Avatar interpolation

[ada-tv]

Fixes #1280

In both demo videos the client being looked at is limited to 5 FPS to make the interpolation more visible.

Master

avatar-lerp-master.webm

This PR

avatar-lerp-pr.webm

[HifiExperiments]

oooo this is super cool

[ada-tv]

Something 7 and I have found in testing is that clients at a higher framerate seem to jitter and ones at low framerates are smooth. I'm not sure yet what the problem might be

[ksuprynowicz]

I sorted out the jitter issue. I'll try to have pull request ready in the evening :)

[ksuprynowicz]

I think I figured it out:
ada-tv#2

[ksuprynowicz]

I got backtrace for the crash:

OtherAvatar::interpolateJoints OtherAvatar.cpp:282
OtherAvatar::simulate OtherAvatar.cpp:423
AvatarManager::updateOtherAvatars AvatarManager.cpp:346
Application::update Application.cpp:2319
Application::idle Application.cpp:1875
Application::event Application_Events.cpp:103
QApplicationPrivate::notify_helper(QObject *, QEvent *) 0x00007fffe7764240
Application::notify Application_Events.cpp:61
QCoreApplication::notifyInternal2(QObject *, QEvent *) 0x00007ffff6ee9f88
QCoreApplicationPrivate::sendPostedEvents(QObject *, int, QThreadData *) 0x00007ffff6eed444
<unknown> 0x00007ffff6f48ba7
<unknown> 0x00007fffddf0166e
<unknown> 0x00007fffddf049ff
g_main_context_iteration 0x00007fffddf05190
QEventDispatcherGlib::processEvents(QFlags<…>) 0x00007ffff6f481ca
QEventLoop::exec(QFlags<…>) 0x00007ffff6ee88ba
QCoreApplication::exec() 0x00007ffff6ef146f
main main.cpp:850
__libc_start_call_main 0x00007fffe6f3df75
__libc_start_main_impl 0x00007fffe6f3e027
_start 0x00005555557d3771

[ksuprynowicz]

The crash happens due to _jointHistory still being empty at the time interpolateJoints is called. Would checking for _jointHistory size and skipping interpolation when the size doesn't match _jointData a reasonable workaround?

[ada-tv]

Would checking for _jointHistory size and skipping interpolation when the size doesn't match _jointData a reasonable workaround?

I think that's the only solution, if there's no history to interpolate from then it can't interpolate

[ksuprynowicz]

After the fix this PR is rock-solid now :)

[ada-tv]

I could hook up rotation interpolation, but I'd need your help getting it to be smooth. My naive impl was stuttering like joints were before your fix.

Position might be harder since avatars have separately networked local and global positions. Some day we need to clean up AvatarData to always just use local positions even when there's no parent, like how avatar rotation is currently. For now I think the forever-ease-out on the global position is good enough until we can get around to that.

[ksuprynowicz]

If you add rotation, I'd love to help on signal processing side as with joint positions :)

[ada-tv]

Now that avatars move smoothly, it's been revealed that #1634 affects entities parented to OtherAvatars too, not just MyAvatar. It was just harder to notice because avatars were already jittery when moving. Weirdly, they seem to stutter when new joint frames are received, but look smooth between each frame. This bug is unrelated to this PR and will have to be fixed later.