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
johnsonx - Monday, May 14, 2007 - link
and to which are you going to admit to?What was that old saying about glass houses and throwing stones? Shouldn't throw them in one? Definitely shouldn't them if you ARE one!
Puddleglum - Monday, May 14, 2007 - link
You mean, while it does compete performance-wise?johnsonx - Monday, May 14, 2007 - link
No, I'm pretty sure they mean DOESN'T. That is, the card can't compete with a GTX, yet still uses more power.INTC - Monday, May 14, 2007 - link
Chadder007 - Monday, May 14, 2007 - link
When will we have the 2600's out in review?? Thats the card im waiting for.TA152H - Monday, May 14, 2007 - link
Derek,I like the fact you weren't mincing your words, except for a little on the last page, but I'll give you a perspective of why it might be a little better than some people will think.
There are some of us, and I am one, that will never buy NVIDIA. I bought one, had nothing but trouble with it, and have been buying ATI for 20 years. ATI has been around for so long, there is brand loyalty, and as long as they come out with something that is competent, we'll consider it against their other products without respect to NVIDIA. I'd rather give up the performance to work with something I'm a lot more comfortable with.
The power though is damning, I agree with you 100% on this. Any idea if these beasts are being made by AMD now, or still whoever ATI contracted out? AMD is typically really poor in their first iteration of a product on a process technology, but tend to improve quite a bit in succeeding ones. I wonder how much they'll push this product initially. It might be they just get it out to have it out, and the next one will be what is really a worthwhile product. That only makes sense, of course, if AMD is now manufacturing this product. I hope they are, they surely don't need to make anymore of their processors that aren't selling well.
One last thing I noticed is the 2400 Pro had no fan! It had a heatsink from Hell, but that will still make this a really attractive product for a growing market segment. Any chance of you guys doing a review on the best fanless cards?
DerekWilson - Wednesday, May 16, 2007 - link
TSMC is manufacturing the R600 GPUs, not AMD.AnnonymousCoward - Tuesday, May 15, 2007 - link
"I bought one, had nothing but trouble with it, and have been buying ATI for 20 years."That made me laugh. If one bad experience was all it took to stop you from using a computer component, you'd be left with a PS/2 keyboard at best.
"...to work with something I'm a lot more comfortable with."
Are you more comfortable having 4:3 resolutions stretched on a widescreen? Maybe you're also more comfortable with having crappier performance than nvidia has offered for the last 6 months and counting? This kind of brand loyalty is silly.
MadBoris - Monday, May 14, 2007 - link
As far as your brand loyalty, ATI doesn't exist anymore. Furthermore AMD executives will got the staff so you can't call it the same.Secondly, Nvidia has been a stellar company providing stellar products. Everyone has some ups and downs. Unfortunately with the hardware and drivers this is ATI's (er AMD's) downs.
This card should do ok in comparison to the GTS, especially as drivers mature. Some reviews show it doing better than GTS640 in most tests, so I am not sure where or how discrepencies are coming about. Maybe hardware compatibility, maybe settings.
rADo2 - Monday, May 14, 2007 - link
Many NVIDIA 8600GT/GTS cards do not have a fan, are available on the market now, and are (probably; different league) much more powerful than 2400 ;) But as you are a fanboy, you are not interested, right?