qball - 3-d rotation controller
void qball(Rectangle r, Mouse *mousep,
void (*redraw)(void), Quaternion *ap)
Qball is an interactive controller that allows arbitrary 3-space rota-
tions 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 dimen-
sions. 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-sup-
plied 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 quater-
nion(9.2)). 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 par-
ticular axis. If ap is null, the rotation is unconstrained. Other-
wise, the rotation will be about the same axis as *ap. This is accom-
plished by projecting points on the sphere to the nearest point also on
the plane through the sphere's center and normal to the axis.
Ken Shoemake, ``Animating Rotation with Quaternion Curves'', SIGGRAPH
'85 Conference Proceedings.