ATi RADEON - The Empire strikes back (4/15)

There is, of course, one feature I would like to mention seperately: Hyper-Z.

This is ATi’s attempt at reducing the amount of traffic on the memory bus, the main bottleneck for graphics cards today.

The starting point is the Z-buffer. You can find it in all of the "classical" 3D cards. Its job is to determine which objects are visible to the viewer and which aren’t because they’re hidden by other objects, and then to eliminate the "invisible" ones. As you can see in the picture below, the 3D card draws much more than you ever actually get to see in the completed frame – this so-called overdraw includes memory hogging textures.

Click to enlarge - Transparent Walls courtesy of (the controversial) ASUS See Through - Much more is rendered than is really visible

Hierachical Z
ATI’s trick is to change the order of the rendering process. Normally, the first step would be to draw all polygons including textures and filtering. It isn’t until the next step that the Z-buffer determines the order in which the objects are arranged, and which therefore are or aren’t visible.Hierarchical Z changes this order by doing the Z-buffer check before the objects are textured. This reduces the number of renderingsteps (texturefiltering) and the amount of memory usage for every frame.

The conventional way - RADEON is doing the Z-Test (Depth Test) before the first step

Z-Compression
This is a simple lossless compression algorithm that sends the Z data over the memory bus in compressed form – in realtime - additionally reducing the traffic on the memory bus.

Fast Z Clear
Once a frame has been rendered, the next step is clearing the Z-buffer in an extra cycle. The conventional way of doing this would be to reset each memory address to 0. Fast Z Clear allows a quick reset of all addresses at once, theoretically making the clearing process 64 times faster.

ATi promises a reduction in memory bandidth usage of up to 20% through the usage of these collective Hyper Z techniques. 3D chips like STMicro’s Kyro can do without these techniques altogether. Their Tile-Based Rendering doen’t produce overdraw in the first place.

Copyright: 07.08.2000 -   RIVA Station 2000 - Lars Weinand
No Copy without Permission!

Translation by Benjamin Kraft

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