Sample shading can be used to specify a minimum number of unique samples to process for each fragment. If sample shading is enabled, an implementation must invoke the fragment shader at least max(minSampleShading × rasterizationSamples, 1) times per fragment. If sampleShadingEnabled is set to TRUE, sample shading is enabled.
If a fragment shader entry point statically uses an input variable decorated with a BuiltIn of SampleId or SamplePosition, sample shading is enabled and a value of 1.0 is used instead of minSampleShading. If a fragment shader entry point statically uses an input variable decorated with Sample, sample shading may be enabled and a value of 1.0 will be used instead of minSampleShading if it is.
If a shader decorates an input variable with Sample and that value meaningfully impacts the output of a shader, sample shading will be enabled to ensure that the input is in fact interpolated per-sample. This is inherent to the specification and not spelled out here – if an application simply declares such a variable it is implementation-defined whether sample shading is enabled or not. It is possible to see the effects of this by using atomics in the shader or using a pipeline statistics query to query the number of fragment invocations, even if the shader itself does not use any per-sample variables.
If there are fewer fragment invocations than covered samples, implementations may include those samples in fragment shader invocations in any manner as long as covered samples are all shaded at least once, and each invocation that is not a helper invocation covers at least one sample.