ATI Radeon HD 2900 XT: Calling a Spade a Spade
by Derek Wilson on May 14, 2007 12:04 PM EST- Posted in
- GPUs
Tessellation and the Future
It's no secret that R600 is AMD's second generation unified shader architecture. The Xbox 360 houses their first attempt at a unified architecture, and the R600 evolved from this. It isn't surprising to learn that some of the non-traditional hardware from the Xenos (the Xbox 360 GPU) found its way into R600.
AMD has included a tessellator on their hardware, which is able to take input geometry and amplify it before passing it on to the vertex shader. This is something that Microsoft is planning on adding to future versions of DirectX as well, but in the meantime developers will need to take special steps to utilize the hardware.
The basic idea behind tessellation is in the subdivision of geometry. There are multiple algorithms for handling this process, and the R600 tessellator is capable of adapting to a developer's specific needs. The tessellator can take a polygon as input and break it up into smaller triangles, creating more vertices for a specific object. Taken on its own, this isn't particularly useful, but this concept can be paired with displacement mapping in order to reshape the tessellated polygon into something more like the actual surface a developer wants to represent (this is called the limit surface).
With low polygon models and lots of pixel shaders, normal maps and textures can approximate the look of more complex geometry, but we're always stuck with the very rough silhouette edge around the object. With more geometry, we could also better use pixel shaders to enhance the geometry present rather than trying to create the illusion of geometry itself.
We can't simply send millions of polygons per character to the graphics card. This isn't because the card can't handle the processing requirements, but rather the bandwidth and latency overhead of sending all this data to the hardware is too high. Tessellation and displacement gives us a way of really using the vertex shading power of unified architectures as well as removing the limitation on polygon count created by overhead.
While geometry shaders can be used for amplification and tessellators can be written as geometry shaders, this process is still way too slow on current programmable hardware. AMD's dedicated tessellator is capable of tessellating up to 15x more data and it can work much faster and more efficiently than a geometry shader set to the same task. With the next version of DX bringing tessellator hardware to all GPUs, developers should be able to focus on more interesting uses for the geometry shader as well.
Having this unit makes porting Xbox 360 games even easier for developers targeting AMD hardware. As most hardware still doesn't support the feature, a more general purpose path will still have to be written, but there wouldn't be any reason to remove what's already there. In these cases, R600 could benefit with greater performance than other hardware.
The downside is that it might be difficult to entice developers not already working with the Xbox 360 to touch the tessellator. It is definitely capable of high performance and terrific detail, but spending time on a feature only a small subset of gamers will be able to experience (for this generation) takes away from time spent making the game better for everyone.
We are always happy to see either hardware or software take a leap and create the first chicken or egg, but we just don't see the tessellator as a big selling point of R600. The technology is great, we're glad it's there, but we will really have to wait and see just how much (if any) real value this adds to the product. We'll leave this section on one final note about a tessellator landscape demo that really brings home what this thing can do.
Facebook
Twitter
RSS
86 Comments
View All Comments
yyrkoon - Tuesday, May 15, 2007 - link
See, the problem here is: guys like you are so bent on saving that little bit of money, by buying a lesser brand name, that you do not even take the time to research your hardware. USe newegg , and read the user reviews, and if that is not enough for you, go to the countless other resources all over the internet.yyrkoon - Tuesday, May 15, 2007 - link
Blame the crappy OEM you bought the card from, not nVIdia. Get an EVGA card, and embrace a completely different aspect on video card life.MSI may make some decent motherboards, but their other components have serious issues.
LoneWolf15 - Thursday, May 17, 2007 - link
Um, since 95% of nvidia-GPU cards on the market are the reference design, I'd say your argument here is shaky at best. EVGA and MSI both use the reference design, and it's even possible that cards with the same GPU came off the same production line at the same plant.DerekWilson - Thursday, May 17, 2007 - link
it is true that the majority of parts are based on reference designs, but that doesn't mean they all come from the same place. I'm sure some of them do, but to say that all of these guys just buy completed boards and put their name on them all the time is selling them a little short.at the same time, the whole argument of which manufacturer builds the better board on a board component level isn't something we can really answer.
what we would suggest is that its better to buy from OEMs who have good customer service and long extensive warranties. this way, even if things do go wrong, there is some recourse for customers who get bad boards or have bad experiences with drivers and software.
cmdrdredd - Monday, May 14, 2007 - link
you're wrong. 99% of people buying these high end cards are gaming. Those gamers demand and deserve the best possible performance. If a card that uses MORE power and costs MORE (x2900xt vs 8800gts) and performs generally the same or slower what is the point? Fact is...ATI's high end is in fact slower than mid range offerings from Nvidia and consumes alot more power. Regardless of what you think, people are buying these based on performance benchmarks in 99% of all cases.AnnonymousCoward - Tuesday, May 15, 2007 - link
No, you're wrong. Did you overlook the emphasis he put on "NOT ALWAYS"?You said 99% use for gaming--so there's 1%. Out of the gamers, many really want LCD scaling to work, so that games aren't stretched horribly on widescreen monitors. Some gamers would also like TVout to work.
So he was right: faster is NOT ALWAYS better.
erwos - Monday, May 14, 2007 - link
It'd be nice to get the scoop on the video decode acceleration present on these boards, and how it stocks up to the (excellent) PureVideo HD found in the 8600 series.imaheadcase - Tuesday, May 15, 2007 - link
I agree! They need to do a whole article on video acceleration on a range of cards and show the pluses and cons of each card in respective areas. A lot of people like myself like to watch videos and game on cards, but like the option open to use the advanced video features.Turnip - Monday, May 14, 2007 - link
"We certainly hope we won't see a repeat of the R600 launch when Barcelona and Agena take on Core 2 Duo/Quad in a few months...."Why, that's exactly what I had been thinking :)
Phew! I made it through the whole thing though, I even read all of those awfully big words and everything! :)
Thanks guys, another top review :)
Kougar - Monday, May 14, 2007 - link
First, great article! I will be going back to reread the very indepth analysis of the hardware and features, something that keeps me a avid Anandtech reader. :)Since it was mentioned that overclocking will be included in a future article, I would like to suggest that if possible watercooling be factored into it. So far one review site has already done a watercooled test with a low-end watercooling setup, and without mods acheived 930MHz on the Core, which indirectly means 930MHz shaders if I understand the hardware.
I'm sure I am not the only reader extremely interested to see if all R600 needs is a ~900-950MHz overclock to offer some solid GTX level performance... or if it would even help at all. Again thanks for the consideration, and the great article! Now off to find some Folding@Home numbers...