Question by silverfox774 · May 24 at 04:17 AM · c#textures rendertexture compute shader

Reading from RWTexture2D in compute shader Unity

I'm learning compute shader with Unity, but I've encountered a bug when transferring RenderTexture to the compute shader. Basically, there're 2 texture being transferred, renderTexture and copyTex. The steps are as follow:

copyTex copy from renderTexture with Graphics.CopyTexture()
Both are transferred to compute shader, and a new renderTexture would be calculated from copyTex
Compute shader finished, renderTexture is used to display, then everything looped again

But for some reason, the copyTex after transferred to the compute shader is blank.

This is the main C# code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class ComputeShaderTest : MonoBehaviour
 {
     public ComputeShader computeShader;
     public RenderTexture renderTexture;
     private RenderTexture copyTex;
 
     [SerializeField] private Image image;
     [SerializeField] private Vector2Int size = new Vector2Int(512, 512);
 
     private Texture2D tex;
 
     void Start()
     {
         InitializeRenderTexture();
         StartCoroutine(SlowTest());
     }
 
     private void InitializeRenderTexture()
     {
         if (!renderTexture)
         {
             renderTexture = new RenderTexture(size.x, size.y, 24);
             renderTexture.enableRandomWrite = true;
             renderTexture.Create();
 
             copyTex = new RenderTexture(size.x, size.y, 24);
             copyTex.enableRandomWrite = true;
             copyTex.Create();
         }
 
         //populate texture2d with all white and black patch in middle
         Vector2Int pos = new Vector2Int(size.x / 2, size.y / 2);
         tex = new Texture2D(size.x, size.y, TextureFormat.RGB24, false);
         for (int i = 0; i < size.x; i++)
         {
             for (int j = 0; j < size.y; j++)
             {
                 tex.SetPixel(i, j, Color.white);
             }
         }
         for (int i = pos.x-10; i < pos.x+10; i++)
         {
             for (int j = pos.y-10; j < pos.y+10; j++)
             {
                 tex.SetPixel(i, j, Color.black);
             }
         }
         tex.Apply();
 
         //copy to rendertexture
         RenderTexture.active = renderTexture;
         Graphics.Blit(tex, renderTexture);
     }
 
     IEnumerator SlowTest()
     {
         UpdateTexture2D();
         while(true)
         {
             yield return new WaitForSeconds(1f);
             TouchTest();        
             Debug.Log("Passed");    
             UpdateTexture2D();
         }        
     }
 
     private void UpdateTexture2D()
     {
         RenderTexture.active = renderTexture;
 
         tex.ReadPixels(new Rect(0, 0, renderTexture.width, renderTexture.height), 0, 0);
         tex.Apply();
         image.material.mainTexture = tex;
     }
 
     private void TouchTest()
     {
         Vector2Int pos = new Vector2Int(size.x/2, size.y/2);
         Graphics.CopyTexture(renderTexture, copyTex);
 
         computeShader.SetInt("posX", pos.x);
         computeShader.SetInt("posY", pos.y);
         computeShader.SetInt("resolution", size.x);
         computeShader.SetTexture(0, "Result", renderTexture);
         computeShader.SetTexture(0, "Copy", copyTex);
 
         computeShader.Dispatch(0, renderTexture.width / 8, renderTexture.height / 8, 1);
     }
 }

And the compute shader:

 #pragma kernel CSMain
 
 RWTexture2D<float4> Result;
 RWTexture2D<float4> Copy;
 
 int posX;
 int posY;
 int resolution;
 
 int dx[9] = {-1, 1, 1, 0, 1, -1, -1, 0, 0};
 int dy[9] = {1, 0, 1, 1, -1, 0, -1, -1, 0};
 
 [numthreads(8,8,1)]
 void CSMain (uint3 id : SV_DispatchThreadID)
 {
     int x = id.x, y = id.y;
     if (x != 0 && x != resolution - 1 && y != 0 && y != resolution - 1)
     {
         if (Copy[id.xy].a > 0.5) //white
         {
             Result[id.xy] = float4(1, 0, 0, 1); //set to red
         }
     }
 }

I can conclude the copyTex is blank because right after copy, I tested it with this piece of code:

 RenderTexture.active = copyTex;

 tex.ReadPixels(new Rect(0, 0, copyTex.width, copyTex.height), 0, 0);
 tex.Apply();
 image.material.mainTexture = tex;

and everything show up fine; but after the compute shader ran and the renderTexture got display, every pixels were red (which means the if statement activated, and all those pixels are previously white).

So texture can't be manipulated to transfer data and I have to use a custom RWStructuredBuffer instead or I mess up somewhere?

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