I was reading up on Bit Arrays to learn more secret bitwise knowledge, and stumbled onto a very clever way to count bits, via Hamming weights and a magic multiplication number. Short story is that this led to efficient fullscreen Anti-Aliasing for 1-bit screen data (colors 0 and 1 only).
Other convolution shaders are easily doable with similar techniques. This one takes every pixel and adds weighted amounts of the neighbors in this way: