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How They Work - Oculus ATW and SteamVR Reprojection

Discussion in 'VR Headsets and Sim Gaming - Virtual Reality' started by noorbeast, Jun 8, 2016.

  1. noorbeast

    noorbeast VR - The Next Generation Staff Member Moderator

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    In VR you need to render two different images to two different screens at very high FPS, which should be maintained to avoid judder, as it can cause VR sickness.

    It is very hardware intensive and very different from a single monitor, where some dropped frames in some places don't really matter and the processing demand is nowhere near as high.

    To put that into context here is a comparison of VR to monitors: http://alex.vlachos.com/graphics/Alex_Vlachos_Advanced_VR_Rendering_GDC2015.pdf

    Shaded Visible Pixels per Second
    ● 720p @ 30 Hz: 27 million pixels/sec
    ● 1080p @ 60 Hz: 124 million pixels/sec
    ● 30” Monitor 2560x1600 @ 60 Hz: 245 million pixels/sec
    ● 4k Monitor 4096x2160 @ 30 Hz: 265 million pixels/sec
    ● VR 1512x1680x2 @ 90 Hz: 457 million pixels/sec

    Oculus uses Asynchronous Time Warp and Valve's SteamVR uses Interleaved Reprojection to help manage the intense requirements.

    Asynchronous Time Warp: https://developer.oculus.com/blog/asynchronous-timewarp-on-oculus-rift/

    With ATW each frame is rendered for the left and right eyes and is processed by ATW before it is displayed. If the rendering is complete it is displayed as synchronous timewarp, but if not and a frame misses the VSync deadline then the previous render is reprojected, shifted for position.

    Interleaved Reprojection: https://steamcommunity.com/app/358720/discussions/0/385429254937377076/

    With SteamVR's Interleaved Reprojection if either the CPU or GPU get too close to using up the available frame time then the compositor will drop into half-time mode where every other frame is reprojected. The result is that the game will be updating at 45hz instead of the normal 90hz.
    • Informative Informative x 2
    Last edited: Jun 8, 2016
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