The Virtual Sphere

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1. Define portion of screen to be projection of virtual sphere.
2. Get two sequential samples of mouse position, S and T.
3. Map 2D point S to 3D unit vector on sphere.
4. Map 2D vector to 3D tangental velocity .
5. Normalize .
6. Axis: .
7. Angle: .
   (Choose so a rotation can be obtained.)
8. Save T to use as S for next time.