Deferred Rendering - Practical Rendering and Computation with Direct3D 11 - page 530

// Light pixel shader that executes once per MSAA subsample
float4 PSMainPerSample( in float4 screenPos : SV_Position,
in uint subSamplelndex : SV_SampleIndex ) : SV_Target0
{
float3 normal;
fioat3 position;
float specularPower;
// Get the G-Buffer values for the current sub-sample, and calculate
// the lighting
GetGBufferAttributes( screenPos.xy, ViewRay, subSamplelndex,
normal, position, specularPower );
return CalcLighting( normal, position, specularPower );
}
Listing 11.18. Per-sample light buffer pixel shader code.
Naturally, running pixel shaders at per-sample frequency incurs a higher overhead
than running them per-pixel. To mitigate this effect, the stencil mask technique mentioned
earlier can be used to mask off pixels that don't need to be shaded per-sample.
For the final pass, we no longer need to output separate values per-sample. Instead,
we want to sample the light buffer for all relevant subsamples, filter the results, and finally
apply the albedo terms. Thus, running the pixel shader at per-pixel frequency and relying
on the coverage and depth/stencil tests is perfectly adequate. However, we must be careful
not to inadvertently apply lighting from subsamples not covered by the triangle currently
being rasterized, or artifacts will occur. To avoid these artifacts, we can make use of the
input coverage mask, much like the lighting pass of a classic deferred Tenderer. The code
in Listing 11.19 demonstrates how to do this.
float4 PSMain( in PSInput input, in uint coverageMask : SV_Coverage ) : SV_Target0
{
// Determine our coordinate for sampling the texture based on
// the current screen position
int2 sampleCoord = int2( input.ScreenPos.xy );
float3 diffuse = 0;
float3 specular = 0;
float numSamplesApplied = 0.0f;
// Loop through the MSAA samples and modulate diffuse and specular
// lighting by the albedo
for ( uint i = 0; i < NUMSUBSAMPLES; ++i )
{
// We only want to apply a lighting sample if the geometry we've
// rasterized passed the coverage test for the corresponding
// MSAA sample point
if ( coverageMask & ( 1 << i ) )