#define convert(x, y) sqrt((double)((x)*(x) + (y)*(y)))
#define D0 32.0
#define N0 4.0
/***************************************************************************
 * Func: filter                                                            *
 *                                                                         *
 * Desc: filters an array of complex data (assumes data is unordered)      *
 *                                                                         *
 * Params: complex_data - pointer to an array of complex data              *
 *         rows - number of rows in the image                              *
 *         cols - number of cols in the image                              *
 ***************************************************************************/
void filter(complex_ptr complex_data, int rows, int cols)
    {
    unsigned long x, y;          /* index for columns and rows */
    unsigned long tempx, tempy;  /* unordered indices */
    int halfcols, halfrows;      /* rows and cols divided by 2 */
    unsigned long index;         /* index into image */
    double butterworth;          /* value of filter function */
    double coordinate;           /* one-dimensional equivalent of x and y */

    halfrows = rows / 2;
    halfcols = cols / 2;

    for(y=0; y<rows; y++)
	{
	if(y >= halfrows)
	    tempy = y - halfrows;
	else
	    tempy = y + halfrows;

	for(x=0; x<rows; x++)
	    {
	    if(x >= halfcols)
		tempx = x - halfcols;
	    else
		tempx = x + halfcols;
	    index = y * cols + x;
	    coordinate = convert((tempx - halfrows), (tempy - halfcols));
	    butterworth = 1.0 / (1.0 + pow(( coordinate / D0), 2*N0));
	    complex_data[index].re *= butterworth;
	    complex_data[index].im *= butterworth;
	    }
	}
    }