GPGPU

General purpose GPU (GPGPU) involves using a graphics card for general purpose computation. It used to be that one would have to perform computations in the pixel shader and render geometry to execute it, which isn't the best abstraction. Recently, there have been developments which allow for proper abstraction: OpenCL, Direct3D 11 Compute, CUDA, etc.

In this example, I have chosen to focus on Direct3D 11 Compute; many of the same ideas apply to the other languages as well. Direct3D 11 Compute is a new type of shader in D3D11 which allows for the explicit usage of shared memory, scattered writes, etc.

As a simple example, say we want to use the compute shader to produce a procedural texture.

RWTexture2D<float4> texrw : register(u0);

[numthreads(1,1,1)]
void CS(uint3 id : SV_DispatchThreadID)
{
    float4 color;

    color.r = id.x / 255.0f;
    color.g = (id.x + id.y) / 510.0f;
    color.b = (sin(id.x * id.y) + 1.0f) / 2.0f;
    color.a = 1.0f;

    texrw[id.xy] = color;
}

Let's take a look at the application-side code.

    ID3D11UnorderedAccessView *nullcsview[] = { NULL };
    g_pd3dDC->CSSetUnorderedAccessViews(0, 1, &texrwv, NULL);
    g_pd3dDC->CSSetShader(cs, NULL, 0);
    g_pd3dDC->Dispatch(256, 256, 1);
    g_pd3dDC->CSSetUnorderedAccessViews(0, 1, nullcsview, NULL);

There isn't much going on here; I didn't even need to use a compute shader for this. However, it does illustrate one important concept: scattered writes. Notice how I am writing a value to an explicit texel in the texrw texture.

Running this compute shader kernel with 256x256x1 groups and using a simple pixel shader to texture a quad, the results are as follows: