When drawing ASCII art, each character can be treated as a pixel. It will have a particular value which determines it's brightness and so on (at least when programatically drawing an ASCII image).
This will determine the values of a large number of pixels, and will generate a table from value (0 -> 255) to a rune value which can be printed on-screen.
To try out the examples, simply go run example/*.go with whichever
file you please. The image-to-ascii.go demonstrates a practical use.