ffx_variable_shading.h compute shader

[pemgithub]

Can you explain how the compute shader and related threadgroups work?

From C++ code:

FFX_VariableShading_GetDispatchInfo(data, AdditionalShadingRates(), w, h);
pCmdLst->Dispatch(w, h, 1);

// coarse tiles are potentially 2x2, so each thread computes 2x2 pixels
if (cb->tileSize == 8)
{
    //each threadgroup computes 4 VRS tiles
    numThreadGroupsX = FFX_VariableShading_DivideRoundingUp(vrsImageWidth, 2);
    numThreadGroupsY = FFX_VariableShading_DivideRoundingUp(vrsImageHeight, 2);
}

From VRSImageGenCS.hlsl (static const uint FFX_VariableShading_ThreadCount1D = 8;)

[numthreads(FFX_VariableShading_ThreadCount1D, FFX_VariableShading_ThreadCount1D, 1)]
void mainCS(
    uint3 Gid  : SV_GroupID,
    uint3 Gtid : SV_GroupThreadID,
    uint  Gidx : SV_GroupIndex)
{
    FFX_VariableShading_GenerateVrsImage(Gid, Gtid, Gidx);
}

From ffx_variable_shading.h:

// sample source texture (using motion vectors)
while (index < FFX_VariableShading_SampleCount)
{
    int2 index2D = 2 * int2(index % FFX_VariableShading_SampleCount1D, index / FFX_VariableShading_SampleCount1D);
    float4 lum = 0;
    lum.x = FFX_VariableShading_GetLuminance(baseOffset + index2D + int2(0, 0));
    lum.y = FFX_VariableShading_GetLuminance(baseOffset + index2D + int2(1, 0));
    lum.z = FFX_VariableShading_GetLuminance(baseOffset + index2D + int2(0, 1));
    lum.w = FFX_VariableShading_GetLuminance(baseOffset + index2D + int2(1, 1));
    ...
    index += FFX_VariableShading_ThreadCount;

For example, suppose we have an image that's 1080x3200 with 8x8 tiles, so the VRS image is 135 x 400. Suppose our example supports up to 2x2 coarse pixel size, so numThreadGroupsX is 68, numThreadGroupsY is 200. Why is it 68x200? 4 VRS tiles (2x2 tiles) per thread. Each threadgroup is 8x8 threads and computes 4 VRS tiles (2x2 VRS tiles is 16x16 pixels) (each thread gets luminance for a 2x2 pixels x 8x8 threads = 16x16 pixels). I think that sort of makes sense...

However, what is while (index < FFX_VariableShading_SampleCount) doing? while (index < 100) { do stuff then index += 64 }. What is this while loop doing? Is it okay to comment it out?

static const uint FFX_VariableShading_ThreadCount1D = 8;
static const uint FFX_VariableShading_SampleCount1D = FFX_VariableShading_ThreadCount1D + 2; // 10
static const uint FFX_VariableShading_SampleCount = FFX_VariableShading_SampleCount1D * FFX_VariableShading_SampleCount1D; // 100
static const uint FFX_VariableShading_ThreadCount = FFX_VariableShading_ThreadCount1D * FFX_VariableShading_ThreadCount1D; // 64