11 - Filling Primitives

Abstract

The most useful fill routines are for filling ellipses and polygons, plus one routine is needed for odd shapes that are not ellipses or polygons. SVGALib also includes a function for filling rectangles, because a rectangle filling function is quite trivial to implement. It is useful for filling primitives to support pattern fill. The functions provided by SVGALib allow the user to set up a pattern much the same way that Microsoft graphics functions do. A bit map in the form of an array of unsigned characters is set up and passed to SVGALib through the SVGA_set_fill_pattern( ) function. Both SVGAJilLrectangle( ) and SVGAJill_ellipse( ) are quite simple functions. Filling polygons is not an easy thing for computers to do. Human beings can look at a polygon and see whether a pixel is inside the polygon or not. A computer does not have access to this world view. Therefore, the polyfill algorithm needs to start at the left-hand side of the polygon and scan across, looking for the right side of the polygon. The data structure used by SVGA_polyfill( ) is a binary tree with the key field being the Y coordinate. Each node in the tree contains a sorted list of X values. The Y values are not needed in the list, because they all use the same Y value, which is the key field for the tree.