Z buffer with degree of visibility test

Abstract

A solution to the problem of determining the degree of visibility for a bounding volume is to first sort the bounding volumes into an order in which they can be tested, after which they are rendered as if they were polygons in their own right. Prior to the testing of the next bounding volume a Bounding Volume Visibility (BVV) mode is enabled. This temporarily suspends write operations to the frame buffer and it resets a TOTAL PIXEL COUNTER and a VISIBLE PIXEL COUNTER. Then the polygon sides of the bounding volume are rendered, with each and every pixel of those polygons being checked for depth, as would be the pixels for 'real' polygons. Each such pixel rendered causes an increment of the TOTAL PIXEL COUNTER, while each such pixel that meets the depth criteria in use causes an increment of the VISIBLE PIXEL COUNTER. The BVV mode is subsequently turned off, and the values of the two counters inspected. The inspecting entity may be software associated with the graphics application. It can decide whether or not to render the image polygons within the bounding volume just tested. The values of the counts in the TOTAL PIXEL COUNTER and the VISIBLE PIXEL COUNTER can be of assistance in this decision. That is, if the count in the VISIBLE PIXEL COUNTER is small in comparison to that in the TOTAL PIXEL COUNTER, then it may be desirable to programmatically ignore the image polygons in that bounding volume. On the other hand, if the counts are anywhere near equal, then it is probably wise to render those image polygons. The BVV mode is then turned back on at the start of the next bounding volume in the sorted list thereof, and so on.