Multisampling

One of the problems with bit-mapped graphics is “aliasing”. This is the phenomenon by which pixels along the edge of a pure shape will seem either to belong to that shape or not so, resulting in an edge which has rectangular errors. Even at fairly high resolutions, this can lead to a low-quality experience. And so schemes have been devised since the beginning of digital graphics, to make this effect less pronounced, even if we do choose raster-graphics.

3D has not been left out. One of the strategies which has existed for some time, is to Super-Sample each screen pixel, let us say by subdividing it by a fixed factor, such as 4×4 sub-pixels. This is also known as “Full-Screen Anti-Aliasing”, or ‘FSAA’. The output of the sub-pixels can be mixed in various ways, to result in a blended color-value for the resulting screen pixel.

But one problem with FSAA has been from the start, that it slows down rendering a whole lot. And so an alternative was devised, called Multi-Sampling.

The main idea behind Multi-Sampling is, that only the screen-pixels that span a triangle-edge, are objectionable in the degree with which they suffer from aliasing. Therefore, most of the screen-pixels are not super-sampled. And, the limited logic of the GPU has a hard time trying to distinguish, which triangle-edges are also model / entity -edges, where aliasing does the most damage. But, because the GPU has specialized logic circuits, which are referred to somewhat incorrectly as one render-output generator, that rasterize a given triangle, those circuits can be expanded somewhat feasibly into also being able to detect which screen-pixels do straddle the edge between two triangles. And then, for the sake of argument, only those may be subdivided into 4×4 sub-samples, each of which is Fragment-Shaded once.

But the logic gets just a bit more complicated. There is no simple way, in which the render-output generator can know, which other triangle a current triangle borders on. This is because in general, once each triangle has been processed, it is forgotten. Once each Geometry Shader input-topology has been processed, it too is forgotten, and the GS proceeds to process the next input-topology with complete amnesia…

Continue reading Multisampling