The Radeon 8500 chip is ATI's answer to the GeForce3. Like the original Radeon, it comes to market months behind the competing NVIDIA product. Also like original Radeon, the 8500 implements nearly everything the competing NVIDIA product can do, plus some additional functionality. This strategy of coming behind NVIDIA and trying to leapfrog them is tricky, but it almost seems to be working for ATI.
The Radeon 8500 is a case in point. The Radeon 8500 and GeForce3 chips are very similar to one another in many ways. They share the same basic configuration: four pixel pipelines with two texture units per pipeline. In its retail boxed form, the Radeon 8500 runs at 275MHz, with a 550MHz (DDR) memory clock, which is just slightly faster than the GeForce3 Ti 500 at 240/500MHz. Both chips implement occlusion detection and other memory bandwidth-saving techniques. Also, the Radeon 8500 GPU implements vertex and pixel shaders much like the GeForce3.
However, ATI claims that the Radeon 8500 implementations of vertex and pixel shaders both include improvements over the "DirectX 8.0" (read: GeForce3) implementations. To give you a sense of where ATI is coming from, let me give you an overview of the Radeon 8500.
The Radeon 8500
By my count, ATI has coined eleven marketing buzzwords in order to describe the Radeon 8500. Like the name RADEON, these terms are supposed to be written in all caps. I DON'T KNOW WHY. Just for the record, here are the terms:
TRUFORM, SMARTSHADER, SMOOTHVISION, CHARISMA ENGINE II, PIXEL TAPESTRY II, VIDEO IMMERSION II, HYPER Z II, HYDRAVISION.
Those are some fine marketing terms, and they really jump out in all caps, don't they? I will try to cover all the concepts wrapped up in these marketing terms, but it's probably best to label the Radeon 8500's features for what they are. Here are some of the highlights:
The Radeon 8500 also includes an entire fixed-function T&L unit (PIXEL TAPESTRY II), which can operate in parallel with the 8500's vertex shader. The GeForce3, by contrast, implements its backward-compatible fixed-function T&L capability as a vertex shader program.
For instance, the Radeon 8500 can apply six textures to a pixel per rendering pass. That statement may seem a bit perplexing, because the Radeon 8500 has only two texture units per rendering pipeline. Here's the distinction: the chip can apply two textures per clock cycle, but it can "loop back" and apply two more textures in the next cycleand two more in the nextall in the same rendering pass. The GeForce3 uses a similar "loopback" method, but it can only apply four textures per pass. In order for the GeForce3 to render shader effects that require six texture operations per pixel, the chip has to do part of the work, write an image out to the frame buffer, and complete its work in a second rendering pass. Multipass rendering saps performance and potentially degrades image quality.
The Radeon 8500's pixel shaders use a different instruction set than the GeForce3's; ATI claims its instruction set is simpler yet more powerful than NVIDIA's. Also, ATI's pixel shader programs can be as long as 22 instructions, while the GeForce3 is limited to 12. Finally, the 8500's pixel shaders can perform operations on texture addresses as well as on color values, potentially allowing much more advanced shader techniques than the GeForce3.
Trouble is, NVIDIA and ATI are backing competing standards that are incompatible with one another. NVIDIA supports polynomial surfaces, and ATI employs N-patches, a.k.a TRUFORM. TRUFORM has the advantage of working reasonably well in current games, provided the developers are willing to enable support for it. No additional information is required, because TRUFORM smoothes out existing 3D models by adding polygon detail.
However, TRUFORM adds detail to 3D models bykind ofguessing. It looks at existing, low-detail 3D models, generates a higher-order surface, and then rebuilds the models with more polygons. It can make a rounded surface look much smoother, but it can make an object built from flat surfaces look, well, puffy.
Still, it's a neat trick, and I'll betcha it works well most of the time.
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