Tessellation: Because The GS Isn't Fast Enough

Microsoft and AMD tend to get the most excited about tessellation whenever the topic of DX11 comes up. AMD jumped on the tessellation bandwagon long ago, and perhaps it does make sense for consoles like the XBox 360. Adding fixed function hardware to quickly and efficiently handle a task that improves memory footprint has major advantages in the living room. We still aren't sold on the need for a tessellator on the desktop, but who's to argue with progress?

Or is it really progressive? The tessellator itself is fixed function rather than programmable. Sure, the input to and output of the tessellator can be manipulated a bit through the Hull Shader and Domain Shader, but the heart of the beast is just not that flexible. The Geometry Shader is the programmable block in the pipeline that is capable of tessellation as well as much more, but it just doesn't have the power to do tessellation on any useful scale. So while most everything has been moving towards programmability in the rendering pipe, we have sort of a step backward here. But why?

The argument between fixed function and programmable hardware is always one of performance versus flexibility and usefulness. In the beginning, fixed function was necessary to get the desired performance. As time went on, it became clear that adding in more fixed function hardware to graphics chips just wasn't feasible. The transistors put into specialized hardware just go unused if developers don't program to take advantage of it. This made a shift toward architectures where expanding the pool of compute resources that could be shared and used for many different tasks became a much more attractive way to go. In the general case anyway. But that doesn't mean that fixed function hardware doesn't have it's place.

We do still have the problem that all the transistors put into the tessellator are worthless unless developers take advantage of the hardware. But the reason it makes sense is that the ROI (return on investment: what you get for what you put in) on those transistors is huge if developers do take advantage of the hardware: it's much easier to get huge tessellation performance out of a fixed function tessellator than to put the necessary resources into the Geometry Shader to allow it to be capable of the same tessellation performance programmatically. This doesn't mean we'll start to see a renaissance of fixed function blocks in our graphics hardware; just that significantly advanced features going forward may still require the sacrifice of programability in favor of early adoption of a feature. The majority of tasks will continue to be enabled in a flexible programmable way, and in the future we may see more flexibility introduced into the tessellator until it becomes fully programmable as well (or ends up just being merged into some future version of the Geometry Shader).

Now don't let this technical assessment of fixed function tessellation make you think we aren't interested in reaping the benefits of the tessellator. Currently, artists need to create different versions of their objects for different LODs (Level of Detail -- reducing or increasing complexity as the object moves further or nearer the viewer), and geometry simulation through texturing at each LOD needs to be done by pixel shaders. This requires extra work from both artists and programmers and costs a good bit in terms of performance. There are also some effects than can only be done with more geometry.

Tessellation is a great way to get that geometry in there for more detail, shadowing, and smooth edges. High geometry also allows really cool displacement mapping effects. Currently, much geometry is simulated through textures and techniques like bump mapping or parallax occlusion mapping or some other technique. Even with high geometry, we will want to have large normal maps for our lighting algorithms to use, but we won't need to do so much work to make things like cracks, bumps, ridges, and small detail geometry appear to be there when it isn't because we can just tessellate and displace in a single pass through the pipeline. This is fast, efficient, and can produce very detailed effects while freeing up pixel shader resources for other uses. With tessellation, artists can create one sub division surface that can have a dynamic LOD free of charge; a simple hull shader and a displacement map applied in the domain shader will save a lot of work, increase quality, and improve performance quite a bit.

If developers adopt tessellation, we could see cool things, and with the move to DX11 class hardware both NVIDIA and AMD will be making parts with tessellation capability. But we may not see developers just start using tessellation (or the compute shader for that matter) right away. Because DirectX 11 will run on down level hardware and at the release of DX11 we will already have a huge number cards on the market capable of running a subset of DX11 bringing with it a better, more refined, programming language in the new version of HLSL and seamless parallelization optimizations, we will very likely see the first DX11 games only implementing features that can run completely on DX10 hardware.

Of course, at that point developers can be fully confident of exploiting all the aspects of DX10 hardware, which they still aren't completely taking advantage of. Many people still want and need a DX9 path because of Vista's failure, which means DX10 code tends to be more or less an enhanced DX9 path rather than something fundamentally different. So when DirectX 11 finally debuts, we will start to see what developers could really do with DX10.

Certainly there will be developers experimenting with tessellation, but these will probably just be simple amplification to get rid of those jagged edges around curved surfaces at first. It will take time for the real advanced tessellation techniques everyone is excited about to come to fruition.

So What's a Tessellator? One Last Thing and Closing Thoughts
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  • Mr Roboto - Thursday, February 26, 2009 - link

    How about this for the Windows 7 release, a DirectX 9 Halo 2 Port For Windows 7 But Needs it DirectX 11 To Run!

    OK so there was and still is no reason that Windows XP couldn't have DirectX 10 or DirectX 11 right? These were just more strong armed tactics by Microsoft to force us to hand them our cash for an aborted OS. Like the 4 year old DirectX 9 Halo port that somehow needed DirectX 10 to run, LOL it still makes me laugh. Whoever though up that idea should be shot. Fucking idiots.

    So how many DX10 games have been released to this day? Ten? How many have been exclusively DX10 like MS wanted? The answer is one the four year old Halo.

    I hope MS continues to hemorrhage money and eventually goes out of business. Nothing would make me happier and it would spur innovation simply by them dying off.
    Reply
  • MadBoris - Tuesday, February 10, 2009 - link

    Unfortunately the API will not be supported until consoles support it.
    Consoles now control game development technical design, thx to Microsofts sly ways.

    Until next gen consoles come out with the next DX API support then DX9 will still be the standard due to multiplatforming. We can barely get a developer to make the proper PC support changes in their multiplatform games, they aren't going to write seperate render paths unless MS pays them to, like with DX10.

    The only redeeming effect is the Nvidia 8800 GPU is one of the best GPU's I bought because it looks like it will last me many years.
    Reply
  • vistaisfine - Friday, February 20, 2009 - link

    personally i hope windows LIVE grows. they need it to. its a slick system that works well. at least in dawn of war 2 and gta IV. i don't think your assesment of how quickly developers are adopting DX 10 is accurate. Alot of gaming companys go for a wide demographic and that usually means creating content that can scale depending on the hardware. the reason why there are so many dx 9 game is because the lowest common demoninator gamer runs a dx 9 card. People are finshing maxing out their AGP builds. (crazy as that sounds). Reply
  • Mr Roboto - Thursday, February 26, 2009 - link

    Are you high? GFWL is an abomination. It has absolutely ruined GTA IV as well as all other games associated with it. It's a straight XBox port and it's total shit. There's no dedicated servers, no anti cheat mechanism, no easy way for matchmaking, constant disconnections, I could go on and on. It's very unfriendly and in typical fashion they treat something as trivial as save games as their own property. Go ahead try backing up your GTA IV save games folder and using them on another PC or game installation. It won't work because Microsoft either didn't think about it or more likely purposefully made it difficult because they think it belongs to them and they're in control of it.

    The only value GFWL has for MS (which I should have seen coming) is they're going to launch a digital distribution service. Punks. GFWL is so goddamn broken and yet they're going to try to sell games through it before it's even in decent form? Nice. You can bet they'll be ultra slow to in responding to critical bugs and spyware ready made to steal your info on launch day.

    Fucking garbage. You're settling for garbage. Valve knows how to do things, development, marketing, listening to their fan base, updating their games above and beyond what any other company would do. Even Steam isn't perfect and I'm not asking MS to become them (an impossibility) but Microsoft has a LONG way to come before they're even in the same conversation.

    I hope it crashes and burns even more than it already has because it's ported console junk.
    Reply
  • jharper12 - Thursday, February 05, 2009 - link

    americunts... that's clever. You got us!! We're idiots! I can't believe we never realized it before! You sir, are pure genius.

    I just feel absolutely terrible that 43 other countries joined us in our latest war. I feel even worse that the entire world economy was brought down by our financial sector that represents about 1% of our $14.33 trillion GDP... I mean I truly feel awful that we are dragging everyone down with us. I wish I could do something about it, but it seems as though everyone believes "americunts are idiots" and then continues to follow us into oblivion. Sorry about that, I wish there was something I could do to help.

    Sincerely,
    Americunt
    Reply
  • Matrix888 - Wednesday, February 04, 2009 - link

    I'll not be bother with the 2000 or so comments here and reply every one of them, but this is my 2 cents about VISTA:

    1) Drivers issues - there was some initial problem with drivers issues especially old hardware and lazy 3rd parties. But this wasn't MS fault. I've installed VISTA since day 1 and the only problem encountered was an X-FI driver from Creative...which isn't available until like 9 to 12 months later (and it's a beta!!!). Tell me which new O/S doesn't have drivers problem? Win98 was forever plagued with drivers problem, WIN XP suffers similar fate initially, but once the driver matures, it was ok. Hell i've waited for 1 year for an X-FI driver for Ubuntu 7....finally released and the next thing i did is upgrade to Ubuntu 8 (by the time the driver was releasead, UBuntu 8.1 is made available) and it bombs my X-FI.

    2) Memory hogging - another urban legend by people who have no ideas. VISTA does not works the same way like XP. What's the point of having idle memory and not used? VISTA pre-cache ur memory to make it better. 2GB of RAM ran just fine.

    3) You need a high spec PC for VISTA - BS. I've ran an Atom 330 PC with VISTA Premium with Aero on just fine. Even stress test it by opening tons of apps (including a Virtual PC running XP) and it multitask great with all your everyday apps...MS office, Firefox, e-mails, bit torrent etc....only 2GB RAM.

    4) Gaming performance suffers - if you have a moderately new hardware, you are not going to see any difference. I ran my games max settings at 1680x1050. Runs great.

    5) UAC? - Cmon. It's only because XP users never use any other OS like Linux or MAC/OS and they can't get this idea in their head it's for their own good. Linux and MAC/OS will prompts u administrator and password when u try to install something or do something funny. It is perfectly fine and u can always turn it off.

    So far, i really have minimal problem with VISTA. VISTA MCE makes a great front-end for HTPC and believe this, the start up is MUCH FASTER than XP. Sure a fresh XP will beat VISTA in terms of boot up time, but once you installed tons of apps into it, XP will slow like a turtle while VISTA will have minimal degradtion and you get response to your desktop much faster.

    I've finally migrated all my PCs and notebooks last year to VISTA and dump XP forever.

    I've no doubt Windows 7 will be great, but to me, it'll be nothing more than VISTA Edition 2. It'll be re-packaging to wow idiots who simply believe all those VISTA rumours without ever using it (or try to run it on a Pentium 3). Seriously, nobody ask you to install a 3.0 engine into a Japanese kei-car!!! :x

    For the rest of us, it'll simply be a moderate update of a already fantastic OS.

    Hasta La Vista!
    Reply
  • volgagerman - Monday, February 02, 2009 - link

    One thing that hasn't been stated yet is that the success of any API is directly related to market forces and timing.

    DirectX 9.0c has been successfull largely for two reasons. One, it took advantage of the market shift that developers would target consoles first and port to PC second in order to extract the largest possible return on investment. Two, many of the gaming engines we enjoy today began their life with the release of DirectX 8. It took 3-4 years for those engines to be built, and they finally meshed with the feature set of DirectX 9, as it was an evolution over 8 and not a true revolution.

    DirectX 10 has not revealed any jaw dropping improvements in shipping titles over 9 because developers are still leveraging those engines for exploitation on the console market. They can easily up the visuals for a port to the PC by removing texture compression, upping resolution support and increasing texture sizes, but the major underlying changes to truly support 10 will not happen in the immediate term.

    That brings me to my next thought. From everything I've read, the next generation of consoles are targeted for release in the 2011-2012 timeframe. A true next-gen game engine takes at least 4 years to fully bake. That means Unreal Engine 4.0, Id Tech 6, next-gen Gamebryo are all now in development to be ready when those next-gen consoles hit. It's a safe bet they are leveraging DirectX 11 to its fullest. To go one step further, I would guess that market forces are not going to change in the future, and game developers will still target consoles first and PCs second. What that means in the 2011-2012 timeframe is a solid install base of DirectX 11 on even low-end PCs by that point. It would be a no-brainer to anticipate Microsoft leveraging it's latest and greatest API in the next generation XBOX. With market forces and timing in alignment, the next-gen gaming engines and the the next-gen consoles are set to take full advantage of DirectX 11. I will expect nothing less than revolutionary changes to the quality of gaming graphics in that time frame.

    Here's one final tidbit. If Microsoft takes Intel's bait and utilizes a 32nm modified version of the Larrabee project for a single chip CPU/GPU solution in the next XBOX, developers will have to shift gears yet again and prepare for a rendering model that is mostly software based. That is beyond DirectX 11 and will lead to even more revolutionary changes...Microsoft would no longer be constrained by the capability of their CPU/GPU, but by the efficiency of the software algorithms to achieve playable frame rates. I look forward to a future on consoles and PCs when a simple patch improves frame rates significantly. No hardware update required. :)
    Reply
  • mechBgon - Monday, February 02, 2009 - link

    Touching on the "rejected" Vista:

    Valve's monthly user survey was updated today. WinXP lost 2.48% in just the last month, and Vista has gained 2.47% of that. The ratio of Vista to WinXP is now 1 to 1.91. I guess you could say Vista is "picking up Steam," ;) and rapidly, too.

    Ten months ago, this AnandTech thread also indicated (much to the OP's annoyance, I'm sure) that even then, the Vista adoption rate among AnandTech Forum users was already about 50%:

    http://forums.anandtech.com/messageview.aspx?catid...">http://forums.anandtech.com/messageview.aspx?catid...

    In light of the readership's tastes, also pretty clearly illustrated in the comments here, I suggest calming the anti-Vista rhetoric and not reading more into things than what's really there.
    Reply
  • Hrel - Sunday, February 01, 2009 - link

    Do you think someone over there at anand could do an article explaining exactly WHY DirectX 10 can't be supported by XP? PLEASE!!!!!! Reply
  • DerekWilson - Friday, February 06, 2009 - link

    because Microsoft chose to tie in driver specific features to DX10 that make a retrofit not technically possible. Microsoft could have chosen to spec out DX10 so that new hardware features were exposed while simply allowing for a new driver model, but they did not: they required the new driver model features.

    this stuff has to do with power management, threading, OS managed/virtual memory, hardware virtualization (sharing between software and even VMs), etc...

    Now that what DX10 is eternally locked into existence as is, porting back is not possible. We're in a similar boat with DX11.

    There is ZERO technical reason that microsoft couldn't have chosen at the outset to enable the Geometry Shader, to extend const/register limits, and to do all the other things that game developers actually use on Windows XP. It would have required hardware companies to implement a distinct WinXP DX10 driver and Vista DX10 driver, but now that can't be done.

    The /why/ of no DX10 on XP is that MS simply decided not to make that an option when developing DX10 and built in features extraneous to game development that require the Vista platform.
    Reply

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