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DK2 and Latency Mitigation

DK2 and Latency Mitigation. Cass Everitt Oculus VR. Being There. Conventional 3D graphics is cinematic Shows you something On a display, in your environment VR graphics is immersive Takes you somewhere Controls everything you see, defines your environment

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DK2 and Latency Mitigation

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  1. DK2 and Latency Mitigation Cass Everitt Oculus VR

  2. Being There • Conventional 3D graphics is cinematic • Shows you something • On a display, in your environment • VR graphics is immersive • Takes you somewhere • Controls everything you see, defines your environment • Very different constraints and challenges

  3. Realism and Presence • Being there is largely about sensor fusion • Your brain’s sensor fusion • Trained by reality • Can’t violate too many hard-wired expectations • Realism may be a non-goal • Not required for presence • Expensive • Uncanny valley

  4. Oculus Rift DK2 • 90°-110° FOV • 1080p OLED screen • 960x1080 per eye • 75 Hz refresh • Low persistence • 1 kHz IMU • Positional tracking

  5. Low Persistence • Stable image as you turn - no motion blur • Rolling shutter • Right-to-left • 3ms band of light • Eyes offset temporally

  6. Positional Tracking • External camera, pointed at user • 80° x 64° FOV • ~2.5m range • ~0.05mm @ 1.5m • ~19ms latency • Only 2ms of that is vision processing

  7. Position Tracking + = technology magic The good news: You don’t need to know.

  8. Image Synthesis • Conventional planar projection • GPUs like this because • Straight edges remain straight • Planes remain planar after projection • Synthesis takes “a while” • So we predict the position / orientation • A long range prediction: ~10-30ms out

  9. Note on Sample Distribution • Conventional planar projection, not great for very wide FOV • Big angle between samples at center of view

  10. AlternativeSample Distributions • Direct render to cube map may be appealing • Tiled renderers could do piecewise linear • Brute force will do in the interim • But not much FOV room left at 100°

  11. Optical Distortion

  12. Distortion Correction

  13. Optical Distortion • HMD optics cause different sample distribution – and chromatic aberration • Requires a resampling pass • Synthesis distribution -> delivery distribution • Barrel distortion to counteract lens’s distortion • Could be built in to a “smarter” display engine • Handled in software today • Requires either CPU, separate GPU, or shared GPU

  14. Display Engine (detour) • In modern GPUs, the 3D synthesis engine builds buffers to be displayed • A separate enginedrives the HDMI / DP / DVI output signal using that buffer • This engine just reads rows of the image • More on this later…

  15. Time Warp • Optical resampling provides an opportunity • Synthesized samples have known location • Global shutter, so constant time • Actual eye orientation will differ • Long range prediction had error • Better prediction just before resampling • Both predictions are for the same target time • So resample for optics and prediction error simultaneously! • Note: This just corrects the view of an “old” snapshot of the world

  16. Time Warp + Rolling Shutter • Rolling shutter adds time variability • But we know time derivative of orientation • Can correct for that as well • Tends to compress sampling when turning right • And stretch out sampling when turning left

  17. Asynchronous Time Warp • So far, we have been talking about 1 synthesized image per eye per display period • @75 Hz, that’s 150 Hz for image synthesis • Many apps cannot achieve these rates • Especially with wide-FOV rendering • Display needs to be asynchronous to synthesis • Just like in conventional pipeline • Needs to be isochronous– racing the beam • Direct hardware support for this would be straightforward

  18. Asynchronous Time Warp • Slower synthesis requires wider FOV • Will resample the same image multiple times • Stuttering can be a concern • When display and synthesis frequencies “beat” • Ultra-high display frequency mayhelp this • Tolerable synthesis rate still TBD • End effect is, your eyes see the best information we have • Regardless of synthesis rate

  19. Questions? • cass.everitt@oculusvr.com • For vision questions: • dov.katz@oculusvr.com

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