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<title>qball(3) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>QBALL(3)</b><td align=right><b>QBALL(3)</b>
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<p><font size=+1><b>NAME </b></font><br>
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qball – 3-d rotation controller<br>
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<p><font size=+1><b>SYNOPSIS </b></font><br>
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<tt><font size=+1>#include <draw.h>
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<tt><font size=+1>#include <geometry.h>
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<tt><font size=+1>void qball(Rectangle r, Mouse *mousep,<br>
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Quaternion *orientation,<br>
void (*redraw)(void), Quaternion *ap)<br>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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<i>Qball</i> 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 <i>r</i>, and diameter the smaller
of <i>r</i>’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.
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Argument <i>mousep</i> is a pointer to the mouse event that triggered
the interaction. It should have some button set. <i>Qball</i> will read
more events into <i>mousep</i>, and return when no buttons are down.
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While <i>qball</i> is reading mouse events, it calls out to the caller-supplied
routine <i>redraw</i>, which is expected to update the screen to reflect
the changing orientation. Argument <i>orientation</i> is the orientation
that <i>redraw</i> should examine, represented as a unit Quaternion (see
<i>quaternion(9.2)).</i> The caller may set it to any
orientation. It will be updated before each call to <i>redraw</i> (and
on return) by multiplying by the rotation specified with the mouse.
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It is possible to restrict <i>qball’s</i> attention to rotations about
a particular axis. If <i>ap</i> is null, the rotation is unconstrained.
Otherwise, the rotation will be about the same axis as <i>*ap</i>. 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.<br>
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<p><font size=+1><b>SOURCE </b></font><br>
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<tt><font size=+1>/usr/local/plan9/src/libgeometry/qball.c<br>
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<p><font size=+1><b>SEE ALSO </b></font><br>
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<a href="../man3/quaternion.html"><i>quaternion</i>(3)</a><br>
Ken Shoemake, “Animating Rotation with Quaternion Curves”, <i>SIGGRAPH
’85 Conference Proceedings.<br>
</i>
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