Make a Texture3D writeable on the GPU in Unity

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29-08-2024

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Making a Texture3D Writeable on the GPU in Unity: A Deep Dive

This article delves into the intricate world of making a Texture3D writeable on the GPU in Unity, tackling a common challenge encountered by developers. We'll unpack the problem, understand the solution, and provide practical examples to illustrate the process.

The Problem:

Many developers face the frustrating error: "Attempting to bind texture as UAV but the texture wasn't created with the UAV usage flag set!" when trying to write to a Texture3D from within a shader. The core issue lies in the fact that Unity's standard Texture3D objects are read-only by default. This means they are meant for data transfer from the CPU to the GPU, not for direct manipulation within the shader.

The Solution:

The solution lies in utilizing Compute Shaders and creating a ComputeBuffer that will hold your 3D data. This approach offers several advantages:

• GPU-side Modification: Data resides directly on the GPU, eliminating the need for constant data transfers between CPU and GPU.
• Performance Optimization: Direct GPU manipulation of data ensures smoother and faster computations.

Step-by-Step Guide:

1. Create a Compute Shader: This shader will perform the desired operations on your Texture3D data.

   Shader "Custom/ComputeShader" 
   {
       Properties 
       {
           _MainTex ("Texture", 2D) = "white" {} 
       }
       SubShader 
       {
           Pass 
           {
               CGPROGRAM
               #pragma vertex vert
               #pragma fragment frag
               #pragma target 3.0
               
               sampler2D _MainTex;
               fixed4 frag (v2f i) : SV_Target
               {
                   return tex2D(_MainTex, i.uv);
               }
               ENDCG
           }
       }
   }
   

2. Create a ComputeBuffer: This buffer will act as a container for your 3D data, making it directly accessible by the Compute Shader.

   public class Example : MonoBehaviour 
   {
       public ComputeShader myComputeShader; // Reference to your compute shader
   
       private ComputeBuffer _buffer; // Our compute buffer
       private int _kernelIndex; // Index of the compute shader kernel
       private int _size = 8 * 8 * 8; // Size of your 3D texture
   
       void Start() 
       {
           // ... [your existing initialization code] ...
   
           // Create ComputeBuffer
           _buffer = new ComputeBuffer(_size, sizeof(float)); 
   
           // Fill the compute buffer with your initial data
           _buffer.SetData(points_f); 
   
           // Get kernel index from the compute shader
           _kernelIndex = myComputeShader.FindKernel("CSMain"); 
       }
   
       void Update()
       {
           // ... [your existing logic] ...
   
           // Send data to compute shader
           myComputeShader.SetBuffer(_kernelIndex, "points", _buffer); 
   
           // Dispatch compute shader
           myComputeShader.Dispatch(_kernelIndex, 8, 8, 8); 
   
           // Read the updated data from the compute buffer
           _buffer.GetData(points_f); // Update your points_f array
           tex.SetPixelData(points_f, 0);
           tex.Apply();
       }
   
       void OnDestroy()
       {
           // Release the ComputeBuffer
           _buffer.Release(); 
       }
   }
   

3. Write the Compute Shader Code: Implement the calculations you want to perform on your Texture3D data within the compute shader.

   RWTexture3D<float> points; // Declares the Texture3D as writeable
   
   [numthreads(8, 8, 8)] // Threads per group
   void CSMain (uint3 id : SV_DispatchThreadID)
   {
       // ... your calculations on 'points' ...
       if (id.x == 0 || id.y == 0 || id.z == 0 || id.x == 7 || id.y == 7 || id.z == 7) 
       {
           points[id] = -1000.0f;
       } 
       else 
       {
           points[id] = 1.0f;
       }
   }
   

Key Points:

• The RWTexture3D declaration in HLSL signals that the texture is writeable.
• The [numthreads] attribute specifies how many threads will execute the compute shader per group.
• You use the SV_DispatchThreadID to access individual points within the Texture3D.
• The ComputeBuffer acts as the bridge between your Unity script and the compute shader, facilitating data transfers.

Caveats:

• Performance: Although the use of Compute Buffers optimizes data manipulation, the initial upload of data can still be costly.
• Complexity: Implementing Compute Shaders adds an extra layer of complexity compared to traditional shader approaches.

Further Optimization:

• Structured Buffer: If you're only writing to individual points, using a StructuredBuffer in your Compute Shader can be more efficient than a full RWTexture3D.

Example Usage:

Imagine you want to apply a blur effect to your Texture3D on the GPU. You could achieve this by using a Compute Shader that iterates over the points in the Texture3D, calculating a weighted average of nearby values for each point.

Final Thoughts:

By utilizing Compute Buffers and Compute Shaders, you can achieve truly GPU-based manipulation of your 3D textures in Unity. This approach unlocks powerful performance optimizations and gives you greater control over your shader computations.

Credits and References:

• Stack Overflow: This article leverages information from a Stack Overflow question, acknowledging the original author's contribution. (Please provide the link to the Stack Overflow question if available.)
• Unity Documentation: [link to relevant Unity documentation on Compute Shaders and Compute Buffers]

This article provides a comprehensive guide to making your Texture3D writeable on the GPU in Unity. We encourage you to experiment and explore the possibilities offered by these powerful tools.