Scan Line Polygon Fill Algorithm

The scanline fill algorithm is an ingenious way of filling in irregular polygons. The algorithm begins with a set of points. Each point is conected to the next, and the line between them is considered to be an edge of the polygon. The points of each edge are adjusted to ensure that the point wih the smaller y value appears first. Next, a data structure is created that contains a list of edges that begin on each scanline of the image. The program progresses from the first scanline upward. For each line, any pixels that contain an intersection between this scanline and an edge of the polygon are filled in. Then, the algorithm progresses along the scanline, turning on when it reaches a polygon pixel and turning off when it reaches another one, all the way across the scanline. 

Text Source: http://www.sccs.swarthmore.edu/users/02/jill/graphics/hw3/hw3.html