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posted by martyb on Friday May 01 2020, @12:17AM   Printer-friendly
from the all-your-bits-are-belong-to-us dept.

DisplayPort Alt Mode 2.0 Spec Released: Defining Alt Mode for USB4

As the tech industry gears up for the launch of the new USB4 standard, a few more parts first need to fall into place. Along with the core specification itself, there is the matter of alternate modes, which add further functionality to USB Type-C host ports by allowing the data pins to be used to carry other types of signals. Keeping pace with the updates to USB4, some of the alt modes are being updated as well, and this process is starting with the granddaddy of them all: DisplayPort Alt Mode.

The very first USB-C alt mode, DisplayPort Alt Mode was introduced in 2014. By remapping the USB-C high speed data pins from USB data to DisplayPort data, it became possible to use a USB-C port as a DisplayPort video output, and in some cases even mix the two to get both USB 3.x signaling and DisplayPort signaling over the same cable. As a result of DisplayPort Alt Mode's release, the number of devices with video output has exploded, and in laptops especially, this has become the preferred mode for driving video outputs when a laptop doesn't include a dedicated HDMI port.

If you're willing to accept Display Stream Compression... New DisplayPort spec enables 16K video over USB-C

VESA press release.

Previously: Forget USB 3.2: Thunderbolt 3 Will Become the Basis of USB 4
DisplayPort 2.0 Announced, Triples Bandwidth to ~77.4 Gbps for 8K Displays
Speed-Doubling USB4 is Ready -- Now We Just Have to Wait for Devices

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  • (Score: 2) by Immerman on Friday May 01 2020, @05:31PM (4 children)

    by Immerman (3985) on Friday May 01 2020, @05:31PM (#989060)

    8k at 60Hz is nice I suppose, for that handful of people that can use such resolution. Okay, yeah, I suppose it'd be nice for a 40+" monitor - I can still make out individual pixels at 4k from up close.

    What I really want to know is, is the spec flexible enough to use the same data rate to deliver 2x4k at 120Hz? That starts being really useful for VR/AR applications. One thin cable that can transmit raw (= low lag) video along with all all the tracking information

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  • (Score: 2) by takyon on Friday May 01 2020, @06:41PM (3 children)

    by takyon (881) <> on Friday May 01 2020, @06:41PM (#989097) Journal

    Probably: []

    Two 4K (3840 × 2160) displays @ 144 Hz and 8 bpc (24 bit/px, SDR) RGB/Y′CBCR 4:4:4 color (uncompressed)

    [...] Three 4K (3840 × 2160) displays @ 90 Hz and 10 bpc (30 bit/px, HDR) RGB/Y′CBCR 4:4:4 color (uncompressed)

    [...] When using only two lanes on the USB-C connector via DP Alt Mode to allow for simultaneous SuperSpeed USB data and video, DP 2.0 can enable such configurations as:

    Three 4K (3840 × 2160) displays @ 144 Hz and 10 bpc (30 bit/px, HDR) RGB/Y′CBCR 4:4:4 color (with DSC)

    Two 4K × 4K (4096 × 4096) displays (for AR/VR headsets) @ 120 Hz and 10 bpc (30 bit/px, HDR) RGB/Y′CBCR 4:4:4 color (with DSC)

    You said "raw", they are saying "with DSC" for 4K×4K @ 120 Hz and simultaneous tracking information. I think 12 bits-per-color might be better for increasing color depth and using HDR at the same time; I'd have to research that.

    My position is that data cables are a nuisance for VR. Everything is going to go wireless or most likely standalone. And part of the way to achieve that will be by using foveated rendering to reduce the necessary on-board performance and bitrates. I don't even know if these video standards are set up to handle foveated rendering (changing mix of resolutions) whatsoever, so some proprietary solutions might end up being used.

    [SIG] 10/28/2017: Soylent Upgrade v14 []
    • (Score: 2) by Immerman on Friday May 01 2020, @09:22PM (2 children)

      by Immerman (3985) on Friday May 01 2020, @09:22PM (#989201)

      I agree that cables are a nuisance - unfortunately there don't seem to be any wireless technologies on the horizon that can supply the required bandwidth, and most compression algorithms introduce additional lag that can be ill afforded.

      Given the speed and erraticness of eye saccades I have my doubts about the long-term comfort of foveated rendering, but it's certainly an intruiging option. In a few generation of rendering hardware, and with fast enough tracking sensors and predictive algorithms (using eye speed and acceleration to predict where it will stop), it just might be possible to avoid "pop-in" as your eye comes to rest on its new target

      I suspect that the video standards would be fine for it, at least assuming a "smart" headset controller. After all, it's basically just coordinated picture-in-picture, with a parallel data stream (easily encoded in unused portions of the image if nothing else) indicating where the focal picture should be positioned. Plus probably some upscaling and graceful blending between them, but that can be handled 100% in the headset, the video stream doesn't need to know it's happening.

      • (Score: 2) by takyon on Saturday May 02 2020, @01:22AM (1 child)

        by takyon (881) <> on Saturday May 02 2020, @01:22AM (#989281) Journal

        Eye tracking could be an easier problem than you would expect. See this earlier comment [] I made. That researcher used an off-the-shelf FOVE headset.

        There's also Tobii, which has made a 1200 Hz eye tracker for researchers and more modest hardware for VR:

        Tobii Pro Launches New Advancement in Eye Trackers for Behavioral Research []

        I don't think that kind of sampling/tracking rate is necessary at all for VR foveated rendering. It remains to be seen if the Hz even needs to be higher than the display's refresh rate.

        What do you do if the tracking is a little slow? Just render more of the frame in high resolution, with a tiny circle in 16K, 8K in a ring outside of that, and so on. The in-between resolutions would "catch" fast eye movement and could be adjusted as necessary by the headset manufacturer. Throw in screen %s for each of the resolutions, and you can come up with the total number of pixels and possibly get an idea of the performance required. 50% reduction in necessary performance has been estimated, but I still think it could cut it by 90% if done right. This would bolster any headset+GPU that adopts the technique, and especially help ARM-based standalone headsets.

        I could even see desktop monitors integrate eye tracking and foveated rendering. Focus on something in the corner of your current display. Depending on the size and the distance from you to the display, you might not be able to see much detail at all in the opposite corner. This kind of thing would require a depth sensor next to the camera, but that technology was consumerized with Microsoft Kinect way back in 2010 and has been used in smartphones more recently.

        [SIG] 10/28/2017: Soylent Upgrade v14 []
        • (Score: 2) by Immerman on Saturday May 02 2020, @03:08AM

          by Immerman (3985) on Saturday May 02 2020, @03:08AM (#989317)

          Hmm, you might be right. I hadn't actually run the numbers before, and I must admit I greatly overestimated the saccade speed - it seems that even with with very generous margins so that you're unlikely to ever focus outside the foveated render the benefits could be dramatic

            Saccade speeds can exceed 500°/s: []
            So a very fast saccade and a rather nasty 20ms of lag could allow allow for a fovea displacement of 500°/s*.02s = 10° away from the "expected" position, or about 10% of the FOV of a current headset, or about 1% of the visible area.

          Meanwhile the resolution of the fovea itself falls off to about 1/5th the peak resolution at 10° from the focal point (20° total) for a 30° foveated region (~10% of the total screen area) would capture just about everything you could possibly see at higher resolution within the next 20ms. And that's assuming just a single jump to up to 5x the background resolution. At a more comfortable 10ms lag, and with just a smidge of saccade-prediction, you could probably narrow the necessary fovea-rendered region dramatically.