ATI Radeon HD 2900 XT: Calling a Spade a Spade
by Derek Wilson on May 14, 2007 12:04 PM EST- Posted in
- GPUs
Beyond the Shader: Coloring Pixels
We can't ignore the last few steps in the rendering pipeline, as AMD has also updated their render back ends (analogous to NVIDIA's ROPs) which are responsible for determining the visibility of each fragment and the final color of each pixel on the screen. Beyond this, the render back ends handle compression and decompression, render to texture functionality, MRTs, framebuffer formats, and usually AA.
Once again, one of the important things to note is that R600 only has four render back ends. This means we will only see 16 pixels complete per clock at maximum, just like the R580. However, AMD has included double the Z/stencil hardware so that we can get up to 32 total Z/stencil ops out of the render back ends to improve stencil shadow operations among other things. Pure fill rate hasn't really mattered in a while, while Z/stencil capability remains important. But will only four render back ends be enough?
Efficiency has been improved on the render back ends, but with the potential of completing 64 threads per clock from the shader hardware, they will need to really work to keep up. R600 has the ability to display floating point formats from 11:11:10 up to 128-bit fp. DX10 requires eight MRTs now, and we've got them. We also get more efficient render to texture features which should help enable more complex effects to process faster.
Z/Stencil Hardware
As far as Z/stencil hardware is concerned, compression has gotten a boost up to 16:1 rather than 8:1 on the X1k series. Depth tests can be limited to a specific range programmatically which can speed up stencil shadows. Our Z-buffer is now 32-bit floating point rather than 24-bit. Hierarchical Z has been enhanced to handle some situations where it was unable to assist in rendering, and AMD has added a hierarchical stencil buffer as well.
AMD is introducing something called Re-Z which is designed to also help with the problem Early-Z has in not being able to handle shaders that update Z data. R600 is able to check Z values before a shader runs as well as after the Z value has been changed in the shader. This allows AMD to throw out pixels that are updated to be out of view without sending them to the render back ends for evaluation.
If we compare this setup with G80, we're not as worried as we are about texture capability. G80 can complete 24 pixels per clock (4 pixels per ROP with six ROPs). Like R600, G80 is capable of 2x Z-only performance with 48 Z/stencil operations per clock with AA enabled. When AA is disabled, the hardware is capable of 192 Z-only samples per clock. The ratio of running threads to ROPs is actually worse on G80 than on R600. At the same time, G80 does offer a higher overall fill rate based on potential pixels per clock and clock speed.
Facebook
Twitter
RSS
86 Comments
View All Comments
GoatMonkey - Monday, May 14, 2007 - link
That's obviously BS. This IS their high end part, it just doesn't perform as well as nVidia's high end part, so it is priced accordingly.poohbear - Monday, May 14, 2007 - link
sweet review though! thanks for including all the important and pertinent cards in your roundup (the 8800gts 320mb inparticular). also love how neutral Anand is in their reviews, unlike some other sites.:pCreig - Monday, May 14, 2007 - link
The R600 is finally here. I'm sure the overall performance is not what AMD was hoping for. Nobody ever shoots to have their newest product be the 2nd best. But pricing it at $399 and including a very nice game bundle will make the HD 2900 XT a VERY worthwhile purchase. I also have the feeling that there is a significant amount of performance increase to be realized through future driver releases ala X1800XT.shady28 - Tuesday, May 15, 2007 - link
Nvidia has gone over the cliff on pricing.
I know of no one personally who has an 88xx series card. I know one who recently picked up an 8600 of some kind, that's it. I have the best GPU of anyone I know.
It's a real shame that there is so much focus on graphics cards that virtually no one buys. These are niche products folks - yet 'who is best' seems to be totally dependent on these niche products. That's patently ridiculous.
It's like saying, since IBM makes the fastest computers in the world (they do), they're the best and you should be buying IBM (or now, lenovo) laptops and desktops.
No one ever said that sort of thing because it's patently ridiculous. Why do people say it now for graphics cards? The fact that they do says a lot about the mentality of sites like AT.
DerekWilson - Tuesday, May 15, 2007 - link
We don't say what you are implying, and we are also very upset with some of NVIDIA's pricing (specifically the 8800 ultra)the 8800 gts 320mb is one of the best values for your money anywhere and isn't crazy expensive -- it's actually the card I'd recommend to anyone who cares about graphics in games and wants good quality and performance at 1600x1200.
I would never tell anyone to buy an 8600 gts because nvidia has the fastest high end card. In fact, in this article, I hope I made it clear that AMD has the opportunity to capitalize on the huge performance gap nvidia left between the 8600 and 8800 series ... If AMD builds a part that performs in this range is priced competitively, they'll have our recommendation in a flash.
Recommending parts based on value at each price or performance segment is something we take pride in and will always do, no matter who has the absolute fastest hardware out there.
The reason our focus was on AMD's fastest part is because they haven't given us any other hardware to test. We will absolutely be talking a lot and in much depth about midrange and budget hardware when AMD makes these parts available to us.
yacoub - Monday, May 14, 2007 - link
$400 is a lot of money. Not terribly long ago the highest end GPU available didn't cost more than $400. Now they hit $750 so you start to think $400 sounds cheap. It's really not. It's a heck of a lot of money for one piece of hardware. You can put together a 650i SLI rig with 2GB of DDR2 6400 and an E4400 for that much money. I know because I just did that. I kept my 7900GT from my old rig because I wanted to see how R600 did before purchasing an 8800GTS 640MB. Now that we've seen initial results I will wait to see how R600 does with more mature drivers and also wait to see the 640MB GTS price come down even more in the meantime.vijay333 - Monday, May 14, 2007 - link
http://www.randomhouse.com/wotd/index.pperl?date=1...">http://www.randomhouse.com/wotd/index.pperl?date=1..."the expression to call a spade a spade is thousands of years old and etymologically has nothing whatsoever to do with any racial sentiment."
yacoub - Monday, May 14, 2007 - link
Yes, a spade was a shovel long before muslims enslaved europeans to do hard labor in north africa and europeans enslaved africans to do hard labor in the 'new world'.vijay333 - Monday, May 14, 2007 - link
whoops...replied to the wrong one.rADo2 - Monday, May 14, 2007 - link
It is not 2nd best (after 8800ULTRA), not 3rd best (after 8800GTX), not 4th best (after 8800GTX-640), but 5th best (after 8800GTS-320), or even worse ;)Bad performance with AA turned on (everybody turns on AA), huge power consumption, late to the market.
A definitive failure.