QBALL(9.2)																QBALL(9.2)

NAME

       qball - 3-d rotation controller

SYNOPSIS

       #include <libg.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 cen-
       ter 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(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 particular axis.  If ap is null, the rotation is  unconstrained.   Other-
       wise,  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(9.2)
       Ken Shoemake, ``Animating Rotation with Quaternion Curves'', SIGGRAPH '85 Conference Proceedings.

																	QBALL(9.2)