Going Deeper: The DX11 Compute Shader and OpenCL/OpenGL

Many developers are excited about the added flexibility of the Compute Shader (also referred to as the CS). This addition to the pipeline steps further from a render-centric API and enables more general purpose algorithms. We see added flexibility in both the type of operations that can be preformed on data and the type of data that can be operated on.

In other pipeline stages, we see limitations imposed that are designed to speed up execution that get in the way of general purpose code. Although we can shoehorn general purpose algorithms into a pixel shader program, we don't have the freedom to use data structures like trees, sharing data between pixels (and thus threads) is difficult and costly, and we have to go through the motions of drawing triangles and mapping solutions onto this.

Enter DirectX11 and the CS. Developers have the option to pass data structures over to the Compute Shader and run more general purpose algorithms on them. The Compute Shader, like the other fully programmable stages of the DX10 and DX11 pipeline, will share a single set of physical resources (shader processors).

This hardware will need to be a little more flexible than it currently is as when it runs CS code it will have to support random reads and writes and irregular arrays (rather than simple streams or fixed size 2D arrays), multiple outputs, direct invocation of individual or groups of threads as per the programmer's needs, 32k of shared register space and thread group management, atomic instructions, synchronization constructs, and the ability to perform unordered IO operations.

At the same time, the CS loses some features as well. As each thread is no longer treated as a pixel, so the association with geometry is lost (unless specifically passed in a data structure). This means that, although CS programs can still use texture samplers, automatic trilinear LOD calculations are not automatic (LOD must be specified). Additionally, depth culling, anti-aliasing, alpha blending, and other operations that have no meaning to generic data cannot be performed inside a CS program.

The type of new applications opened up by the CS are actually infinite, but the most immediate interest will come from game developers looking to augment their graphics engines with fancy techniques not possible in the Pixel Shader. Some of these applications include A-Buffer techniques to allow very high quality anti-aliasing and order independent transparency, more advanced deferred shading techniques, advanced post processing effects and convolution, FFTs (fast Fourier transforms) for frequency domain operations, and summed area tables.

Beyond the rendering specific applications, game developers may wish to do things like IK (inverse kinematics), physics, AI, and other traditionally CPU specific tasks on the GPU. Having this data on the GPU by performing calculations in the CS means that the data is more quickly available for use in rendering and some algorithms may be much faster on the GPU as well. It might even be an option to run things like AI or physics on both the GPU and the CPU if algorithms that always yield the same result on both types of processors can be found (which would essentially substitute compute power for bandwidth).

Even though the code will run on the same hardware, PS and CS code will perform very differently based on the algorithms being implemented. One of the interesting things to look at is exposure and histogram data often used in HDR rendering. Calculating this data in the PS requires several passes and tricks to take all the pixels and either bin them or average them. Despite the fact that sharing data is going to slow things down quite a bit, sharing data can be much faster than running many passes and this makes the CS an ideal stage for such algorithms.

A while back we took a look at OpenCL, and we know that OpenCL will be able to share data structures with OpenGL. We haven't yet gotten a developer's take on comparing OpenCL and the DX11 CS, but at first blush it seems that the possibilities opened up for game developers and graphics processing with DX11 and the Compute Shader will also be possible with OpenGL+OpenCL. Although the CS can be used as a general purpose hardware accelerated GPU computing interface, OpenCL is targeted more at that arena and its independence from Microsoft and DirectX will likely mean wider adoption as a GPU compute language for general purpose tasks.

The use of OpenGL has declined significantly in the game developer community over the last five years. While OpenCL may enable DX11 like applications to be written in combination with OpenGL, it is more likely that this will be the venue of workstation applications like CAD/CAM and simulations that require visualization. While I'm a fan of OpenGL myself, I don't see the flexibility of OpenCL as a significant boon to its adoption in game engines.

Drilling Down: DX11 And The Multi-Threaded Game Engine So What's a Tessellator?


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  • Hrel - Sunday, February 1, 2009 - link

    This is one of the most poorly written articles I've ever read on anandtech. It's like the author couldn't organize his thoughts properly. Also, the speculation was riddled with subjective assumptions. I'm not sure if the author just doesn't know this topic very well or if he hadn't slept in 3 days, but this could have been done much better. Great topic though, and interesting subject matter. Reply
  • GourdFreeMan - Sunday, February 1, 2009 - link

    Derek, the DX10 geometry shader was never really intended to do tessellation, and really should not be thought of as a generalized tessellator. It was designed to offer a generalized hardware implementation of vertex effects such as skinning, vertex blending and tweening (see the dolphin demo in the DX SDX for what I am refering to here).

    If it becomes desirable at some point in time in the future to offer fully programable tessellation, then vertex shader, hull shader, tessellator, domain shader and geometry shader could all be merged into another compute shader earlier in the pipeline to do generalized vertex manipulation.

    Of course, it is also possible that the existing tessellator will prove more efficient as fixed function hardware, and only minor functionality improvements will be added.
  • eXistenZ - Sunday, February 1, 2009 - link


    I just wanted to add, that ATi graphic cards have tesslator included since Radeon 8500, but i can be wrong...
    I remember "Truform" technology, which is working in Serious Sam, or Return to Castle Wolfenstein, and Counter Strike 1.6 (it is not working now in Counter Strike).

    I want to know, if author of this article forgot about it, or im wrong about this technology.

    Sorry for my english, im from Slovakia :)
  • haukionkannel - Sunday, February 1, 2009 - link

    There has been a tessalation unit in ATI cards for some time. It's not the same as is reguired in DX11, but guite near. I think that it was mentioned in the article...

    From what I know is that DX10 has been slow because in most games it's just dx9 with some clued features from dx10 above it. With pure dx10 codepath it would have been faster, but that would have left all those XP-customers out, and would not have been sound economically...
    The author hopes that Win7 win encourage the transfer from XP, so there will be larger amount of DX10 and DX11 platforms. So it would become enonomically possible to make DX11 based games (just leaving out some pure DX11 features so that older dx10 cards could handle the games.) So actually when dx11 games comes out, they would be actually first to make use of all dx10 features...
    Well there are so many dx9 machines in the world that even that will take time. So we will see poor dx10, dx11 performance until the XP customers are not needed by game companies, and even then there are those pure console transfers without any optimization like GTA...
    I hope that "Chattered horisont" from Futuremark shows what DX10 can really do. It is goint to be pure DX10 game, so it can use advantages that dx10 can offer. On the other hand it can be next Crysis that looks really good, but makes your hardware moan for more power. We will see...
  • yyrkoon - Sunday, February 1, 2009 - link

    "On the flipside, DX11 will be able to run on down level hardware."

    Um . . . Eh ? English ?

    "This may not significantly speed up the graphics subsystem (especially if we are already very GPU limited), but this does increase the ability to more easily explicitly massively thread a game and take advantage of the increasing number of CPU cores on the desktop. "

    ... and significantly slow things down even further.

    " These code resources are huge and can be hard to manage without OOP (Object Oriented Programming) constructs. But there are some differences to how things work in other OOP languages. "

    I think you would find many experienced programmers who would say that OOP is a way of programming, not necessarily a language type, and I would have to agree with them. Now if you mean languages that *support* OOP, then sure, I can live with that.

    Also, one other minor thing that kind of bothers me. You speak of Directx 6, but was Directx 6 an actual redistributable ? I definitely do not remember it, but I *do* remember Directx 5, Direct 7, 8, . . . and even that thing MS claims never existed . . . WinG.
  • DerekWilson - Friday, February 6, 2009 - link

    down level hardware == hardware that meets a lower DX spec (like DX10 hardware).

    allowing games to be more mulithreaded using a fine grained synchronization scheme ala DX11 should not slow things down if developers take advantage of it correctly (which will be much easier than doing your own management here).

    yes i did mean languages that support the OOP model.

    DX6 was a Win98 thing ... it existed and actually was (iirc) the first version of DX to be hardware accelerated ... at least that's how I remember it.

    DX4, on the other hand, never existed -- MS skipped from DX3 to DX5.
  • frozentundra123456 - Sunday, February 1, 2009 - link

    I was initially unhappy with both Vista and DX10. However, I have come to accept Vista, but dont know if it is that much improved over WinXP. I only have Vista because I bought a new computer with that OS intstalled. I dont really know of anything I do with Vista that could not be done with XP. The only advantage to Vista is that it is supposedly more secure than XP, but I never had any major security problems with XP, nor have I had any with Vista.
    DX10 is still more of a disappointment to me. It requires too many resources and does not seem to offer corresponding improvements in visual quality. Nearly every game I have that is DX10 compliant, I run in DX9 mode because the performance improvement in DX9 more than makes up for the slight visual improvement with DX10. (Yes, I know I need a better graphics card.) I have an HD2600 pro, which was supposedly a "mid range" DX10 card when it came out, and it is virtually worthless for trying to play in DX10 mode, as I stated above.
    I wonder if DX9 will still be supported when DX11 comes out. If not, they had better make DX11 run better on low to midrange hardware than DX10, or there will be a lot of unhappy users.
  • epyon96 - Saturday, January 31, 2009 - link

    Since Derek claims that Direct X 11 is simply a superset of Dx10, why does Microsoft release it simply as 10.2 instead? I am curious what makes a Direct X version and what determines an incremental move forward. Reply
  • ltcommanderdata - Saturday, January 31, 2009 - link

    I'd like to know that too. Since to me DX9.0c (SM3.0) seems to have been a pretty major step forward from DX9.0 (SM2.0), even a whole new shader model, yet it was only given a letter subscript. It should have at least been DX9.1.

    My cynical view? It's all marketing and Microsoft appeasing hardware vendors for their own benefit. For example DX8.1 was supposed to be a decent step forward, going from SM1.1 to SM1.4 with longer shaders and other features. Yet nVidia refused to support SM1.4 and managed to convince Microsoft to call SM1.3 DX8.1 compliant even though it's closer to SM1.1 than SM1.4. My suspicion is that Microsoft agreed with nVidia, because at that time nVidia was making the GPU for the XBox and Microsoft needed them.

    A similar situation occurred with DX9.0c and SM3.0. This time ATI wasn't going to offer immediate support for SM3.0 in their GPUs. So in order for ATI's X8xx generation to not look so far behind, SM3.0 was only marketed as DX9.0c instead of DX9.1 or something more major. Why would Microsoft appease ATI? Conveniently, ATI was making the GPU for Microsoft's next-gen XBox 360, so Microsoft needed them.

    This might not actually be true, but it's interesting that the swings in XBox GPU choice corresponds with Microsoft's degree of emphasis on DirectX capability.

    In the case of DX11, I think there is sufficient new capabilities with Tessellation and Compute Shaders to justify a major number increase. I believe what Derek means is that DX11 is a superset of DX10 in the same way DX9 is a superset of DX8. They both offer backwards compatibility. In contrast, DX10 is not compatible with DX9 and Vista actually has separate DX10 and DX9 APIs (and third Vista specific DX9.0L) while DX8, DX7, etc can run on the DX9 API.
  • GourdFreeMan - Sunday, February 1, 2009 - link

    Microsoft originally had some soft guidelines in this respect. Letter releases were to represent minor changes in the API such as the range and precision allowed for constants, max number of loop iterations in pixel and vertex shaders, etc. Point releases would permit added functionality to stages of the rendering pipeline. Version releases could include changes to the rendering pipeline itself. In practice, point and letter releases have been to support vendor-specific functionality, and version releases have set a baseline for all vendors.

    Microsoft's guildelines fit for all DirectX changes except 9.0c, which was really a vendor-specific change to fit the nVIDIA 6000 series hardware. (ATi did not have SM3.0 cards until its next hardware generation).

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