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GPU and Compositor


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Relationship GPU and Compositor

Hello, now i am VR application developer using HTC VIVE.

During development, i have some question.


 1.  I connected the hdmi cable to the main board. (Desktop to link box) 

      so, 'compositor is not available' Error is presentating SteamVR.

      i connected the hdmi cable to external GPU , VIVE is working.

     This means thant the warning "compositer is not available" is displayed because the built-in
     graphics do not support vive's resolution?


2. i test simple VR application used notebook equipment GPU is 940MX.
    simple VR application is maded by unity3d and just one plane.

    but i connect vive, error is display "compositer is not available".

    i already know minum PC spec, I think a very simple vr program should work. There is no
    interaction between the controller and the object, and there is nothing to load the gpu.

    I understand if the vr program is running and there is a delay.
    However, there was an error that the compositor could not be used and it was a pity 
    that the HMD itself could not be used.
  What exactly is the compositor role?
  Does it work only when I use a graphics card that supports 2160x1200 resolution of HTC VIVE?


3. Can I connect multiple base stations to implement a more extended room scale?

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Hi! Those are some great questions. I can give some very basic answers, but I think  or someone else might be able to explain in more detail.

1. Yes, that is essentially the case. VR is inherently a very resource intensive process, and only high end GPUs can provide the processing power needed to render the displays in all their complexity.

2. In Jeff W. Murray's book, 'Building Virtual Reality with Unity and Steam VR' he describes it simply: "The Compositor is in charge of synchronization, distortion, prediction and other issues that would otherwise be a huge challenge to get working nicely for comfortable VR experiences." In otherwords, without the Compositor... no VR. Of course, it's a very powerful bit of software and requires a lot of power behind it to work properly.

3. No, with the current generation of Lighthouse tracking, you can only use 2 base stations. Lighthouse tracking 2.0 will open this up a bit, but that will be a whole new generation of hardware and it's not out yet for a while. You can read more about that, here: https://steamcommunity.com/games/steamvrtracking/announcements/detail/1264796421606498053

I hope I answered your questions at least somewhat,

Thank you,

-John C

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A couple of things to note. The 940mx is about 4-8 times less powerful than the GTX 970 (our min spec) depending on which type of computation that is being bench marked. It is severely underpowered for use with VR hence why the compositor yeilds errors. The reason so much GPU power is needed is in part the resolution of the Vive, the fact that it must render stereoscopically, and also the fact that the headset needs to update at 90hz/FPS in order to provide a smooth enough experience that motion sickness and other unwanted physical discomfort is eliminated. In fact, the Vive's full bandwidth is still not something that most 9 series GPUs can even begin to approach and there is some scaling that happens by default (Google "Vive supersampling" to learn more...).

2) I can't describe the compositor's role any better than Synthesis' explanation. I'd recommend watching these two videos on VR rendering that go into depth about the role of the compositor.  

  •  - (2015)
  •  - (2016 update that covers async. timewarp and other recent advancements)
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