FalconFly
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Technically, the CPU will never 'take over rendering'...
The point at which the CPU or any other component will start idle cycles due to another component still busy is highly varying and changes per Application used. That's why there isn't any known, fixed limit, such as "a Voodoo5 stops scaling @ AthlonXP 2500+" for example. In some scenarios, scaling will cease as low as 500MHz, in others we'll still see mild improvements exceeding 2500MHz.
glQuake I for example will likely scale linear with CPU and eventually top-off at the Fillrate limit. Depending on Resolution, latter will come sooner or later.
Unreal Tournament for example will almost never hit the fillrate limit, its first limit will be either CPU Software T&L, Driver Geometry throughput or corresponding PCI/AGP Data rate limit.
Going further uphill, UT2003 for example will never hit fillrate limit, depending on Level and Texture Detail it'll be all either VRAM limit or CPU/PCI/AGP or Driver Geometry throughput limit.
Eventually (Doom3 / Quake 4), you'll hit a scenario with exclusive Geometry and VRAM limits, almost regardless of CPU used (it'll still creep up due to heavy T&L workloads, but basically scale very bad compared to amount of CPU power added).
Those 3dfx Drivers nor Cards were never made to send such insane amounts of T&L Data through the Driver onto the Card. Still, they do quite an amazing job, considering how relatively well it still works out.
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