| chbint_c |
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Table of contents
Procedure
chbint_c ( Interpolate a Chebyshev expansion )
void chbint_c ( ConstSpiceDouble cp [],
SpiceInt degp,
ConstSpiceDouble x2s [2],
SpiceDouble x,
SpiceDouble * p,
SpiceDouble * dpdx )
AbstractReturn the value of a polynomial and its derivative, evaluated at the input `x', using the coefficients of the Chebyshev expansion of the polynomial. Required_ReadingNone. KeywordsINTERPOLATION MATH POLYNOMIAL Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- cp I degp+1 Chebyshev polynomial coefficients. degp I Degree of polynomial. x2s I Transformation parameters of polynomial. x I Value for which the polynomial is to be evaluated p O Value of the polynomial at `x' dpdx O Value of the derivative of the polynomial at X Detailed_Input
cp is an array of coefficients of a polynomial with
respect to the Chebyshev basis. The polynomial to be
evaluated is assumed to be of the form:
cp[degp]*T(degp,s) + cp[degp-1]*T(degp-1,s) + ...
+ cp[1]*T(1,s) + cp[0]*T(0,s)
where T(i,s) is the i'th Chebyshev polynomial
evaluated at a number `s' whose double precision
value lies between -1 and 1. The value of `s' is
computed from the input variables x2s[0], x2s[1] and `x'.
degp is the degree of the Chebyshev polynomial to be
evaluated.
x2s is an array of two parameters. These parameters are
used to transform the domain of the input variable `x'
into the standard domain of the Chebyshev polynomial.
x2s[0] should be a reference point in the domain of `x';
x2s[1] should be the radius by which points are
allowed to deviate from the reference point and while
remaining within the domain of `x'. The value of
`x' is transformed into the value `s' given by
s = ( x - x2s[0] ) / x2s[1]
Typically x2s[0] is the midpoint of the interval over
which `x' is allowed to vary and x2s[1] is the radius of
the interval.
The main reason for doing this is that a Chebyshev
expansion is usually fit to data over a span
from A to B where A and B are not -1 and 1
respectively. Thus to get the "best fit" the
data was transformed to the interval [-1,1] and
coefficients generated. These coefficients are
not rescaled to the interval of the data so that
the numerical "robustness" of the Chebyshev fit will
not be lost. Consequently, when the "best fitting"
polynomial needs to be evaluated at an intermediate
point, the point of evaluation must be transformed
in the same way that the generating points were
transformed.
x is the value for which the polynomial is to be
evaluated.
Detailed_Output
p is the value of the polynomial to be evaluated. It
is given by
cp[degp]*T(degp,s) + cp[degp-1]*T(degp-1,s) + ...
+ cp[1]*T(1,s) + cp[0]*T(0,s)
where T(i,s) is the i'th Chebyshev polynomial
evaluated at a number s = ( x - x2s[0] )/x2s[1]
dpdx is the value of the derivative of the polynomial at `x'.
It is given by
1/x2s[1] [ cp[degp]*T'(degp,s)
+ cp[degp-1]*T'(degp-1,s)
+ ...
+ cp[1]*T'(1,s)
+ cp[0]*T'(0,s) ]
where T(i,s) and T'(i,s) are the i'th Chebyshev
polynomial and its derivative, respectively,
evaluated at a number s = ( x - x2s[0] )/x2s[1]
ParametersNone. Exceptions
Error free.
1) No tests are performed for exceptional values (`degp' negative,
etc.). This routine is expected to be used at a low level in
ephemeris evaluations. For that reason it has been elected as
a routine that will not participate in error handling.
FilesNone. Particulars
This routine computes the value of a Chebyshev polynomial
expansion and the derivative of the expansion with respect to `x'.
The polynomial is given by
cp[degp]*T(degp,s) + cp[degp-1]*T(degp-1,s) + ...
+ cp[1]*T(1,s) + cp[0]*T(0,s)
where
s = ( x - x2s[0] ) / x2s[1]
and
T(i,s) is the i'th Chebyshev polynomial of the first kind
evaluated at `s'.
Examples
The numerical results shown for this example may differ across
platforms. The results depend on the SPICE kernels used as
input, the compiler and supporting libraries, and the machine
specific arithmetic implementation.
1) Depending upon the user's needs, there are 3 routines
available for evaluating Chebyshev polynomials.
chbval_c for evaluating a Chebyshev polynomial when no
derivatives are desired.
chbint_c for evaluating a Chebyshev polynomial and its
first derivative.
chbder_c for evaluating a Chebyshev polynomial and a user
or application dependent number of derivatives.
Of these 3 the one most commonly employed by SPICE software
is chbint_c as it is used to interpolate ephemeris state
vectors; this requires the evaluation of a polynomial
and its derivative. When no derivatives are desired one
should use chbval_c, or when more than one or an unknown
number of derivatives are desired one should use chbder_c.
The code example below illustrates how this routine might
be used to obtain points for plotting a polynomial
and its derivative.
Example code begins here.
/.
Program chbint_ex1
./
#include <stdio.h>
#include "SpiceUsr.h"
int main( )
{
/.
Local variables.
./
SpiceDouble dpdx;
SpiceDouble x;
SpiceDouble p;
SpiceInt degp;
/.
Set the coefficients of the polynomial and its
transformation parameters
./
SpiceDouble cp [7] = { 1.0, 3.0, 0.5, 1.0,
0.5, -1.0, 1.0 };
SpiceDouble x2s [2] = { 0.5, 3.0 };
degp = 6;
x = 1.0;
chbint_c ( cp, degp, x2s, x, &p, &dpdx );
printf( "Value of the polynomial at x=1: %10.6f\n", p );
printf( " First derivative at x=1 : %10.6f\n", dpdx );
return ( 0 );
}
When this program was executed on a Mac/Intel/cc/64-bit
platform, the output was:
Value of the polynomial at x=1: -0.340878
First derivative at x=1 : 0.382716
Restrictions
1) One needs to be careful that the value
(x-x2s[0]) / x2s[1]
lies between -1 and 1. Otherwise, the routine may fail
spectacularly (for example with a floating point overflow).
Literature_References
[1] W. Press, B. Flannery, S. Teukolsky and W. Vetterling,
"Numerical Recipes -- The Art of Scientific Computing,"
chapter 5.4, "Recurrence Relations and Clenshaw's Recurrence
Formula," p 161, Cambridge University Press, 1986.
[2] T. Rivlin, "The Chebyshev Polynomials," Wisley, 1974.
[3] R. Weast and S. Selby, "CRC Handbook of Tables for
Mathematics," 4th Edition, CRC Press, 1976.
Author_and_InstitutionJ. Diaz del Rio (ODC Space) Version-CSPICE Version 1.0.0, 01-NOV-2021 (JDR) Index_Entriesinterpolate a chebyshev expansion |
Fri Dec 31 18:41:02 2021