static void
Qt_ToBallPoints (Quat q,
HVect *arcFrom,
HVect *arcTo)
{
double s;
s = sqrt(q.x*q.x + q.y*q.y);
if (s == 0.0) *arcFrom = V3_(0.0, 1.0, 0.0);
else *arcFrom = V3_(-q.y/s, q.x/s, 0.0);
arcTo->x = q.w*arcFrom->x - q.z*arcFrom->y;
arcTo->y = q.w*arcFrom->y + q.z*arcFrom->x;
arcTo->z = q.x*arcFrom->y - q.y*arcFrom->x;
if (q.w < 0.0) *arcFrom = V3_(-arcFrom->x, -arcFrom->y, 0.0);
}
static HVect
ConstrainToAxis (HVect loose,
HVect axis)
{
HVect onPlane;
register double norm;
onPlane = V3_Sub(loose, V3_Scale(axis, V3_Dot(axis, loose)));
norm = V3_Norm(onPlane);
if (norm > 0.0)
{
if (onPlane.z < 0.0) onPlane = V3_Negate(onPlane);
return ( V3_Scale(onPlane, 1/sqrt(norm)) );
}
/* else drop through */
/* ================= */
if (axis.z == 1) onPlane = V3_(1.0, 0.0, 0.0);
else onPlane = V3_Unit(V3_(-axis.y, axis.x, 0.0));
return (onPlane);
}
/* Halve arc between unit vectors v0 and v1. */
HVect V3_Bisect(HVect v0, HVect v1)
{
HVect v = {0, 0, 0, 0};
float Nv;
v = V3_Add(v0, v1);
Nv = V3_Norm(v);
if (Nv < 1.0e-5) {
v = V3_(0, 0, 1);
} else {
v = V3_Scale(v, 1/sqrt(Nv));
}
return (v);
}
