Parts of this section were originally published in our RADEON 64MB DDR Guide:

After an application draws a 3D scene, the depth buffer (or Z-buffer) has to be cleared so that the next frame can be drawn. Traditionally, the processor would write 0's straight across the buffer, consuming just as much bandwidth as an actual depth value would. To combat this expensive waste, ATI developed a method of clearing the Z-buffer without writing values to each of the memory locations called Fast Z Clear. We can do the math to get a better idea of just how much memory is being saved using the first component of HyperZ.

Let's say we're running Quake III at 1600x1200 in 32-bit color. Using the results from our Quake III MAX quality test on Test System #1, 30.6 frames per second, we derive the following equation:

(1600x1200x(32/8)x30.6)=235MB/s

In addition, since the processor already knows the value written to each memory location, there is no need to execute the first fetch for the Z comparison. This effectively doubles the result of our equation, meaning the RADEON saves 470MB/s of Z data from being transferred.