1c_ftcurvs1(3NCARG)               NCAR GRAPHICS              c_ftcurvs1(3NCARG)
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NAME

6       c_ftcurvs1  -  calculate  values for a smoothing spline for data in the
7       plane.
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FUNCTION PROTOTYPE

10       int c_ftcurvs1(int, float [], float [], int, float [],
11                      int, float, float, float [], float []);
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SYNOPSIS

14       int c_ftcurvs1(int n, float xi[], float yi[], int dflg,
15                      float d[], int m, float xl, float xr,
16                      float xo[], float yo[])
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DESCRIPTION

19       n           The number of input data points. (n > 1)
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21       xi          An array containing the X-coordinates for the  input  data.
22                   These need not be increasing.
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24       yi          An array containing the Y-coordinates for the input data.
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26       dflg        A  switch  for  interpreting  the  value of d (as described
27                   below).  If dflg=0, then d is an  array  of  length  n  (an
28                   error  estimate for each input data value); if dflg=1, then
29                   d is a scalar that serves as an error  estimate  for  every
30                   single data item.
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32       d           A  user-specified  value containing the observed weights. D
33                   may be either an array or a scalar, depending on the  value
34                   of dflg.
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36       m           The number of output values.
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38       xl          A lower limit for an interval of interpolation.  The inter‐
39                   val between zero and one interpolates the entire curve, any
40                   value  outside  this  range  results in extrapolation.  The
41                   interval [xl,xr] is divided into m equal values to  produce
42                   the interpolated curve.
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44       xr          An  upper  limit  for  an  interval  of interpolation.  The
45                   interval between  zero  and  one  interpolates  the  entire
46                   curve,  any  value outside this range results in extrapola‐
47                   tion.  The interval [xl,xr] is divided into m equal  values
48                   to produce the interpolated curve.
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50       xo          The interpolated X values.
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52       yo          The interpolated Y values.
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RETURN VALUE

55       c_ftcurvs1 returns an error value as per:
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57       = 1 if n is less than 2.
58       = 2 if smt is negative.
59       = 3 if eps is negative or greater than 1.
60       = 5 if d is negative.
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USAGE

63       This  function computes an interpolatory smoothing spline under tension
64       through a sequence of data values in the plane.
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66       Two parameters and one function argument are used to control the degree
67       of smoothness -- the parameters are smt, and eps and the function argu‐
68       ment is d.
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70       The argument d is a value indicating the degree of  confidence  in  the
71       accuracy  of the input function values -- it should be an approximation
72       of the standard deviation of error. Effectively the value of d controls
73       how  close  the smoothed curve comes to the input data points.  If d is
74       small, then the interpolated curve will pass close to the  input  data.
75       The larger the value of d, the more freedom the smooth curve has in how
76       close it comes to the input data values.
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78       The parameter smt is a more subtle global smoothing parameter; smt must
79       be  non-negative.  For  small values of smt, the curve approximates the
80       tension spline and for larger values of smt, the curve is  smoother.  A
81       reasonable  value  for smt is (float) n. smt can be set with the proce‐
82       dure c_ftsetr.
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84       The parameter eps controls the precision to which smt  is  interpreted;
85       eps  must  be  between  0. and 1. inclusive. A reasonable value for eps
86       sqrt( 2./(float) n ). eps can be set with the procedure c_ftsetr.
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88       c_ftcurvs1 is called after all of the desired values for control param‐
89       eters  have been set using the procedures c_ftseti, c_ftsetr, c_ftsetc.
90       Control parameters that apply to c_ftcurvs are: sig, smt, eps, sf2.
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92       The value for the parameter sig specifies the tension  factor.   Values
93       near  zero  result  in a cubic spline; large values (e.g. 50) result in
94       nearly a polygonal line. A typical value is 1. (the default).
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ACCESS

97       To use c_ftcurvs1, load the NCAR Graphics library ngmath.
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SEE ALSO

100       fitgrid_params, c_ftseti, c_ftsetr, c_ftsetc.
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102       Complete documentation for Fitgrid is available at URL
103       http://ngwww.ucar.edu/ngdoc/ng/ngmath/fitgrid/fithome.html
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106       Copyright (C) 2000
107       University Corporation for Atmospheric Research
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109       The use of this Software is governed by a License Agreement.
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113UNIX                              August 2002               c_ftcurvs1(3NCARG)
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