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Old 07-10-08, 11:32 PM   #61
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Join Date: Mar 2003
Location: Tulsa
Posts: 5,101
Default Re: The right price for the GTX200 family...

AA Z Cycles is a condition of the ROPS. Its affiliated with bandwith but not entirely. Bandwith helps maximum amounts of fillrate/texel rate through the pipeline as fast as possible. When you use fewer cycles. You actually need less bandwith to do the same task. For example. The GTX 280 has a huge texel fillrate for bilinear and AF. To achieve its bilinear texel fillrate. It needs bandwith to feed texture units. Thats why. Despite the 9800GTX having a super texturing performance. In relation to the GTX 260. It cant match its performance due to a lack of bandwith. Of course thats a pretty simplistic explanation. There are obviously other factors. Just having more bandwith doesnt neccasarily mean your going to use it all. There are all kinds of things which can impact bandwith advantages and disadvantages.

Running out of memory is completely unrelated to the two though. The 8800 GTX had a large bandwith advantage over the 9800GTX. But it didnt always translate into better performance despite its superior pixel and bandwith. Typically when the 9800GTX failed in comparison to the 8800GTX it was actually the memory bottlenecking the card.

AA Passes relate to how many times the AA data must cycle through the ROPS to reach final rendering. In the old days went like this.

2x 1 Cycle
4x 2 Cycles
6x 3 cycles ((For ATI))

The Geforce 8800GTX did both 2x and 4x in 1 cycle. So 2x and 4x AA performance were largely identical when transparency SS or memory bottlenecks or zfill bandwith bottlenecks didnt come into play. Prior Nvidia cards improved did 2x in one cycle and 4x in another. Thats why the G80 was so good at running AA.

I'm not sure how many cycles ATI is doing with there 8xAA. But this could easily be tested with a fillrate tester that tests Z-Fillrate. The more your fillrate halves. The more cycles it takes. Over at beyond3d I saw some tests with the HD3850 Zfillrate. And it was taking twice as many cycles at 4xAA as the X1900XT. This was not broken. Just by design. Now I cant/wont confirm that with 100% certainty but it seems obvious to me ATI has improved the AA cycle routine of their ROPS.
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