underdog, my feeling is that the quaternion, which expresses a (fourth) rotation about an axis is exactly what you would want to manipulate to counteract the "rolling" effect, provided the quaternion axis is coincident with the object origin to camera (or point-at target) vector. You might have to experiment and/or do lots of research to understand the relationship between the quaternion rotation angle theta and the world orientation of the camera. I'm entirely unconvinced it could be as easy as resetting theta to zero without touching the axis (in this specific case) but that shouldn't be too hard to test.

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