Method and apparatus for controlling compressed z information in a video graphics system that supports anti-aliasing

Abstract

A method and apparatus for managing compressed Z information in a video graphics system that supports anti-aliasing is described. Each pixel in the display frame is represented with a primary Z value, a secondary Z value, a first and second color, and a pixel mask that indicates how the Z values and colors apply to the samples of the pixel. The primary Z values for the pixels in a pixel block are then compressed using a compression algorithm and stored in a Z buffer in a compressed format. A secondary mask that indicates which pixels in the pixel block have valid secondary Z values is also stored in the Z buffer, along with the secondary Z values and the pixel masks in an uncompressed format. A Z mask value for each pixel block in the frame is stored in a Z mask memory, where the Z mask for each pixel block indicates the level of compression of the Z information the corresponding pixel block. When Z information for a pixel block is required for processing operations, a cache is first examined to determine if the Z information for the pixel block is included in the cache. If the Z information is not included in the cache, the Z mask memory is consulted to determine the level of compression of the Z information for the particular pixel block. Based on the indication provided by the Z mask memory as to the level of compression, a predetermined amount of buffered Z information is retrieved from the Z buffer, and when the Z information is in compressed format, it is decompressed. The fetched Z information is then stored in the cache for use in video graphics processing.