Formula behind auto vertical tilt checkbox?

[thwalker]

We're trying to build some USD scenes from scratch and the USD camera doesn't have an auto vertical tilt checkbox, it's just a float value. If we're rendering in V-Ray, how could we determine the vertical tilt amount without having to go into 3ds Max necessarily?

[dlparisi]

I think it's the tan() value of the camera's angle from the xy plane (i.e., tan (0) =0 (a level camera), tan(20)=.364. It's easy to compute for level camera but once a camera has any roll I think it takes that in to account as well.