Filters are a post-calculation effect applied to the resulting image. They
can do things like motion blurring, edge detection, emulation of palettes
or truecolor on displays that can't handle them, and such things. There is
a tutorial chapter about them.
Filter command
Edge detection
Edge detection2
Starfield
Random dot stereogram
Interlace filter
Motion blur
Emboss
Palette emulator
Antialiasing
Truecolor emulator