ATI's Next Gen Geometry Enhancement

ATI officially announced its TRUEFORM rendering technology today. ATI has made no secret of this technology and you can find plenty of ATI speak on the subject here , including some indignant digs at Nvidia's counter-marketing to the game developer community:

"It is a disservice to you for our competition to use you as a lever against us by trying to trick you into ignoring a compelling feature which is a standard part of DirectX (need we also mention the propaganda against using user clip planes, EMBM or a third texture?)."

Quote from ATI Developer Relations Newsletter - April 2001

This all leads us to believe that ATI might be starting to get more aggressive with its next generation hardware, even though the company may not be in a position to deliver that hardware to support its claims until at least the end of the year.

As far as the technology is concerned, TRUEFORM takes the standard triangle data fed into the 3D pipeline of a graphics chip, and adds curve point-normal (PN) triangles into the equation to create a different level of shading. ATI takes the standard triangle primitive data of a model, and adds a triangular Bezier patch over it. This, in effect, requires the ATI hardware to create a curved surface composed of a number of smaller triangles as a patch over the original triangle.

By adding the curved patch on the original triangle, ATI's hardware can create a more appealing shading of the model. So, instead of calculating the shading of a simple triangle, the hardware is creating a curved surface that gives a greater amount of detail to the area. In the figure below, (from left to right ) (a) Input triangulation, (b) Gouraud shaded input triangulation, (c) geometric component of the PN triangles (shaded according to surface normal variation) (d) curved PN triangles (shaded with independently constructed quadratically varying normals) are demonstrated, and you can see the clear advantages of ATI's approach. Just look at the lighting fo the forehead from one image to the next to see the impact.

You can find a technical paper PDF that explains the theory behind curved PN triangles here , including more images, and an in-depth discussion of the mathematics.

From the same presentation, the diagram below shows the curve PN triangle tessellation process' position in the 3D pipeline.

This technology doesn't have to be proprietary to ATI because, the feature is available in DirectX 8.0 as a higher-order primitive much the same as rectangular and triangular (RT) patches, or B-splines. ATI has implemented the use of PN triangles in hardware, and is obviously aware of the need to create more awareness for its technology and expertise. So far, Nvidia has had a great deal of leeway and input into DirectX, helped by its involvement in the Xbox.

As for performance issues, higher-order primitives obviously means more vertices, meaning more data has to be generated from triangle setup. ATI has a number of arguments regarding performance:Your CPU is going to be the limiting factor on how many vertices are fed to the graphics chip so, if the CPU is fast enough, the bottleneck is not the graphics processor, which has the cycles to give.Game developers could view this approach as a form of "geometry compression". ATI's argument here is that since the lowest primitive delivered is a single, flat triangle, PN triangles could be used as a mechanism for gaining greater levels of detail (LOD) out of a model, or scene, with minimal impact on storage and bandwidth costs. In effect, if you want to up the quality of your image, increase the LOD, you can use ATI's hardware to do that in graphics hardware using PN curves, and not sacrifice too much in the way of setup.

It's too early to tell what the impact is going to be on the actual consumer of ATI TRUEFORM products. Not only does ATI need to win over game developers, but it will have to show practical benefits in actual game play. But, it is certainly an interesting technology, and a strong indication of ATI's technology positioning for its next generation hardware.