There are two interpolation qualifiers within the GLSL that allow the programmer to fine-tune some aspects of multisampling. They are sample and centroid.
Before we can get into how sample and centroid work, we need a bit of background. Let's consider the way that polygon edges are handled without multisampling. A fragment is determined to be inside or outside of a polygon by determining where the center of that pixel lies. If the center is within the polygon, the pixel is shaded, otherwise it is not. The following image represents this behavior. It shows pixels near a polygon edge without MSAA. The line represents the edge of the polygon. Gray pixels are considered to be inside the polygon. White pixels are outside and are ...
