Geografic-Information-System/raster/r.proj/bilinear_f.c

/*
 * Name
 *  bilinear_f.c -- use bilinear interpolation with fallback for given row, col
 *
 * Description
 *  bilinear interpolation for the given row, column indices.
 *  If the interpolated value (but not the nearest) is null,
 *  it falls back to nearest neighbor.
 */

#include <grass/gis.h>
#include <grass/raster.h>
#include "r.proj.h"

void p_bilinear_f(struct cache *ibuffer,	/* input buffer                  */
		void *obufptr,	/* ptr in output buffer          */
		int cell_type,	/* raster map type of obufptr    */
		double col_idx,	/* column index          */
		double row_idx,	/* row index                     */
	    struct Cell_head *cellhd	/* cell header of input layer    */
    )
{
    /* start nearest neighbor to do some basic tests */
    int row, col;		/* row/col of nearest neighbor   */
    FCELL cell;

    /* cut indices to integer */
    row = (int)floor(row_idx);
    col = (int)floor(col_idx);

    /* check for out of bounds - if out of bounds set NULL value     */
    if (row < 0 || row >= cellhd->rows || col < 0 || col >= cellhd->cols) {
        Rast_set_null_value(obufptr, 1, cell_type);
        return;
    }

    cell = CVAL(ibuffer, row, col);
    /* if nearest is null, all the other interps will be null */
    if (Rast_is_f_null_value(&cell)) {
        Rast_set_null_value(obufptr, 1, cell_type);
        return;
    }

    p_bilinear(ibuffer, obufptr, cell_type, col_idx, row_idx, cellhd);
    /* fallback to nearest if bilinear is null */
    if (Rast_is_f_null_value(obufptr))
        Rast_set_f_value(obufptr, cell, cell_type);
}