Compute the rotation angle around a given point, needed to reach the
zenith position.
At a zenith position, the camera direction is going througth the earth
center and the frustrum bottom ray is going through the chosen pivot
point.
The bottom-center of the screen is a good candidate for the pivot point.
Compute the rotation angle around a given point, needed to reach the zenith position. At a zenith position, the camera direction is going througth the earth center and the frustrum bottom ray is going through the chosen pivot point. The bottom-center of the screen is a good candidate for the pivot point.