NURBS Curve
A nonuniform rational B-spline curve defined by
 |
where 
is the order, 
are the
B-spline basis functions, 
are control
points, and the weight 
of 
is the last
ordinate of the homogeneous point 
. These curves
are closed under perspective transformations and can represent
conic sections exactly.
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{1/4, -1/2, 1} cross {1/3, 1, -2/3}
