Graphics subsystem with fast clear capability

Abstract

A graphics subsystem in which a very fast clear operation is performed without the need to address each pixel, and without using memories which include a hardware fast-clear capability. This is implemented by using a reference frame counter: the window is divided up into n regions, where n is the range of the frame counter (i.e. n=2.sup.p, where p is the number of bits in the frame counter). Every time the application issues a clear command, the reference frame counter is incremented (and allowed to roll over if it exceeds its maximum value), and only the n.sup.th region is cleared. The clear updates the depth and/or stencil buffers to the new values and the frame count buffer with the reference value. This region is much smaller than the full region the application thinks it is clearing, so takes less time and hence gives the speed increase. When the local buffer is subsequently read and the frame count is found to be the same as the reference frame count, the local buffer data is used directly. The result of this arrangement is that the cost of clearing the depth and stencil buffers can be amortized over a number of clear operations (as issued by the application).