qball – 3–d rotation controller
void qball(Rectangle r, Mouse *mousep,
Qball is an interactive controller that allows arbitrary 3–space
rotations to be specified with the mouse. Imagine a sphere with
its center at the midpoint of rectangle r, and diameter the smaller
of r's dimensions. Dragging from one point on the sphere to another
specifies the endpoints of a great–circle arc. (Mouse
points outside the sphere are projected to the nearest point on
the sphere.) The axis of rotation is normal to the plane of the
arc, and the angle of rotation is twice the angle of the arc.
Argument mousep is a pointer to the mouse event that triggered the interaction. It should have some button set. Qball will read more events into mousep, and return when no buttons are down.
While qball is reading mouse events, it calls out to the caller–supplied routine redraw, which is expected to update the screen to reflect the changing orientation. Argument orientation is the orientation that redraw should examine, represented as a unit Quaternion (see quaternion(2)). The caller may set it to any orientation. It will be updated before each call to redraw (and on return) by multiplying by the rotation specified with the mouse.
It is possible to restrict qball's attention to rotations about
a particular axis. If ap is null, the rotation is unconstrained.
Otherwise, the rotation will be about the same axis as *ap. This
is accomplished by projecting points on the sphere to the nearest
point also on the plane through the sphere's center and normal
Ken Shoemake, ``Animating Rotation with Quaternion Curves'', SIGGRAPH '85 Conference Proceedings.