## 在Unity built-in RP中擷取motion vector

> 記錄一些我自己查不太到資料、曾經被困擾很久的小問題們

這篇文章主要會說明如何從unity取得motion vector並匯出成圖片。

網路上大部分查到的資料會是使用URP或HDRP，但是unity其實有內建的`_CameraMotionVectorsTexture`可以在built-in RP中得到motion vector的資料。

首先需要enable motion vectors和depth才能使用`_CameraMotionVectorsTexture`。

    motionVectorCamera = GetComponent<Camera>();
    motionVectorCamera.depthTextureMode = DepthTextureMode.Depth | DepthTextureMode.MotionVectors;

創建一個RenderTexture來保存。

    currentFrameTexture = new RenderTexture(Screen.width, Screen.height, 0, RenderTextureFormat.ARGBFloat);
    currentFrameTexture.enableRandomWrite = true;
    currentFrameTexture.Create();

然後在onRenderImage()

private void OnRenderImage(RenderTexture src, RenderTexture dest){
        CaptureFrame(src, dest, motionVectorMaterial, PATH);
}

private void CaptureFrame(RenderTexture src, RenderTexture dest, Material shaderProgram, string filepath){

        // render the current frame's render to the new RenderTexture
        Graphics.Blit(src, currentFrameTexture, shaderProgram);

        // save the frame's render to a Texture2D
        Texture2D tex = new Texture2D(currentFrameTexture.width, currentFrameTexture.height, TextureFormat.RGBAFloat, false);
        tex.ReadPixels(new Rect(0, 0, currentFrameTexture.width, currentFrameTexture.height), 0, 0);
        tex.Apply();
        
        // save those data to exr file
        byte[] exrBytes = tex.EncodeToEXR();
        System.IO.File.WriteAllBytes(filepath + ".exr", exrBytes);

        Destroy(tex);
        
        Graphics.Blit(src, dest);
    }

這邊是匯出成EXR檔，也可以改成PNG或TXT之類的。

這邊用到了一個shaderProgram的Material，接下來就來看shader的部分：

Shader "Custom/MotionVectorShader"
{
    Properties
    {
        _MainTex("Current Frame", 2D) = "white" {}
    }

    SubShader
    {
        Tags { "RenderType"="Opaque" }
        Cull Off ZWrite Off ZTest Always
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            sampler2D _CameraMotionVectorsTexture;
            float4 _CameraMotionVectorsTexture_TexelSize;
            float4 _CameraMotionVectorsTexture_ST;
            
            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _CameraMotionVectorsTexture);
                return o;
            }

            half4 frag(v2f i) : SV_Target
            {
                // Fetch motion vectors from Unity's built-in motion vector buffer
                float2 motionVector = tex2D(_CameraMotionVectorsTexture, i.uv).rg;
                float2 color = tex2D(_MainTex, i.uv).rg;
                
                // for visualizing
                motionVector = motionVector * 10 + 0.5;

                return half4(motionVector.x, motionVector.y, 0, 1);
            }
            ENDCG
        }
    }
}

這樣就成功地從`_CameraMotionVectorsTexture`拿到motion vector的資料了。