Still new to quaternions, so:

I’ve almost got a spaceship control from TrackballControls.

I’m confused why approach one with vectors (almost) works, but approach two with quaternions, is failing entirely currently

with vectors:

CAMERA.getWorldDirection( cameraDir )

SHIP.lookAt( cameraDir )

with quaternions:

var invQtrn = CAMERA.quaternion.inverse()

SHIP.setRotationFromQuaternion( invQtrn )

Vectors works smoothly but an axis is “forgotten” between it and the camera (gimbal lock ?)

Quaternions should be the same idea, turn ship directly away from camera, but it kind of jerks around in odd directions.

Original problem with vectors to give an idea:

If I do a normal (not inversed) quaternion, the ship stays level with the camera, just facing the wrong direction.

With the inverse quaternion it doesn’t seem to bear any more relation to the camera.