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Could SLI allow extremely high resolution dual screens?

captaintripscaptaintrips Posts: 313
Art3mis
edited May 2013 in General
Just out of curiosity, wondering if Oculus plans to realease a dual-screen oculus in the future (two independent screens at full resolution per each eye), and if so... could there be any possibility of making each screen rendered by a single GPU? One GPU focuses on a screen 1920x1080 (or higher) and the other GPU does the same with the other? I know SLI profiles allow you to choose what each GPU can do now... such as one handling the graphics and the other handling the MSAA, or both handling graphics, or whatnot.

I remember reading a while back that was one of the biggest hurdles with VR was the bandwidth and graphics power required to rendered two separate images at higher resolutions per each image, however it seems plausible simply having SLI, or more specifically, having a sepereate GPU to handle each of the two screens separately would resolve that?

My only contradiction on this thought would be possible latency between GPU's and if microstutter would affect anything, but I'm not technical enough in how cross gpu architecture works to understand if or how that may play a role.

Thoughts or comments?

Comments

  • nellusnellus Posts: 113
    mmm, i'm not an expert of cross-gpu but i wanna make a reflection.
    In this moment if i plug two gpu i've not bouble performance, 'cause the sli (or cross fire) don't render one frame per gpu, but render one frame togheter with a boost of 60% or little more.
    i don't know if the programmers can solve this, not right now.

    Correct me if i'm in fault. tx.
  • tlopestlopes Posts: 163
    SLI currently operates in one of several different modes:
    Split Frame Rendering (SFR), the first rendering method. This analyzes the rendered image in order to split the workload 50/50 between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry. For example, in a scene where the top half of the frame is mostly empty sky, the dividing line will lower, balancing geometry workload between the two GPUs. This method does not scale geometry or work as well as AFR, however.
    Alternate Frame Rendering (AFR), the second rendering method. Here, each GPU renders entire frames in sequence – one GPU processes even frames, and the second processes odd frames, one after the other. When the slave card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master card, which then outputs the completed frames. Ideally, this would result in the rendering time being cut in half, and thus performance from the video cards would double. In their advertising, NVIDIA claims up to 1.9x the performance of one card with the dual-card setup. While AFR may produce higher overall framerates than SFR, it may result in increased input latency due to the next frame starting rendering in advance of the frame before it. This is identical to the issue that was first discovered in the ATI Rage Fury MAXX board in 1999.[1] This makes SFR the preferred SLI method for fast paced action games.
    SLI Antialiasing. This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern (for example, slightly up and to the right), and the second GPU uses a pattern offset by an equal amount in the opposite direction (down and to the left). Compositing both the results gives higher image quality than is normally possible. This mode is not intended for higher frame rates, and can actually lower performance, but is instead intended for games which are not GPU-bound, offering a clearer image in place of better performance. When enabled, SLI Antialiasing offers advanced antialiasing options: 2 GPUs are capable of up to 64x while a 3 GPU setup supports up to 96x antialiasing. A quad-GPU system can reach up to 128x.[2]

    Since most programs designed to run for VR on the Rift are performing CPU-GPU synchronization each frame (that is, to lower latency, they wait until the GPU is done rendering the frame before starting the next one), that seems to me to throw the usefulness of AFR right out the window. SFR is still interesting though, and it lends itself moderately well to rendering the scene twice (you could render the scene once per videocard and they should both finish at roughly the same time). In fact, if the split were vertical instead of horizontal, then that'd work fairly well for speeding up rendering side-by-side frames for VR. However, I bet that many SLI and Crossfire drivers would have to be heavily modified to support this style of rendering. It's a cool idea, and I hope that the graphics cards manufacturers take it into account when they start tailoring their hardware and software towards VR more.
  • IVI4ttIVI4tt Posts: 9
    The other issue with SLI or CrossfireX (especially CrossfireX) is microstutter. While it's merely annoying on a monitor, I suspect on a HMD it will be deeply unpleasant.

    And the only solution to multi-GPU microstutter is to deliberately add in latency between frames -- something we definitely don't want.
  • What would be cool is if it were two different displays in the headset maybe software could allow for each display to be driven by one of two GPUs, that would be pretty cool. Not sure if it would be difficult to have the frames timed or something, probably.
  • theodorebriggstheodorebriggs Posts: 9
    NerveGear
    It should be possible to use the SFR approach for the rift, however I suspect it will take some major tweaking and maybe even a hardware generation it get it working better than one card. The good part is the two frames should be very similar unless there are objects very close. The bad part is they would likely need to extensively rework the image combination step to split each horizontal line between the cards. There is a significant chance this would require a change to the hardware (or at least firmware) of the cards. Keeping them synced up may also lead to a latency hit if one side has significantly more work to do than the other. So, don't hold your breath and plan on a single card for VR for a while.
  • jwilkinsjwilkins Posts: 580
    Art3mis
    It seems to me that two different GPU rendering to two different displays is not SLI, but just the normal state of affairs.
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  • 320x200320x200 Posts: 125
    Hiro Protagonist
    jwilkins wrote:
    It seems to me that two different GPU rendering to two different displays is not SLI, but just the normal state of affairs.

    What happens to v-sync with multi-monitor setups? I'm not sure what exactly it would feel like to have one eye lagging behind the other...
  • jwilkinsjwilkins Posts: 580
    Art3mis
    It couldn't be off by more than 8 milliseconds off at 60fps. Do you think that is detectable?
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  • 320x200320x200 Posts: 125
    Hiro Protagonist
    jwilkins wrote:
    It couldn't be off by more than 8 milliseconds off at 60fps. Do you think that is detectable?

    It would depend on the visible delta between frames. If you have a bright 1-frame machine gun muzzle flash consistently going off in one eye and then the other over and over I would bet that's pretty noticeable.
  • jwilkinsjwilkins Posts: 580
    Art3mis
    I don't think so. 30 flashes per second would be perceived as flicker. I don't think you could tell the flicker was alternating between eyes.
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  • 320x200320x200 Posts: 125
    Hiro Protagonist
    jwilkins wrote:
    I don't think so. 30 flashes per second would be perceived as flicker. I don't think you could tell the flicker was alternating between eyes.

    Well, possibly at that speed, but you want to consider large deltas with a wide range of frequencies. To stick with the muzzle flash example, 30 per second is well over the firecap of many weapons in cod so I don't think you can discount the problem just by assuming the delta will either be infrequent or hyper-frequent.

    Simple test to run with the Rift, just manually mess up the eyes, but I've not tried it myself. Given recent issues I've hit with sound perception I don't discount any chance of human perception picking up unnatural things as feeling "off" or "somehow wrong". Yeah, I know the ear is a lot faster than the eye, but still, doing unnatural things, even quickly, is often perceptible.
  • jwilkinsjwilkins Posts: 580
    Art3mis
    I think you are absolutely right and we need to actually test it. We can only discuss this so much :) My problem right now is that I don't have a Rift or a high speed camera (to confirm the actual delay between eyes instead of trusting I programmed it correctly). It would still be anecdotal until we could study more people's reaction. Unfortunately I can't do that unless I go through a lot of red tape, but an individual or company can experiment on people with impunity :lol:
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  • AntonycAntonyc Posts: 5
    NerveGear
    Been thinking alot about sli support specifically for the rift

    I reckon if NVidia could implement a sli method, that renders the 3d image side by side with a vertical split
    each card would drive one half of the screen, this would be the best solution for everyone

    it would work for 1 screen and also 2 screens as you would just set them up as 2 screen surround
    imagine dual 1080p or even 1440p screens :)

    I think this would also give us the best performance, I currently use sli with 3 screens and a resolution of 5760*1080
    and my two gtx580 handle most modern games with ease at that resolution, so I imagine I have a lot of headroom as far as the rift dev kit is concerned

    it would work better for 2 screens as they should be perfectly in sync going via sli

    I can see it being beneficial for NVidia, as I imagine sli using side by side vertical split method would be the primary way of powering a hmd like the rift, they'd probably sell quite a extra cards because of this
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