System for rapidly performing scan conversion with anti-aliasing upon outline fonts and other graphic elements

Abstract

A system rapidly rasterizes high resolution shapes, such as outline fonts, for use in a lower resolution pixel image. For individual pixels a line coverage value is determined for each of at least two sampling lines running in different directions, such as at right angles, within the pixel. The pixel's line coverage value for each line is a function of the degree to which the line is covered by any shapes within the pixel. Then a coverage value is determined for the pixel, itself, as a non-linear function of it's one or more line coverage values running in each of the different directions. Commonly the non-linear function causes the pixel's coverage value to vary more rapidly with variations in the line coverage value of that one of its sampling line which is closest to being half covered within the pixel. Such rasterization can be performed even more rapidly by using two passes, a first which calculates a pixel coverage value for all pixels as a function of the coverage within each pixel of one or more lines of the first set of parallel lines which run in a first direction, and a second pass which varies the pixel coverage value set in the first pass only if there is an intersection between the outline of a shape being rendered and a one or more lines of the second set of parallel lines which run in a second direction.