Object3D.getWorld*() with cameras do not work when WebXR is presenting.

[omgitsraven]

Description of the problem

If an object has an onBeforeRender function which calls getWorldPosition on the camera argument, and the user is in VR, then that object (and any rendered after it) will stick to the user's face, as though the VR camera jumped to the origin mid-render.

Example: https://ravenworks.ca/temp/threeOBRcamerabug/

Three.js version

r112 (and some earlier, though I'm not sure exactly where it broke; this used to work in 2017, but it doesn't now.)

Browser

my demo only runs in Chrome because of Three dropping WebVR support, but the same bug happens in Firefox if using A-Frame (that's how I discovered the bug)

OS

only tried on Windows 10

Hardware Requirements (graphics card, VR Device, ...)

Oculus Rift

[Mugen87]

There is an unfortunate dependency when using Object3D.getWorldPosition() in this context. This method enforces a recomputation of the local and world matrices which will produce a wrong result since these matrices are actually set by WebXRManager. Since position, rotation and scale are not set (they are in fact out-of-sync), a recomputation produces wrong results (um, I'm having a déjà vu here).

You can avoid this problem by doing this in your onBeforeRender() callback:

cameraWorldPosition.setFromMatrixPosition( camera.matrixWorld );

or by setting camera.matrixAutoUpdate to false in your app. However, if you do this, you have to manually call Object3D.updateMatrix() by yourself whenever you transform the camera via position, rotation and scale.

Besides, avoid to create an instance of Vector3 every time onBeforeRender() is executed. Declare cameraWorldPosition once and reuse it.

[omgitsraven]

Ahh, thank you for the workaround!

(And yeah, I just put that Vector3 there so the demo would be as brief as possible…)