Instead, one can render multiple samples around a fragment being rendered.  The number of samples is configurable, as well as their locations.

Multi-sampling anti-aliasing (MSAA) generates several fragments with slightly different sub-pixel locations, and averages the result.

MSAA runs the fragment program just once per fragment rasterised.

The locations are pseudo-randomly positioned throughout the pixel region.


Source: AnandTech

No multi-sampling vs. 8x MSAA (Zoomed 200%)

High