Compute Shader Creation

This guide walks you through the process of creating and utilizing compute shaders in DirectX. Compute shaders offer a powerful way to leverage the parallel processing capabilities of the GPU for general-purpose computation, not just graphics rendering.

What are Compute Shaders?

Unlike traditional graphics pipeline shaders (vertex, pixel, geometry, hull, domain), compute shaders are designed for arbitrary computations. They can read and write to various resources like textures, buffers, and UAVs (Unordered Access Views), enabling a wide range of non-graphics tasks such as:

• Physics simulations
• Image processing and filtering
• Particle systems
• Data sorting and analysis
• Machine learning inference
• Ray tracing acceleration structures

Creating a Compute Shader

Compute shaders are written using High-Level Shading Language (HLSL). The core function of a compute shader is typically defined with the num_threads attribute, which specifies the group size the shader will execute in.

Basic HLSL Compute Shader Structure

// Define the thread group size
[num_threads(8, 8, 1)]
void CSMain(uint3 dispatchThreadID : SV_DispatchThreadID)
{
    // 'dispatchThreadID' is the unique ID of the current thread within the dispatch call.
    // You can use this ID to access specific elements in your data structures.

    // Example: Accessing a buffer and performing a computation
    // Assume 'myBuffer' is a StructuredBuffer or RWStructuredBuffer
    // and 'myTexture' is a Texture2D or RWTexture2D

    // Read from a buffer
    // MyDataType data = myBuffer[dispatchThreadID.x];

    // Perform computation...
    // float result = data.value * some_constant;

    // Write to a buffer or texture
    // myBuffer[dispatchThreadID.x] = newValue;
    // myTexture[dispatchThreadID.xy] = computedColor;
}
            

Key Elements:

• [num_threads(x, y, z)]: This attribute defines the number of threads that will be executed concurrently within a thread group. The values typically range from 1 to 256 for each dimension, but the total number of threads per group cannot exceed 256.
• SV_DispatchThreadID: This semantic identifies the unique ID of the thread within the entire dispatch call. It's a 3-component unsigned integer vector (uint3) representing the thread's index in the x, y, and z dimensions.
• Input/Output Parameters: Compute shaders can take various input resources (e.g., StructuredBuffer<T>, Texture2D<T>) and write to output resources (e.g., RWStructuredBuffer<T>, RWTexture2D<T>).

Dispatching a Compute Shader

In your C++ application code, you'll bind the compute shader and its associated resources to the graphics pipeline and then issue a dispatch call.

Example C++ Dispatch Code Snippet:

// Assume 'pComputeShader' is a pointer to your compiled compute shader object
// and 'pComputeState' is a pointer to your graphics pipeline state object

// Bind the compute shader and pipeline state
m_deviceContext->CSSetShader(pComputeShader.Get(), nullptr, 0);
m_deviceContext->SetPipelineState(pComputeState.Get());

// Bind resources (e.g., buffers, textures) to shader resource views (SRVs)
// and unordered access views (UAVs)
// m_deviceContext->CSSetShaderResources(...)
// m_deviceContext->CSSetUnorderedAccessViews(...)

// Define the number of thread groups to dispatch
// This is typically calculated based on your data size and shader's thread group size
UINT numGroupsX = (dataSizeX + THREAD_GROUP_SIZE_X - 1) / THREAD_GROUP_SIZE_X;
UINT numGroupsY = (dataSizeY + THREAD_GROUP_SIZE_Y - 1) / THREAD_GROUP_SIZE_Y;
UINT numGroupsZ = (dataSizeZ + THREAD_GROUP_SIZE_Z - 1) / THREAD_GROUP_SIZE_Z;

// Dispatch the compute shader
m_deviceContext->Dispatch(numGroupsX, numGroupsY, numGroupsZ);

// Unbind resources and shader to prevent unintended side effects
// m_deviceContext->CSSetShaderResources(0, 0, nullptr);
// m_deviceContext->CSSetUnorderedAccessViews(0, 0, nullptr, nullptr);
// m_deviceContext->CSSetShader(nullptr, nullptr, 0);
            

Resource Binding and Semantics

The connection between your HLSL shader and your C++ code is established through resource binding. You'll use specific views to expose your application's data to the compute shader:

• Shader Resource Views (SRVs): For reading data.
• Unordered Access Views (UAVs): For reading and writing data.
• Constant Buffers: For passing constant parameters to the shader.

The semantic SV_DispatchThreadID, along with SV_GroupThreadID and SV_GroupID, are crucial for coordinating work across threads within a group and across the entire dispatch.

API Reference

Further Reading

• Compute Shader Optimization Techniques
• Practical Compute Shader Examples
• Understanding Shader Types