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<title>GNU Scientific Library – Reference Manual: 2D Interpolation Example programs</title>
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<p>
Previous: <a href="2D-Higher_002dlevel-Interface.html#g_t2D-Higher_002dlevel-Interface" accesskey="p" rel="previous">2D Higher-level Interface</a>, Up: <a href="Interpolation.html#Interpolation" accesskey="u" rel="up">Interpolation</a> [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<h3 class="section">28.15 2D Interpolation Example programs</h3>
<p>The following example performs bilinear interpolation on the unit
square, using <em>z</em> values of <em>(0,1,0.5,1)</em> going clockwise
around the square.
</p>
<div class="example">
<pre class="verbatim">#include <stdio.h>
#include <stdlib.h>
#include <gsl/gsl_math.h>
#include <gsl/gsl_interp2d.h>
#include <gsl/gsl_spline2d.h>
int
main()
{
const gsl_interp2d_type *T = gsl_interp2d_bilinear;
const size_t N = 100; /* number of points to interpolate */
const double xa[] = { 0.0, 1.0 }; /* define unit square */
const double ya[] = { 0.0, 1.0 };
const size_t nx = sizeof(xa) / sizeof(double); /* x grid points */
const size_t ny = sizeof(ya) / sizeof(double); /* y grid points */
double *za = malloc(nx * ny * sizeof(double));
gsl_spline2d *spline = gsl_spline2d_alloc(T, nx, ny);
gsl_interp_accel *xacc = gsl_interp_accel_alloc();
gsl_interp_accel *yacc = gsl_interp_accel_alloc();
size_t i, j;
/* set z grid values */
gsl_spline2d_set(spline, za, 0, 0, 0.0);
gsl_spline2d_set(spline, za, 0, 1, 1.0);
gsl_spline2d_set(spline, za, 1, 1, 0.5);
gsl_spline2d_set(spline, za, 1, 0, 1.0);
/* initialize interpolation */
gsl_spline2d_init(spline, xa, ya, za, nx, ny);
/* interpolate N values in x and y and print out grid for plotting */
for (i = 0; i < N; ++i)
{
double xi = i / (N - 1.0);
for (j = 0; j < N; ++j)
{
double yj = j / (N - 1.0);
double zij = gsl_spline2d_eval(spline, xi, yj, xacc, yacc);
printf("%f %f %f\n", xi, yj, zij);
}
printf("\n");
}
gsl_spline2d_free(spline);
gsl_interp_accel_free(xacc);
gsl_interp_accel_free(yacc);
free(za);
return 0;
}
</pre></div>
<p>The results of the interpolation are shown in the following plot,
where the corners are labeled with their fixed <em>z</em> values.
</p>
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