Workbench.tv’s Joe Clay offers a quick demonstration for creating glitch-type of effects in After Effects. Joe uses a black and white matte for the time displacement that will affect an animated layer. Before that matte, Joe adds a posterize effect that can limit the amount of colors that are being used.
This can make for some really interesting effects when using the time displacement effect. It is also an easy and quick way to make a matte, using gradients and the posterize effect.
A matte just needs to be a greyscale image. This means that it can be generated by any means. As an example, you can create interesting glitch effects using the Fractal Noise and time displacements.