Fill 'er up! Graphics filling algorithms

One of the most important and useful ideas in 2D computer graphics is the fill. Simple in concept but powerful in practice, filling is so basic a technique that there's a fill tool in just about every 2D drawing system sold today. Probably one of the first important uses of fills was to color in 2D cartoon characters, but that's just the tip of the iceberg. It's hard to know who wrote the first fill routine, but it may have been Dick Shoup, who implemented a fill technique for his Superpaint system in the mid-1970s. The next big step was Alvy Ray Smith's 24-bit flood-fill algorithm, which also included tint fill. Change came fast after that point, culminating in the sophisticated filling tools we have today. In this article, I discuss some fill techniques that I've cooked up over time to solve different jobs. I focus on techniques for identifying fill pixels.