# Read input data into a compute shader in URP

> Allocate a buffer that reads input data into a compute shader in a render pass.

To read input data into the compute shader, do the following:

1. Create a graphics buffer using `CreateBuffer`, then add a handle to it in your pass data. For example:

   ```lang-cs
   // Declare a buffer handle
   BufferHandle m_InputBufferHandle;

   // Add the handle to your pass data
   class PassData
   {
       public BufferHandle input;
       // ...
   }

   // Create the buffer in RecordRenderGraph
   public override void RecordRenderGraph(RenderGraph renderGraph, ContextContainer frameData)
   {
       // Create the input buffer as a structured buffer of 20 ints
       BufferDesc desc = new BufferDesc
       {
           name   = "InputBuffer",
           count  = 20,
           stride = sizeof(int),
           target = GraphicsBuffer.Target.Structured
       };

       m_InputBufferHandle = renderGraph.CreateBuffer(desc);
   }
   ```

2. Create placeholder data in the buffer to read from for this example:

   ```lang-cs
   // Placeholder input data for the compute shader
   private List<int> inputData = new List<int>();

   // Initialize in the render pass constructor
   public ComputePass()
   {
       for (int i = 0; i < 20; i++)
       {
           inputData.Add(i);
       }
   }
   ```

3. Attach the buffer handle and data to the pass data.

   ```lang-cs
   using (var builder = renderGraph.AddComputePass("ComputePass", out PassData passData))
   {
       passData.input = m_InputBufferHandle;
       passData.bufferData = inputData;
   }
   ```

4. Use the `UseBuffer` method to declare the buffer as an input of this pass in the render graph system. For example:

   ```lang-cs
   builder.UseBuffer(passData.input, AccessFlags.Read);
   ```

5. In your `SetRenderFunc` method, use the [`SetBufferData`](https://docs.unity3d.com/ScriptReference/Rendering.CommandBuffer.SetBufferData.html) API to upload the input data to the buffer and the [`SetComputeBufferParam`](https://docs.unity3d.com/ScriptReference/Rendering.CommandBuffer.SetComputeBufferParam.html) API to attach the buffer to the compute shader. For example:

   ```lang-cs
   // Upload input data to the Render Graph buffer
   context.commandBuffer.SetBufferData(passData.input, passData.bufferData);

   // Attach buffer to the compute shader

   // The first parameter is the compute shader
   // The second parameter is the function that uses the buffer
   // The third parameter is the RWStructuredBuffer input variable to attach the buffer to
   // The fourth parameter is the handle to the input buffer
   context.commandBuffer.SetComputeBufferParam(passData.computeShader, passData.computeShader.FindKernel("Main"), "inputData", passData.input);
   ```

## Example

For a full example, refer to the example called **Compute** in the [Universal Render Pipeline (URP) package samples](/engine/6000.5/manual/render-pipelines/universal-render-pipeline/introduction/installing-and-configuring-urp/creating-a-urp-project/package-samples.md).

## Additional resources

* [Compute shaders](/engine/6000.5/manual/materials-and-shaders/shaders/writing-custom/class-compute-shader.md)
* [Writing shaders](/engine/6000.5/manual/materials-and-shaders/shaders/writing-custom/shader-writing.md)
* [Write to and read from a compute shader in a render pass](/engine/6000.5/manual/render-pipelines/universal-render-pipeline/customizing-urp/render-graph/compute-shader/run.md)