Request for an Alternate eye rendering mode

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    Hi Ralf,

    I remember reading in Reddit, in the topic of the GTA VR mod, that you had the code of an alternate eye rendering mode that were present in old versions of VorpX.

    Could be possible to implement it in VorpX again? It works very good, and with HMDs like the Valve Index with high refresh rate people report very good results with GTA on that mode. Although it has its flaws, like sickness when steering in a vehicle and some more rendering issues, that can be warned, as that rendering works as good as a native VR mode. At least for me, GTA VR gives lots of presence. And of course, a nice framerate.


    Considering the inherent glitches due to both eyes being rendered at a different time, the fact that it falls apart completely with any framerate below the headset refresh rate and the incompatibility with some headsets don’t make the approach look overly intriguing to me TBH. The only real “advantage” is that it’s way easier to implement than proper single frame stereo.

    Don’t get fooled by the framerate a game’s FPS counter might display. The effective stereo framerate is just half of that since each eye only gets updated every other frame. 90fps shown by an FPS counter with alternate eye stereo means 45fps per eye. In other words: performance wise proper single frame stereo at 45fps is the same as alternate eye stereo at 90fps. The actual performance gain after factoring out the misleading doubled framerate you see with an FPS counter would be about 10% in most cases since the method has slightly less overhead.

    Might come at some point as an option, but weighing the potential 10% effective performance gain against the inherent issues doesn’t really make it a priority.


    I have played more than 100h of GTA5 via MOD with alternate eye rendering, and it allows you to have better resolution and/or better frame rate, but the moment you start turning your head, is awfull, really awfull.

    If it’s the only way to have the game on VR, is ok for me, but if you can have single frame stereo with half the framerate, I prefer it.


    The most important point here is that you don’t get twice the framerate with alternate eye rendering in the first place.

    A game’s FPS counter will show twice the framerate, but that’s highly misleading. Since each eye only gets updated every other frame the actual framerate per eye is half of what the FPS counter shows. Alternate eye rendering has more or less the same framerate per eye as single frame stereo, just with added glitches due to both eyes not being rendered at the same time, which can be fairly brutal if either you or stuff in the game moves fast.

    For a developer it’s way easier to implement, but from a user’s perspective there isn’t really any advantage over proper single frame stereo except for a minor FPS boost since it has a little less overhead. There probably would be some exceptions in games with high draw call counts, which might benefit more performance wise. But considering all the inherent issues introduced by the left and right eye being drawn at a different time alternate eye rendering is nothing to get excited about in general. Quite the opposite actually.

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