Draw a shape on the screen around the center point, then press space to activate the fill from the center of the screen.
This is an example of a recursive flood fill algorithm ( http://en.wikipedia.org/wiki/Flood_fill ). In terbo mode it goes quicker, but not as quick as it should because the added speed causes errors which means it takes for ever (it still works perfectly though). If you run it at normal speed you can see how it is meant to do it.
