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.

CFAA and No Fixed Resolve Hardware The New Video Decode Pipeline: UVD


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  • wjmbsd - Monday, July 02, 2007 - link

    What is the latest on the so-called Dragonhead 2 project (aka, HD 2900 XTX)? I heard it was just for OEMs at first...anyone know if the project is still going and how the part is benchmarking with newest drivers? Reply
  • teainthesahara - Monday, May 21, 2007 - link

    After this failure of the R600 and likely overrated(and probably late) Barcelona/Agena processors I think that Intel will finally bury AMD. Paul Ottelini is rubbing his hands with glee at the moment and rightfully so. AMD now stands for mediocrity.Oh dear what a fall from grace.... To be honest Nvidia don't have any real competition on the DX10 front at any price points.I cannot see AMD processors besting Intel's Core 2 Quad lineup in the future especially when 45nm and 32 nm become the norm and they don't have a chance in hell of beating Nvidia. Intel and Nvidia are turning the screws on Hector Ruiz.Shame AMD brought down such a great company like ATI. Reply
  • DerekWilson - Thursday, May 24, 2007 - link

    To be fair, we really don't have any clue how these cards compete on the DX10 front as there are no final, real DX10 games on the market to test.

    We will try really hard to get a good idea of what DX10 will look like on the HD 2000 series and the GeForce 8 Series using game demos, pre-release code, and SDK samples. It won't be a real reflection of what users will experience, but we will certainly hope to get a glimpse at performance.

    It is fair to say that NVIDIA bests AMD in current game performance. But really there are so many possibilities with DX10 that we can't call it yet.
  • spinportal - Friday, May 18, 2007 - link

    From the last posting of results for the GTS 320MB round-up
    http://www.anandtech.com/video/showdoc.aspx?i=2953...">Prey @ AnandTech - 8800GTS320
    we see that the 2900XT review chart pushes the nVidia cards down about 15% across the board.
    http://www.anandtech.com/video/showdoc.aspx?i=2988...">Prey @ AnandTech - ATI2900XT
    The only difference in systems is software drivers as the cpu / mobo / mem are the same.

    Does this mean ATI should be getting a BIGGER THRASHING BEAT-DOWN than the reviewer is stating?
    400$ ATI 2900XT performing as good as a 300$ nVidia 8800 GTS 320MB?

    Its 100$ short and 6 months late along with 100W of extra fuel.

    This is not your uncle's 9700 Pro...
  • DerekWilson - Sunday, May 20, 2007 - link

    We switched Prey demos -- I updated our benchmark.

    Both numbers are accurate for the tests I ran at the time.

    Our current timedemo is more stressful and thus we see lower scores with this test.
  • Yawgm0th - Wednesday, May 16, 2007 - link

    The prices listed in this article are way off.

    Currently, 8800GTS 640MB retails for $350-380, $400+ for OC or special versions. 2900XT retails for $430+. In the article, both are listed as $400, and as such the card is given a decent review in the conclusion.

    Realistically, this card provides slightly inferior performance to the 8800GTS 640MB at a considerably higher price point -- $80-$100 more than the 8800GTS. I mean, it's not like the 8800Ultra, but for the most part this card has little use outside of AMD and/or ATI fanboys. I'd love for this card to do better as AMD needs to be competing with Nvidia and Intel right now, but I just can't see how this is even worth looking at, given current prices.
  • DerekWilson - Thursday, May 17, 2007 - link

    really, this article focuses on architechture more than product, and we went with MSRP prices...

    we will absolutly look closer at price and price/performance when we review retail products.
  • quanta - Tuesday, May 15, 2007 - link

    As I recalled, the Radeon HD 2900 only has DVI ports, but nowhere in DVI documentation specifies it can carry audio signals. Unless the card comes with adapter that accepts audio input, it seems the audio portion of R600 is rendered useless. Reply
  • DerekWilson - Wednesday, May 16, 2007 - link

    the card does come with an adapter of sorts, but the audio input is from the dvi port.

    you can't use a standard DVI to HDMI converter for this task.

    when using AMD's HDMI converter the data sent out over the DVI port does not follow the DVI specification.

    the bottom line is that the DVI port is just a physical connector carrying data. i could take a DVI port and solder it to a stereo and use it to carry 5.1 audio if I wanted to ... wouldn't be very useful, but I could do it :-)

    While connected to a DVI device, the card operates the port according to the DVI specification. When connected to an HDMI device through the special converter (which is not technically "dvi to hdmi" -- it's amd proprietry to hdmi), the card sends out data that follows the HDMI spec.

    you can look at it another way -- when the HDMI converter is connected, just think of the dvi port as an internal connector between an I/O port and the TMDS + audio device.
  • ShaunO - Tuesday, May 15, 2007 - link

    I was at an AMD movie night last night where they discussed the technical details of the HD 2900 XT and also showed the Ruby Whiteout DX10 Demo rendered using the card. It looked amazing and I had high hopes until I checked out the benchmark scores. They're going to need more than free food and popcorn to convince me to buy an obsolete card.

    However there is room for improvement of course. Driver updates, DX10 and whatnot. The main thing for me personally will be driver updates, I will be interested to see how well the card improves over time while I save my pennies for my next new machine.

    Everyone keeps saying "DX10 performance will be better, yadda yadda" but I also want to be able to play the games I have now and older games without having to rely on DX10 games to give me better performance. Nothing like totally underperforming in DX9 games and then only being equal or slightly better in DX10 games compared to the competition. I would rather have a decent performer all-round. Even saying that we don't even know for sure if DX10 games are even going to bring any performance increases of the competition, it's all speculation right now and that's all we can do, speculate.


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