Method and apparatus for compositing colors of images with memory constraints for storing pixel data

Abstract

A method and an apparatus determine a color for pixels in a graphics system in which images are defined by pixels. Multiple fragments of an image may be visible in any given pixel. Each visible fragment has a fragment value that includes the color of that fragment. For such given pixel, up to a predetermined number of the fragment values are stored. When a new fragment is visible in the given pixel, one of the fragment values is discarded to determine which fragment values are stored and subsequently used to generate the color of the pixel. The discarded fragment value may be the new fragment value or one of the stored fragment values. Various strategies can be used to determine which fragment value is discarded. One such scheme selects the stored fragment value with the greatest Z-depth. Another scheme selects the stored fragment value that produces the smallest color difference from the new fragment value. Still another scheme selects the new fragment value when one of the fragments is in front of the new fragment and the stored fragment value of that fragment produces the smallest color difference from the new fragment value.