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Sample Types

Themost straightforward sampling-based approach to anti-aliasing animage is to perform all computations that would be done for a "real"pixel for each sample. While this is highly effective at removing allkinds of aliasing artifacts, it is also a very computationallyexpensive method, as it increases the pixel shading, rasterization,memory bandwidth and memory capacity requirements N-fold for Nsamples. Techniques where all computations are carried out for eachindividual sample in this fashion are calledsuper-samplinganti-aliasing (SSAA).

Aroundthe turn of the century,multi-sampleanti-aliasing (MSAA) was introduced ingraphics hardware as an optimization of super-sampling. Unlike theSSAA case, with MSAA each pixel is only shaded once. However, depthand stencil values are computed once per sample, allowing for thesame anti-aliasing quality regarding geometry edges as SSAA at asignificantly lower performance cost. Also, further performanceimprovements, especially regarding memory bandwidth usage, arepossible if Z and colour buffer compression are supported – this isthe case on all modern GPU architectures. Due to the way sampling isoptimized, transparency, texture and shader aliasing cannot bedirectly addressed using MSAA.

Athird type of sampling was introduced by NVIDIA in 2006 withcoveragesampling anti-aliasing (CSAA).LikeMSAA decouples shading from the per-sample evaluation of depth andstencil values, CSAA introduces additionalcoveragesamples, which do not store any colour, depth or stencil values butonly a binary coverage value. These binary samples are used to guidethe blending of existing MSAA samples. As such, CSAA modes addcoverage samples to MSAA modes, but it does not make sense to performcoverage sampling without also producing multiple MSAA samples. ThreeCSAA modes exist on modern NVIDIA hardware: 8xCSAA (4xMSAA / 8coverage samples), 16xCSAA (4x/16), 16xQCSAA (8x/16) and 32xCSAA(8x/32). AMD offers a similar implementation with 4x EQAA (2x/4),8xEQAA (4x/8) and 16xEQAA (8x/16). The additional coverage samplesusually only have a small impact on performance.

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