Authors
Thomas Porter, Tom Duff
Lucasfilm
Portals
Summary
A separate component is needed to retain the matte information, the extent of coverage of an element at a pixel. In a full color rendering of an element, the RGB components retain only the color. In order to place the element over an arbitrary background, a mixing factor is required at every pixel to control the linear interpolation of foreground and background colors. In general, there is no way to encode this component as part of the color information. For anti-aliasing purposes, this mixing factor needs to be of comparable resolution to the color channels. Let us call this an alpha channel, and let us treat an alpha of 0 to indicate no coverage, 1 to mean full coverage, with fractions corresponding to partial coverage.
Abstract
Most computer graphics pictures have been computed all at once, so that the rendering program takes care of all computations relating to the overlap of objects. There are several applications, however, where elements must be rendered separately, relying on compositing techniques for the anti-aliased accumulation of the full image. This paper presents the case for four-channel pictures, demonstrating that a matte component can be computed similarly to the color channels. The paper discusses guidelines for the generation of elements and the arithmetic for their arbitrary compositing.