Unity Get Camera Motion Vectors in Custom Pass for Universal Render Pipeline (URP)

GetCameraMotionVectors.hlsl

#ifndef GETCAMERAMOTIONVECTORS_INCLUDE
#define GETCAMERAMOTIONVECTORS_INCLUDE

#if defined(USING_STEREO_MATRICES)
        float4x4 _PrevViewProjMStereo[2];
#define _PrevViewProjM  _PrevViewProjMStereo[unity_StereoEyeIndex]
#define _ViewProjM unity_MatrixVP
#else
float4x4 _ViewProjM;
float4x4 _PrevViewProjM;
#endif


void GetCameraMotionVectors_float(float4 UV, float3 positionOS, out float2 velocity)
{
	velocity = 0;
	#ifndef SHADERGRAPH_PREVIEW
	float4 positionCS = TransformObjectToHClip(positionOS);
	float4 projPos = positionCS * 0.5;
	projPos.xy = projPos.xy + projPos.w;
	
	half depth = 0.0;
	#if UNITY_REVERSED_Z
		depth = 1.0;
	#endif
	
	float3 viewPos = ComputeViewSpacePosition(projPos.xy, depth, unity_CameraInvProjection); 
	
	float3 viewDirectionTemp = mul(unity_CameraInvProjection, float4(UV.xy * 2 - 1, 0.0, -1));
	float3 viewVector = mul(unity_CameraToWorld, float4(viewDirectionTemp, 0.0));
	float3 rayDirection = viewVector / length(viewVector);
	
	float4 worldPos = float4(mul(unity_CameraToWorld, float4(viewPos, 1.0)).xyz, 1.0);
	float4 prevPos = worldPos;

	float4 prevClipPos = mul(_PrevViewProjM, prevPos);
	float4 curClipPos = mul(_ViewProjM, worldPos);

	float2 prevPosCS = prevClipPos.xy / prevClipPos.w;
	float2 curPosCS = curClipPos.xy / curClipPos.w;
	
	velocity = prevPosCS - curPosCS;
	
	#if UNITY_UV_STARTS_AT_TOP == 0
		velocity.y = -velocity.y;
	#endif
	
	
	velocity.xy *= 0.5;
	#endif
}
#endif

WriteGpuVpToShader.cs

using UnityEngine;
using UnityEngine.Rendering;

// Note: This form of assigning view projection matrices to shader has been removed from Altos.
// Now we assign the view projection matrices directly to specific material in the Render Pass. 

public class WriteGpuVpToShader : MonoBehaviour
{
    private Matrix4x4 gpuVPLast;
    private Camera cam;

    private void Start()
    {
        RenderPipelineManager.beginCameraRendering += OnBeginCameraRendering;
        cam = Camera.main;
        cam.depthTextureMode |= DepthTextureMode.MotionVectors;
    }

    void OnBeginCameraRendering(ScriptableRenderContext context, Camera camera)
    {
        var gpuProj = GL.GetGPUProjectionMatrix(cam.projectionMatrix, true);
        var gpuView = cam.worldToCameraMatrix;
        var gpuVP = gpuProj * gpuView;

        Shader.SetGlobalMatrix("_ViewProjM", gpuVP);

        if (gpuVPLast != null)
            Shader.SetGlobalMatrix("_PrevViewProjM", gpuVPLast);

        gpuVPLast = gpuVP;
    }

    void OnDestroy()
    {
        RenderPipelineManager.beginCameraRendering -= OnBeginCameraRendering;
    }
}