Spline Interpretation ABC Introduction and outline Based mostly on Wikipedia.

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Spline Interpretation ABC Introduction and outline Based mostly on Wikipedia

Spline Interpretation Spline is a flat flexible strip of thin narrow piece of wood, metal, or plastic used in drawing curved. Mathematically, spline means curve fitting mostly using certain piecewise low degree polynomials. The main reason to use low degree polynomial is to avoid Runge’s phenomenon.

Runge’s Phenomenon

Runge’s Phenomenon Red: data; blue: 5 th order; green: 9 th order

Runge’s Phenomenon

Definition of Spline

Linear Spline

Quadratic Spline

Cubic Spline

Hermite Spline

Hermite spline

In general, cubic spline does not guarantee monotonicity even for monotonic data. Hermite monotonic spline, however, guarantees monotonicity between data points. The advantage is the elimination of overshoots. The prices to pay are these: – we no longer have the smoothness of continuity in curvature and –we have to inject another subjective idea of monotonic and/or variations.

Comparisons between Cubic Natural and Hermite Splines An Example

Data

Envelopes of Maxima

Envelopes of Minima

Mean of Max and Min Envelopes

Difference between Mean of Max and Min Envelopes

First Proto-IMF

B-Spline Something used extensively by Computer Aided Design (CAD), but we have found limited use of it.

Bézier Curve

Cubic Bezier Bases

Bézier Curve

An example

Rational Bézier Curve

B-Spline A B-spline is a generalization of the Bézier Curve; A B-spline with no internal knots is a Bézier Curve. It is generated through Cox-de Boor recursion formula. A further generalization is NURBS: NonUniform Rational B-Spline. See for example, David F. Rogers: An Introduction to NURBS. Morgan Kaufmann, San Francisco, 2001

B-Spline

Properties of B-Spline A B-spline could have degree n; control points, p; and knots, m, each independent to others, but m ≥ p. It is much more flexible. For n=3, p=10, and m=14, we have B-spline at left and Bezier curve at right.

Properties of B-Spline In general, the lower the degree, the closer the curve follows the control points. For the same numbers of control points and knots, but different degrees, the spline results are very different: left, n=7; center, n=5; right, n=3.

Properties of B-Spline B-spline is a piecewise curve. We must have m=n+p+1. Clamped spline has to pass the beginning and end points. Strong convex hull property Variation diminishing property Affine invariance But B-Spline does not pass through the control points!

Variation Diminishing Property: no straight line (resp., plane) intersects a B-spline curve more times than it intersects the curve's control polygon.

Summary The differences between different splines seems to be huge. Based on principles of least interferences and maximum smoothness, we selected natural spline as the base for EMD. Different splines (except B-spline) should not change the scales of the IMFs, but it would change the shape and energy levels. EMD is unique with respect to all parameters selected.