Continuing from part 2 of this series, the tension in a Cardinal spline is controlled by the s-parameter. Given a four-tuple of control points or knots,
[Pa, Pb, Pc, Pd]
cubic Hermite interpolation is applied with start-tangent s(Pc – Pa) and end-tangent s(Pd – Pb) .
Theoretically, s could vary from zero to positive infinity. A very long tangent vector, indicated by a large s-value causes the curve to follow the tangent very closely in and out of the join point. A very small tangent vector, indicated by a small s-value, causes the curve to approach and leave the join point more like a straight line.
Intuitively, this is the opposite of how we expect tension to behave. Larger tension should ‘pull’ the curve closer to a straight-line interpolation of the knots, with exactly straight lines as a limiting case. This gives rise to the convention of a formal tension parameter, T, that is inversely related to s.
The typical convention is to define s = (1-T)/2 or T = 1 – 2s. Note that the zero-tension case corresponds to s = 1/2 or a relatively ‘loose’ movement into and out of each join. This special case corresponds to the Catmull-Rom spline. For this reason, the C-R spline is sometimes called the zero-tension or neutral Cardinal spline.
The tension parameter will be discussed in more detail in subsequent posts in this series. Tangent vectors are arbitrary at the initial and terminal knots, just as with the Catmull-Rom spline. Fortunately, there are several methods for automatically assigning these tangents. The C-R spline in Degrafa implements two methods; point duplication and point reflection. Line segment reflection can be added as a third option. It is also possible to allow designers to set these tangents interactively.
Local control via knot placement, tension control, and adjustment of initial/terminal tangent vectors provides artists with a variety of methods to design specific curves using Cardinal splines.
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