mean that we must not change the
+// EncodeVelocity / DecodeVelocity code for now, i.e it must do nothing like it is doing currently.
+// Note2: Motion blur code of posptrocess stack do * 2 - 1 to uncompress velocity which is not expected, TAA is correct.
 // Design note: We assume that velocity/distortion fit into a single buffer (i.e not spread on several buffer)
 void EncodeVelocity(float2 velocity, out float4 outBuffer)
 {

com.unity.render-pipelines.high-definition/HDRP/Material/MaterialUtilities.hlsl

     return rawOcclusionMask;
 }
 
+// Calculate velocity in Clip space [-1..1]
 float2 CalculateVelocity(float4 positionCS, float4 previousPositionCS)
 {
     // This test on define is required to remove warning of divide by 0 when initializing empty struct

com.unity.render-pipelines.high-definition/HDRP/RenderPipelineResources/CameraMotionVectors.shader

         #include "../ShaderPass/FragInputs.hlsl"
         #include "../ShaderPass/VaryingMesh.hlsl"
         #include "../ShaderPass/VertMesh.hlsl"
+        #include "../Material/Builtin/BuiltinData.hlsl"
 
         struct Attributes
         {
             return output;
         }
 
-        float4 Frag(Varyings input) : SV_Target
+        void Frag(Varyings input, out float4 outColor : SV_Target0)
         {
             float depth = LOAD_TEXTURE2D(_CameraDepthTexture, input.positionCS.xy).x;
 
 
             float4 prevClipPos = mul(_PrevViewProjMatrix, prevPos);
             float4 curClipPos = mul(_NonJitteredViewProjMatrix, worldPos);
-            float2 prevHPos = prevClipPos.xy / prevClipPos.w;
-            float2 curHPos = curClipPos.xy / curClipPos.w;
-            float2 previousPositionCS = (prevHPos + 1.0) / 2.0;
-            float2 positionCS = (curHPos + 1.0) / 2.0;
+            float2 previousPositionCS = prevClipPos.xy / prevClipPos.w;
+            float2 positionCS = curClipPos.xy / curClipPos.w;
-        #if UNITY_UV_STARTS_AT_TOP
-            previousPositionCS.y = 1.0 - previousPositionCS.y;
-            positionCS.y = 1.0 - positionCS.y;
-        #endif
-
-            return float4(positionCS - previousPositionCS, 0.0, 1.0);
+            // Convert from Clip space (-1..1) to NDC 0..1 space
+            float2 velocity = (positionCS - previousPositionCS);
+#if UNITY_UV_STARTS_AT_TOP
+            velocity.y = -velocity.y;
+#endif
+            // Convert velocity from Clip space (-1..1) to NDC 0..1 space
+            // Note: ((positionCS * 0.5 + 0.5) - (previousPositionCS * 0.5 + 0.5)) = (velocity * 0.5)
+            EncodeVelocity(velocity * 0.5, outColor);
         }
 
     ENDHLSL

com.unity.render-pipelines.high-definition/HDRP/ShaderPass/ShaderPassVelocity.hlsl

     // TODO: How to allow overriden velocity vector from GetSurfaceAndBuiltinData ?
     float2 velocity = CalculateVelocity(inputPass.positionCS, inputPass.previousPositionCS);
 
-    EncodeVelocity(velocity, outColor);
+    // Convert from Clip space (-1..1) to NDC 0..1 space
+    // Note: ((positionCS * 0.5 + 0.5) - (previousPositionCS * 0.5 + 0.5)) = (velocity * 0.5)
+    EncodeVelocity(velocity * 0.5, outColor);
 
     // Note: unity_MotionVectorsParams.y is 0 is forceNoMotion is enabled
     bool forceNoMotion = unity_MotionVectorsParams.y == 0.0;