proj/maps_avgs/apps/ccinterp.c

/*
 *  Cubic convolution interpolation, based on GONG software, received Apr-88
 *  Interpolation by cubic convolution in 2 dimensions with 32-bit float data
 *
 *  Reference:
 *    R.G. Keys in IEEE Trans. Acoustics, Speech, and Signal Processing, 
 *    Vol. 29, 1981, pp. 1153-1160.
 *
 *  Usage:
 *    float ccint2 (float *f, int nx, int ny, double x, double y)
 *    double ccint2d (double *f, int nx, int ny, double x, double y)
 *
 *  Bugs:
 *    Currently interpolation within one pixel of edge is not
 *      implemented.  If x or y is in this range or off the picture, the
 *      function value returned is NaN.
 */

#include <math.h>
/*  Calculate the interpolation kernel.  */
static void ccker (double *u, double s) {
  double s2, s3;

  s2= s * s;
  s3= s2 * s;
  u[0] = s2 - 0.5 * (s3 + s);
  u[1] = 1.5*s3 - 2.5*s2 + 1.0;
  u[2] = -1.5*s3 + 2.0*s2 + 0.5*s;
  u[3] = 0.5 * (s3 - s2);
}
                    /*  Cubic convolution interpolation  */
float ccint2 (float *f, int nx, int ny, double x, double y) {
  double  ux[4], uy[4];
  double sum;
  int ix, iy, ix1, iy1, i, j;

  if (x < 1. || x >= (float)(nx-2) || y < 1. || y >= (float)(ny-2))
    return 0.0/ 0.0;   // a quiet NaN Value

  ix = (int)x;
  ccker (ux,  x - (double)ix);
  iy = (int)y;
  ccker (uy,  y - (double)iy);

  ix1 = ix - 1;
  iy1 = iy - 1;
  sum = 0.;
  for (i = 0; i < 4; i++)
    for (j = 0; j < 4; j++)
      sum = sum + f[(iy1+i) * nx + ix1 + j] * uy[i] * ux[j];
  return (float)sum;
}

double ccint2d (double *f, int nx, int ny, double x, double y) {
  double  ux[4], uy[4];
  double sum;
  int ix, iy, ix1, iy1, i, j;

  if (x < 1. || x >= (float)(nx-2) || y < 1. || y >= (float)(ny-2))
    return 0.0/ 0.0;   // a quiet NaN Value

  ix = (int)x;
  ccker (ux,  x - (double)ix);
  iy = (int)y;
  ccker (uy,  y - (double)iy);

  ix1 = ix - 1;
  iy1 = iy - 1;
  sum = 0.;
  for (i = 0; i < 4; i++)
    for (j = 0; j < 4; j++)
      sum = sum + f[(iy1+i) * nx + ix1 + j] * uy[i] * ux[j];
  return sum;
}

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