I used to write very short programs for the Amiga that would scan through all the pixels on the screen and assign a color to them based on whatever math functions I felt like writing. They took hours, sometimes days to run, but the results were often interesting, sometimes radial gradients, sometimes random noise, sometimes things that looked like musical notes on a random background. Then I'd load a color table and color cycle the resulting image. Sometimes this created movement in the image, sometimes it animated the noise so it looked like old-fashioned TV static. Just did it for fun, only took a few minutes to write the code, and sometimes the results were great. It was often very difficult to predict what the final image would look like, unless I started with a program I'd run before.