Z buffer bandwidth reductions via split transactions

In a computer graphics system wherein objects are represented by one or more primitives and wherein pixel data associated with a set S of pixels represents a view of the objects, an apparatus for identifying whether a given primitive is visible at the plurality of pixels, the apparatus comprising:

• a buffer that stores, for each pixel P belonging to set S, a depth value Z.sub.old associated with the pixel P, wherein Z.sub.old comprises a plurality of portions including a most significant portion and at least one less significant portion, wherein said buffer comprises a plurality of contiguous blocks each storing corresponding portions of the depth value Z.sub.old for a given set SP of consecutive pixels belonging to set S;
• scan conversion logic that determines a set SQ of consecutive pixels belonging to set S that cover the given primitive, and that computes, for each pixel Q within the set SQ of consecutive pixels, a depth value Z.sub.new of the given primitive at the pixel Q, wherein the computed depth value Z.sub.new comprises a plurality of portions including a most significant portion and at least one less significant portion, and wherein the portions of the computed depth value Z.sub.new correspond to the portions of the stored depth values;
• and
• fetch logic, coupled to comparison logic and the buffer, for fetching from the buffer the most significant portion of the depth value Z.sub.old of the set SQ of consecutive pixels;
• and
• wherein the comparison logic compares, for each pixel Q within the set SQ of consecutive pixels, the most significant portion of the depth value Z.sub.old of the particular pixel fetched from the block of the buffer associated with the set SQ of consecutive pixels to the most significant portion of the computed depth value Z.sub.new of the particular pixel to determine if one of three conditions is satisfied:
• a) the given primitive is visible at the particular pixel,
• b) the given primitive is hidden at the particular pixel, or
• c) it is undetermined whether the given primitive is either visible or hidden at the particular pixel;
• If condition a) is satisfied, the comparison logic controls the buffer to store the computed depth value Z.sub.new of the particular pixel in the blocks of the buffer associated with the particular pixel;
• If condition b) is satisfied, the processing ends with respect to the particular pixel;
• and
• If condition c) is satisfied, the fetch logic and comparison logic repeat the fetching operation and compare operation, respectively, for the next less significant portion of the depth values Z.sub.old and Z.sub.new associated with the particular pixel until the least significant portion of the depth values Z.sub.old and Z.sub.new has been processed.