Anti-aliased Ray Tracing with Covers

Abstract

A fast and effective object space method for antialiasing ray-traced pictures is introduced. Traditionally, anti-aliasing has been done using super-sampling. However, this is costly since it requires casting large numbers of rays to obtain sample densities above the displayed pixel density. It is also wasteful since much of the information in these samples is discarded when they are filtered to yield the anti-aliased pixels. We avoid these problems by performing the filtering in the object space using the geometry of ray-surface intersections rather than by casting extra rays. In addition, we only perform filtering at a pixel if edges are nearby. We detect these edges by observing the order in which the pixel s rays pass through cover surfaces constructed just inside and outside the surface of each object. Shadows, reflections and refractions can be anti-aliased using this method and a variety of object types can be handled including ellipsoids, polyhedra, and objects formed using set operations.Our anti-aliasing gives a high image quality that can only be approached by using super-sampling densities at least four times the display pixel density. Moreover, since the overhead of our method is small, it would take three to four times as long to render an anti-aliased image using super-sampling than it would with our method. Furthermore, covers allow sampling densities less than the displayed pixel density. When this is done, anti-aliased images can be rendered twice as fast as with no anti-aliasing and six to eight times as fast as when super-sampling is used for anti-aliasing.