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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> &nbsp; [<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 &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;

#include &lt;gsl/gsl_math.h&gt;
#include &lt;gsl/gsl_interp2d.h&gt;
#include &lt;gsl/gsl_spline2d.h&gt;

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 &lt; N; ++i)
    {
      double xi = i / (N - 1.0);

      for (j = 0; j &lt; N; ++j)
        {
          double yj = j / (N - 1.0);
          double zij = gsl_spline2d_eval(spline, xi, yj, xacc, yacc);

          printf(&quot;%f %f %f\n&quot;, xi, yj, zij);
        }
      printf(&quot;\n&quot;);
    }

  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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