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SPLINE

SPLINE

The SPLINE function performs cubic spline interpolation. It accepts single- or double-precision input.

This routine is written in the IDL language. Its source code can be found in the file spline.pro in the lib subdirectory of the IDL distribution.

Examples

The commands below show a typical use of SPLINE:

`; X values of original function:`
`X = [2.,3.,4.]`
` `
`; Make a quadratic`
`Y = (X-3)^2`
`;Values for interpolated points:`
`T = FINDGEN(20)/10.+2`
` `
`; Do the interpolation:`
`Z = SPLINE(X,Y,T)`

Syntax

Result = SPLINE( X, Y, T [, Sigma] [, /DOUBLE] )

Return Value

Returns the result of the cubic spline interpolation. If X, Y, or T are of type double precision, then the computations will be done using double precision, and a double-precision result will be returned. Otherwise, single precision will be used, and a single-precision result will be returned.

Arguments

X

The abscissa vector. Values must be strictly increasing (no duplicate values). X must have three or more points.

Y

The vector of ordinate values corresponding to X. Y should have the same number of elements as X.

T

The vector of abscissa values for which the ordinate is desired. The values of T must be strictly increasing (no duplicate values).

Sigma

The amount of “tension” that is applied to the curve. The default value is 1.0. If sigma is close to 0, (e.g., .01), then effectively there is a cubic spline fit. If sigma is large, (e.g., greater than 10), then the fit will be like a polynomial interpolation.

Keywords

DOUBLE

Set this keyword to perform computations using double-precision arithmetic and to return a double-precision result.

Version History

 Original Introduced 6.1 Added DOUBLE keyword