Plan 9 - man page for qball (plan9 section 9)
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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 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 pro-
jected 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
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 ori-
entation is the orientation that redraw should examine, represented as a unit Quaternion
(see quaternion(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
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 to the axis.
Ken Shoemake, ``Animating Rotation with Quaternion Curves'', SIGGRAPH '85 Conference Pro-
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