QBALL(2) QBALL(2)
NAME
qball - 3-d rotation controller
SYNOPSIS
#include <draw.h>
#include <geometry.h>
void qball(Rectangle r, Mouse *mousep,
Quaternion *orientation,
void (*redraw)(void), Quaternion *ap)
DESCRIPTION
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 trig-
gered 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 to the axis.
SOURCE
/sys/src/libgeometry/qball.c
SEE ALSO
quaternion(2)
Ken Shoemake, ``Animating Rotation with Quaternion Curves'',
SIGGRAPH '85 Conference Proceedings.
Page 1 Plan 9 (printed 12/22/24)