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.
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dragonsqrrl - Thursday, August 25, 2011 - link
You forgot c).-if you're an ATI fanboy
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."
strikeback03 - Wednesday, May 16, 2007 - link
What about in Euchre, where a spade can be a club (and vice versa)?johnsonx - Monday, May 14, 2007 - link
Just wait until AT refers to AMD's marketing budget as 'niggardly'...bldckstark - Monday, May 14, 2007 - link
What do shovels have to do with race?Stan11003 - Monday, May 14, 2007 - link
My big hope out all of this that the ATI part forces the Nvidia parts lower so I can use my upgrade option from EVGA to get a nice 8800 GTX instead of my 8800 GTS ACS3 320. However with a quad core and a decent 2GB I have no gaming issues at all. I play at 1600x1200(when that become a low rez?) and everything is butter smooth. Without newer titles all this hardware is a waist anyways.Gul Westfale - Monday, May 14, 2007 - link
the article says that the part is not a failure, but i disagree. i switched from a radeon 1950pro to an nvidia geforce 8800GTS 320MB about a mont ago, and i paid only $350US for it. now i see that it still outperforms the new 2900...one of my friends wanted to wait to buy a new card, he said he hoped that the ATI part was going to be faster. now he says he will just buy the 8800GTS 320, since ATI have failed.
if they can bring out a part that competes well with the 8800GTS and price it similarly or lower then it would be worth buying, but until then i will stick with nvidia. better performance, better price, and better drivers... why would anyone buy the ATI card now?
ncage - Monday, May 14, 2007 - link
My conclusion is to wait. All of the recent GPU do great with dx9...the question is how will they do with dx10? I think its best to wait for dx10 titles to come out. I think crysis would be a PERFECT test.wingless - Monday, May 14, 2007 - link
I agree with you. Crysis is going to be the benchmark for these DX10 cards. Its hard to tell both Nvidia and AMD's DX10 performance with these current, first generation DX10 titles (most of which have a DX9 version) because they don't fully take advantage of all the power on both the G80 or R600 yet. Its true that Crysis will have a DX9 version as well but the developer stated there are some big differences in code. I'm an Nvidia fanboy but I'm disappointed with the Pure Video and HDMI support on the 8800 series cards. ATI got this worked out with their great AVIVO and their nice HDMI implementation but for now Nvidia is still the performance champ with "simpler" hardware. The G80 and R600 continue the traditions of their manufacturers. Nvidia has always been about raw power and all out speed with few bells and whistles. ATI is all about refinement, bells and whistles, innovations, and unproven new methods which may make or break them.All I really want to wait for is to see how developers embrace CUDA or ATI's setup for PHYSICS PROCESSING! Both companies seem to have well thought out methods to do physics and I cant wait to see that showdown. AGEIA and HAVOK need to hop on-board and get some software support for all this good hardware potential they have to play with. Physics is the next big gimmick and you know how much we all love gimmicks (just like good 'ole 3D acceleration 10 years ago).
poohbear - Monday, May 14, 2007 - link
they dont make a profit from high end parts that's why they're not bothering w/ it? that's AMD's story? so why bother having an FX line w/ their cpus?