David McGriffy wrote:
I got a GearVR recently and it does work better than Google Cardboard. Much
of this is just comfort, I think, but I understand that it has its own
gyros, mostly because they are faster. 'Heresay' is that the Oculus Rift
samples at least 1000Hz. I have actually written an audio rate rotate that
could handle this, but it does seem like overkill. At normal head turning
rates, I find interpolating the rotation within each block to be enough.
http://blogs.valvesoftware.com/abrash/latency-the-sine-qua-non-of-ar-and-vr/
(Written by a super-expert of VR)
Assuming accurate, consistent tracking (and that’s a big if, as I’ll
explain one of these days), the enemy of virtual registration is
latency. < If too much time elapses between the time your head starts
to turn and the time the image is redrawn to account for the new pose,
the virtual image will drift far enough so that it has clearly wobbled
(in VR), or so that is obviously no longer aligned with the same
real-world features (in AR). >
(Elaboration:)
Suppose you rotate your head at 60 degrees/second. That sounds fast,
but in fact it’s just a slow turn; you are capable of moving your head
at hundreds of degrees/second. Also suppose that latency is 50 ms and
resolution is 1K x 1K over a 100-degree FOV. Then as your head turns,
the virtual images being displayed are based on 50 ms-old data, which
means that their positions are off by three degrees, which is wider
than your thumb held at arm’s length. Put another way, the object
positions are wrong by 30 pixels. Either way, the error is very
noticeable.
In other words: There are speed/delay problems, there are sync problems,
there might be more...
You can do prediction to move the drawing position to the right place,
and that works pretty well most of the time. Unfortunately, when there
is a sudden change of direction, the error becomes even bigger than
with no prediction.
Tracking latency is highly dependent on the system used. An IMU (3-DOF
gyro and 3-DOF accelerometer) has very low latency – on the order of 1
ms – but drifts. < In particular, position derived from the
accelerometer drifts badly, because it’s derived via double
integration from acceleration. > Camera-based tracking doesn’t drift,
but has high latency due to the need to capture the image, transfer it
to the computer, and process the image to determine the pose; that can
easily take 10-15 ms. < Right now, one of the lowest-latency
non-drifting accurate systems out there is a high-end system from NDI,
which has about 4 ms of latency, so we’ll use that for the tracking
latency. >
It would be far easier and more generally applicable to have the
display run at 120 Hz, which would immediately reduce display latency
to about 8 ms, bringing total latency down to 12-14 ms.
(s. PlaystationVR...)
If you ever thought that AR/VR was just a simple matter of showing an
image on the inside of glasses or goggles, I hope that by this point
in the blog it’s become clear just how complex and subtle it is to
present convincing virtual images – and we’ve only scratched the surface.
Wise words... (No, really!)
But not to get stuck and despair: Motion-tracked audio seems to be
sooooo much easier to realize...
Best regards,
Stefan
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