spline Programspline
GNU spline is a program for interpolating between the data points
in one or more datasets. Each dataset would consist of values for an
independent variable and a dependent variable, which may be a vector of
specified fixed length. When discussing interpolation, we call these
variables `t' and `y', respectively. To emphasize:
t is a scalar, but in general the dependent variable
y may be a vector.
The simplest case is when there is a single input file, which is in ASCII format, and the vector y is one-dimensional. This is the default. For example, the input file could contain the dataset
0.0 0.0 1.0 1.0 2.0 0.0
which are the coordinates (t,y) of the data points (0,0), (1,1), and (2,0). Data points do not need to be on different lines, nor do the t and y coordinates of a data point need to be on the same line. However, there should be no blank lines in the input if it is to be viewed as forming a single dataset. Also, by default the t coordinate should be monotonically increasing, so that y may be viewed as a function of t.
You would construct a spline (the graph of an `interpolating function') passing through the points in this dataset by doing
spline input_file > output_file
To produce a Postscript plot of the spline with the graph
utility, you would do
spline input_file | graph -T ps > output.ps
To display a spline on an X Window System display, you could do
echo 0 0 1 1 2 0 | spline | graph -T X
Notice that the last example avoids the use of the input file
altogether. spline will read from standard input if no files are
specified on the command line, or if the special file name `-'
is specified.
What exactly does spline do? First, it fits a curve (the graph
of an interpolating function) through the points in the dataset. Then
it splits the interval over which the independent variable t
ranges into 100 sub-intervals, and computes the y values at
each of the 101 subdivision points. It then outputs each of the
pairs (t, y). These are the coordinates of 101 points that lie
along a curve that interpolates between the points in the dataset. If
there is more than one dataset in the input (separated by blank lines),
each dataset is interpolated separately.
You may use the `-n' option to replace `100' by any other positive integer. You may also use the `-t' option to specify an interpolation interval that differs from the default (the interval over which the independent variable ranges). For example, the command
echo 0 0 1 1 2 0 | spline -n 20 -t 1.0 1.5 > output_file
will produce a dataset consisting of 21 (rather than 101) data points, with t values spaced regularly between 1.0 and 1.5 (rather than between 0.0 and 2.0). The data points will lie along a curve passing through (0,0), (1,1), and (2,0). This curve will be a parabola.
In general, the interpolating function will be a piecewise cubic spline. That is, between each pair of adjacent `knots' (points in the input dataset), y will be a cubic function of t. This function will differ, depending on which pair of knots y lies between. At each knot, both the slope and curvature of the cubic pieces to either side will match. In mathematical terms, the interpolating curve will be twice continuously differentiable.
spline supports `adding tension' to the interpolating curve.
A nonzero value for the tension can be specified with the `-T'
option. For example, a spline under considerable tension can be
computed and displayed by doing
echo 0 0 1 0 2 0 | spline -T 10 | graph -T X
As the tension parameter is increased to positive infinity, the spline will converge to a polygonal line. You are meant to think of the spline as being drawn taut. Actually, tension may be negative as well as positive. A spline with negative tension will tend to bow outward, in fact to oscillate sinusoidally. But as the tension decreases to negative infinity, the spline, though oscillatory, will again converge to a polygonal line.
If the tension is positive, its reciprocal will be the maximum range of the independent variable t over which the spline will `like to curve'. Increasing the tension far above zero will accordingly force the spline to consist of short curved sections, centered on the data points, and sections that are almost straight. It follows that tension is a `dimensionful' quantity. If the tension is nonzero, then when the values of the independent variable are multiplied by some common positive factor, the tension should be divided by the same factor to obtain a scaled version of the original spline. If the tension is zero (the default, or cubic spline case), then the computation of the spline will be unaffected by linear scaling of the data.
In mathematical terms, a spline under tension will satisfy the differential equation @ifnottex y""=sgn(tension)*(tension^2)y" between each successive pair of knots. If the tension equals zero, which is the default, the fourth derivative of y with respect to t will equal zero at every point. In this case, y as a function of t will reduce to a cubic polynomial between each successive pair of knots. But if the tension is nonzero, y will not be a polynomial function of t. It may be expressed in terms of exponential functions, however.
Irrespective of whether or not the spline is under tension, you may specify the `-p' option if you wish the spline to be a periodic function of t. This will only work if the y values for the first and last points in the dataset are equal. Otherwise, it would make no sense to compute a periodic interpolation.
It is sometimes useful to interpolate between data points at the same
time as they are generated by an auxiliary program. That is, it
is useful for spline to function as a real-time filter.
spline does not normally act as a filter, since computing an
interpolating curve that is as smooth as possible is a global task. But
if the `-f' option is specified, spline will indeed function
as a filter. A different interpolation algorithm (cubic Bessel
interpolation, which is local rather than global) will be used. If
`-f' is specified, `-p' may not be specified. Also, if
`-f' is specified then an interpolation interval (a range of
t values) must be requested explicitly with the `-t'
option.
Cubic Bessel interpolation is inherently less smooth than the construction of a global cubic spline. If the `-f' option is specified, the slope of the spline at each knot will be chosen by fitting a parabola through that knot, and the two adjacent knots. The slopes of the two interpolating segments to either side of each interior knot will match at that knot, but typically their curvatures will not. In mathematical terms, the interpolating curve will be continuously differentiable, but in general not twice continuously differentiable. This loss of differentiability is the price that is paid for functioning as a real-time filter.
spline
The preceding section explains how spline can be employed to
interpolate a function y of a scalar variable t, in the
case when y is a scalar. In this section we explain how to
perform more sophisticated interpolations. This includes
multidimensional interpolations, and interpolations that are splinings
of curves, rather than of functions.
spline can handle the case when y is a vector of
arbitrary specified dimensionality. The dimension can be specified with
the `-d' option. For example, an input file could contain the
multidimensional dataset
0.0 0.0 1.0 1.0 1.0 0.0 2.0 0.0 1.0
which are the coordinates (t,y) of the data points (0,0,1), (1,1,0), and (2,0,1). You would construct a spline (the graph of an interpolating function) passing through the points in this dataset by doing
spline -d 2 input_file > output_file
The option `-d 2' is used because in this example, the dependent variable y is a two-dimensional vector. Each of the components of y will be interpolated independently, and the output file will contain points that lie along the graph of the resulting interpolating function.
When doing multidimensional splining, you may use any of the options
that apply in the default one-dimensional case. For example, the
`-f' option will yield real-time cubic Bessel interpolation. As
in the one-dimensional case, if the `-f' option is used then the
`-t' option must be used as well, to specify an interpolation
interval (a range of t values). The -p option
will yield a periodic spline, i.e., the graph of a periodic
vector-valued function. For this, the first and last dataset
y values must be the same.
spline can also be used to draw a curve through arbitrarily
chosen points in the plane, or in general through arbitrarily chosen
points in d-dimensional space. This is not the same as splining,
at least as the term is conventionally defined. The reason is that
`splining' refers to construction of a function, rather than the
construction of a curve that may or may not be the graph of a function.
Not every curve is the graph of a function.
The following example shows how you may `spline a curve'. The command
echo 0 0 1 0 1 1 0 1 | spline -d 2 -a -s | graph -T X
will construct a curve in the plane through the four points (0,0), (1,0), (1,1), and (0,1), and graph it on an X Window System display. The `-d 2' option specifies that the dependent variable y is two-dimensional. The `-a' option specifies that t values are missing from the input, and should be automatically generated. By default, the first t value is 0, the second is 1, etc. The `-s' option specifies that the t values should be stripped from the output.
The same technique may be used to spline a closed curve. For example, doing
echo 0 0 1 0 0 1 0 0 | spline -d 2 -a -s -p | graph -T X
will construct and graph a closed, lozenge-shaped curve through the three points (0,0), (1,0), and (0,1). The construction of a closed curve is guaranteed by the `-p' (i.e., `--periodic') option, and by the repetition of the initial point (0,0) at the end of the sequence.
When splining a curve, whether open or closed, you may wish to substitute the `-A' option for the `-a' option. Like the `-a' option, the `-A' option specifies that t values are missing from the input and should be automatically generated. However, the increment from one t value to the next will be the distance between the corresponding values of y. This scheme for generating t values, when constructing a curve through a sequence of points, is the scheme that is used in the well known FITPACK subroutine library. It is probably the best approach when the distances between successive points fluctuate considerably.
A curve through a sequence of points in the plane, whether open or closed, may cross itself. Some interesting visual effects can be obtained by adding negative tension to such a curve. For example, doing
echo 0 0 1 0 1 1 0 0 | spline -d 2 -a -s -p -T -14 -n 500 | graph -T X
will construct a closed curve through the three points (0,0), (1,0), and (0,1), which is wound into curlicues. The `-n 500' option is included because there are so many windings. It specifies that 501 points should be generated, which is enough to draw a smooth curve.
spline command-line options
The spline program will interpolate vector-valued functions of a
scalar variable t, and curves in d-dimensional space.
The algorithms used by spline are similar to those discussed in
D. Kincaid and [E.] W. Cheney, Numerical Analysis (2nd
ed., Brooks/Cole, 1996), section 6.4, and C. de Boor, A
Practical Guide to Splines (Springer-Verlag, 1978), Chapter 4.
Input file names may be specified anywhere on the command line. That is, the relative order of font names and command-line options does not matter. If no file names are specified, or the file name `-' is specified, the standard input is read.
An input file may contain more than a single dataset. Unless the `-a' or `-A' options are used (see below), each dataset is expected to consist of a sequence of data points, given as alternating t and y values. t is the scalar independent variable, and y is the vector-valued dependent variable. The dimensionality of y is specified with the `-d' option (the default is 1).
If the input file is in ASCII format (the default), its datasets are separated by blank lines. An input file may also contain any number of comment lines, which must begin with the comment character `#'. Comment lines are ignored. They are not treated as blank, i.e., they do not interrupt a dataset in progress.
The options to spline are listed below. There are three sorts of
option:
Options that take an argument are followed, in parentheses, by the type and default value of the argument.
The following options specify the type of interpolation to be performed on each dataset.
spline can be used as a real-time filter. The slope of the
interpolating curve at each point in a dataset will be chosen by fitting
a quadratic function through that point and the two adjacent points in
the dataset. If `-f' is specified then the `-t' option,
otherwise optional, must be used as well. Also, if `-f' is
specified then the `-k', `-p', and `-T' options may not
be used.
If `-f' is not specified, then a different (global)
interpolation algorithm will be used.
The following options specify the format of the input file(s) and the output file.
FLT_MAX, which is the largest possible single precision floating
point number. On most machines this is approximately 3.4x10^38.
DBL_MAX, which is the largest possible double
precision floating point number. On most machines this is
approximately 1.8x10^308.
INT_MAX, which is the largest
possible integer. On most machines this is 2^31-1.
spline).
spline).
The following options request information.
spline and the plotting utilities
package, and exit.
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