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DirectCompute Lecture Series 101: Introduction to DirectCompute

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In this lecture, Chas Boyd, the Program Manager Architect of Windows Graphics, gives you an introduction to DirectCompute. He covers some background on DirectCompute, gives examples of applications, gives an overview of the DirectCompute API, and then covers the principles of basic usage.

The Roundtable discussion for DirectCompute can be found here and the slides are available from here.

For more information about DirectX, check the DirectX Developer Center and the Windows Developer Center.

You can also download the Direct X SDK.

The following videos continue the series:

·         DirectCompute 110: Memory Patterns

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  • I really like the lecture series approch, and this series specifically has been good so far. The GPU capabilities being exposed via the new Windows graphics stack and DirectX APIs in Vista and Win7 are just awesome.

  • Allan LindqvistaL_ Kinect ftw

    awsome stuff Smiley but why isnt it visible on the front page?

     

    i have a general directx question though,

    you set only one compute shader to the directx context object you create, and from what ive seen you only set one pixel and vertex shader to the context also. but how does that work when you have a whole bunch of shaders you want to execute? in a game i imagine there are tons of shaders given all the object you want to render, are those linked together into i single giant shader or do you, for each object, set the shader to the context, then call draw, then set the shaders for the next object?

     

    also, how does the input assembler stage fit into all of this?

     

    i know its slightly off topic in a discussion on compute shaders but it seems that would be relevant when thinking about displaying data from the shader. Smiley

  • IA stage is the first stage of the DirectX 10/11 graphics pipeline and as its name says, will assemble vertices from a stream of indices (if you have a DrawIndexed* call) and one or multiple streams of vertices (think of SOA or AOS concept, an array being
    a stream). IA stage will be responsible of feeding the next stage, the VS stage, with a vertex that has the requested input layout (a C-like structure).

    The IA stage also handle the different primitive topologies such as triangles list, triangle strips, points list, line strips, as well as adjacent primitives, etc.

    You execute only one shader at a time (from the DX API perspective). E.g.

    SetComputeShader(...)

    Dispatch

    SetComputeShader(...) 

    Dispatch

    ...

    or

    Set{VS|PS|GS}Shader(...)

    Draw*

    Set{VS|PS|GS}Shader(...)

    Draw*

    ...

    In a game there's a lot of shader, that's right. Some engines use what you can call state sorting. E.g. sort all objects that use this particular kind of shader (I.e. a leather shader or metal shader), or also same fixed function state like blend modes,
    depth/stencil modes, etc.. then during the draw pass of the game's rendering engine :

    foreach listOfObject in listsOfObjectSortedByState

      SetStates(listOfObject.GetStates())

      foreach object in listOfObject
         object.Draw()

    The goal here is to do minimal state changes (because of the performance cost of them). Which states are used for sorting is game or 3D engine dependent.

    Some games use also an übershader approach (one giant shader, with a lot of branch inside). So they don't have to sort objects by shader since everything is shaded the same way with some permutation (each object just have to update a constant buffer for
    saying, if normal mapping is on or off, what kind of lighting model is used, if lighting is on, and so on) :

    if( textureEnabled )
    { // fetch texels

    }
    if( normalMappingEnabled )
    { // fetch normal from normal map

    }

    if( lightingEnabled )
    {// do lighting stuff

    }

    Usually GPUs don't handle very well branchy code, but I know that IHV's drivers have optimizations for rebuilding a shader internaly without branch if the conditional expressions of the if branches are known at draw call time. It takes some overhead of course.
    So games render at the start of a new level the whole level (with an orthographic projection and no culling) in an offscreen buffer (this produce visual crap, so you likely don't want gamers to see it). This ensure that everything that could be seen in this
    level will be drawn during this special pass.

    This pass produces the whole permutation set of state combinations and the gfx driver can build efficient internal representation of the über shader for each possible combination. The offscreen buffer is then thrown away and the normal rendering loop can
    happen (without potential FPS drop).

    Note also that now with DX11 you have Dynamic shader linkage mechanism that will permit to have shader code that use abstract interface at compile time, and do let the resolution happen during runtime. This mechanism is also available for DirectCompute (compute
    shaders) which is great.

     

  • Gus Classgclassy Helping developers rock at coding for Windows!

    aL_  I'm working on making this visible on the front page, hopefully the next couple of posts for this series will be visible there.

  • Allan LindqvistaL_ Kinect ftw

    ah, i see, thanks for the detailed reply Smiley

     

    so the IA stage sort of makes a mesh (or graph) out of lists of verts and edges and returns a pointer that you use later on after setting the shaders for that object? or do you call the IA stage for each object before setting the GS,VS and PS shader? your explination got me closer but i feel like on the cusp of getting whats going on there Smiley

     

    also, are you Chas? i guess my questions are pretty beginner level, but you seem to know a great deal about directx Smiley

  • Allan LindqvistaL_ Kinect ftw

    cool, the world needs to know Smiley directx has a whole lot of power but i think its a mystery to a lot of people

  • so the IA stage sort of makes a mesh (or graph) out of lists of verts and edges and returns a pointer that you use later on after setting the shaders for that object?

    Sort of. But there's no notion of list of edges, only indices that describe how the mesh is build up, according to a topology (triangle list, strips) and one or multiple streams of vertices. There's also no pointer in HLSL. NEVER. Smiley

     

    or do ypu call the IA stage for each object before setting the GS, VS and PS?

    You don't call a stage, only set states to a stage. But that's right you have to set IA stage properly before drawing each object (but as well as all the other stages), saying what is the topology, vertex layout, VBs and IB you are binding, etc.

     

    also, are you Chas?

    Lemme see.. putting a hand on the back of my neck.. No ponytail, i'm afraid. No beard either. This process proves right off the bat that I'm not Chas Boyd. Smiley

    I have some experience regarding DX, though. Few years ago I wrote a runtime for doing OpenGL CAD style graphics for a CAD vendor, implemented it on top of D3D10. This was challenging. There's a demo of it on channel9, done by an ex-workmate during Anantha's talk at PDC08. Search for something like "Write your Graphics Engine to shine on modern hw".

  • Allan LindqvistaL_ Kinect ftw

    i see.. i think i get it, but i'll look at some more samples and docs Smiley

    thanks for filling me in Smiley

  • You're welcome. If you have questions on DX 11 you can always ask at http://forums.xna.com/forums/76.aspx.

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