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.
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Roy2001 - Tuesday, May 15, 2007 - link
The reason is, you have to pay extra $ for a power supply. No, most probably your old PSU won't have enough milk for this baby. I will stick with nVidia in future. My 2 cents.Chaser - Tuesday, May 15, 2007 - link
Such a revealing tech article. Thanks for other sources Tom.
archcommus - Tuesday, May 15, 2007 - link
$300 is the exact price point I shoot for when buying a video card, so that pretty much eliminates AMD right off the bat for me right now. I want to spend more than $200 but $400 is too much. I'm sure they'll fill this void eventually, and how that card will stack up against an 8800 GTS 320 MB is what I'm interested in.H4n53n - Tuesday, May 15, 2007 - link
Interesting enough in some other websites it wins from 8800 gtx in most games,especially the newer ones and comparing the price i would say it's a good deal?I think it's just driver problems,ati has been known for not having a very good driver compared to nvidia but when they fixed it then it'll windragonsqrrl - Thursday, August 25, 2011 - link
lol...fail. In retrospect it's really easy to pick out the EPIC ATI fanboys now.Affectionate-Bed-980 - Tuesday, May 15, 2007 - link
I skimmed this article because I have a final. ATI can't hold a candle to NV at the moment it seems. Now while the 2900XT might have good value, I am correct in saying that ATI has lost the performance crown by a buttload (not even like X1800 vs 7800) but like they're totally slaughtered right?Now I won't go and comment about how the 2900 stacks up against competition in the same price range, but it seems that GTSes can be acquired for cheap.
Did ATI flop big here?
vailr - Monday, May 14, 2007 - link
I'd rather use a mid-range older card that "only" uses ~100 Watts (or less) than pay ~$400 for a card that requires 300 Watts to run. Doesn't AMD care about "Global Warming"?Al Gore would be amazed, alarmed, and astounded !!
Deusfaux - Monday, May 14, 2007 - link
No they dont and that's why the 2600 and 2400 don't existochentay4 - Monday, May 14, 2007 - link
Let me start with this: i always had a nvidia card. ALWAYS.Faster is NOT ALWAYS better. For the most part this is true, for me, it was. One year ago I boght a MSI7600GT. Seemed the best bang for the buck. Since I bought it, I had problems with TVout detection, TVout wrong aspect ratios, broken LCD scaling, lot of game problems, inexistent support (nv forum is a joke) and UNIFIED DRIVER ARQUITECTURE. What a terrible lie! The latest official drivers is 6 months ago!!!
Im really demanding, but i payed enough to demand a 100% working product. Now ATi latest offering has: AVIVO, FULL VIDEO ACC, MONTHLY DRIVER UPDATES, ALL BUGS I NOTICED WITH NVIDIA CARD FIXED, HDMI AND PRICE. I prefer that than a simple product, specially for the money they cost!
I will never buy a nvidia card again. I'm definitely looking forward ATis offering (after the joke that is/was 8600GT/GTS).
Enough rant.
Am I wrong?
Roy2001 - Tuesday, May 15, 2007 - link
Yeah, you are wrong. Spend $400 on a 2900XT and then $150 on a PSU.