About Anti-Aliasing

In the discipline of signal processing, there is a specific sampling frequency called the Nyquist frequency. When sampling a signal at a frequency lower than the Nyquist frequency, the signal reconstructed from these samples is different to the original. The difference between the original and the reconstructed signals manifests as artefacts. This effect is called “aliasing”.

Two of the most common manifestations of aliasing in computer graphics are discussed here. The first is “jaggies” (figure 1 below) where what is intended to be a diagonal straight line appears as a jagged staircase. This is introduced by the “rasterizer stage” of the graphics card rendering “in between” pixels as either on or off.

Aliasing and anti-aliasing.

The second is sampling aliasing. This is when a high frequency (rapidly changing) texture is being used to render something in the distance, such as the checkerboard pattern in Figure 2, and appears to create visible artefacts known as moiré patterns.

Aliasing artifacts in a compressed space.

There are several algorithms used to combat the amount of visible aliasing and get rid of the staircase artefacts. However, these algorithms might incur a moderate performance cost depending on the hardware that it is being deployed on.

  • Super-sampling anti-aliasing (SSAA). This is a brute-force technique that renders the whole scene in a resolution that is an integer multiple (x2, x4) than the final one, and then down samples the whole framebuffer to produce the desired final resolution. The result is excellent quality and improves both jaggies and texture sampling artefacts, but the technique is obviously extremely expensive. The costs of rendering – including raster ops, fragment ops and associated bandwidth – are multiplied by the square of the resolution multiplier i.e. x4 for x2 SSAA, x16 for x4 etc

  • Multi-sampling anti-aliasing (MSAA). This technique increases the number of samples taken per pixel by rendering the image to a buffer, which is able to store multiple samples per pixel. It then resolves that buffer to produce an output that matches the viewport resolution. This is very efficient on PowerVR hardware as the resolve is performed on-chip, saving precious memory bandwidth. MSAA improves jaggies but not sampling artefacts.

  • Shader-based techniques such as fast approximate anti-aliasing (FXAA) or sub-pixel morphological anti-aliasing (SMAA). Both of these techniques use analytics to detect and blur sharp geometric features. They are also post processing algorithms which are performed in screen space, and generally, have a fixed cost (a single full-screen pass) but require more memory bandwidth, which is usually at a premium on mobile and embedded devices.