Geografic-Information-System/lib/vector/Vlib/remove_areas.c

/*!
   \file remove_areas.c

   \brief Vector library - clean geometry (remove small areas)

   Higher level functions for reading/writing/manipulating vectors.

   (C) 2001-2009 by the GRASS Development Team

   This program is free software under the GNU General Public License
   (>=v2).  Read the file COPYING that comes with GRASS for details.

   \author Radim Blazek
 */

#include <grass/config.h>
#include <stdlib.h>
#include <grass/gis.h>
#include <grass/vector.h>
#include <grass/glocale.h>

/*!
   \brief Remove small areas from the map map.

   Centroid of the area and the longest boundary with adjacent area is
   removed.  Map topology must be built GV_BUILD_CENTROIDS.

   \param[in,out] Map vector map
   \param thresh maximum area size for removed areas
   \param[out] Err vector map where removed lines and centroids are written
   \param removed_area  pointer to where total size of removed area is stored or NULL

   \return number of removed areas 
 */
int
Vect_remove_small_areas(struct Map_info *Map, double thresh,
			struct Map_info *Err, double *removed_area)
{
    int area, nareas;
    int nremoved = 0;
    struct ilist *List;
    struct ilist *AList;
    struct line_pnts *Points;
    struct line_cats *Cats;
    double size_removed = 0.0;

    List = Vect_new_list();
    AList = Vect_new_list();
    Points = Vect_new_line_struct();
    Cats = Vect_new_cats_struct();

    nareas = Vect_get_num_areas(Map);
    for (area = 1; area <= Vect_get_num_areas(Map); area++) {
	int i, j, centroid, dissolve_neighbour;
	double length, size;

	if (area <= nareas)
	    G_percent(area, nareas, 1);
	G_debug(3, "area = %d", area);
	if (!Vect_area_alive(Map, area))
	    continue;

	size = Vect_get_area_area(Map, area);
	if (size > thresh)
	    continue;
	size_removed += size;

	/* The area is smaller than the limit -> remove */

	/* Remove centroid */
	centroid = Vect_get_area_centroid(Map, area);
	if (centroid > 0) {
	    if (Err) {
		Vect_read_line(Map, Points, Cats, centroid);
		Vect_write_line(Err, GV_CENTROID, Points, Cats);
	    }
	    Vect_delete_line(Map, centroid);
	}

	/* Find the adjacent area with which the longest boundary is shared */

	Vect_get_area_boundaries(Map, area, List);

	/* Create a list of neighbour areas */
	Vect_reset_list(AList);
	for (i = 0; i < List->n_values; i++) {
	    int line, left, right, neighbour;

	    line = List->value[i];

	    if (!Vect_line_alive(Map, abs(line)))	/* Should not happen */
		G_fatal_error(_("Area is composed of dead boundary"));

	    Vect_get_line_areas(Map, abs(line), &left, &right);
	    if (line > 0)
		neighbour = left;
	    else
		neighbour = right;

	    G_debug(4, "  line = %d left = %d right = %d neighbour = %d",
		    line, left, right, neighbour);

	    Vect_list_append(AList, neighbour);	/* this checks for duplicity */
	}
	G_debug(3, "num neighbours = %d", AList->n_values);

	/* Go through the list of neghours and find that with the longest boundary */
	dissolve_neighbour = 0;
	length = -1.0;
	for (i = 0; i < AList->n_values; i++) {
	    int neighbour1;
	    double l = 0.0;

	    neighbour1 = AList->value[i];
	    G_debug(4, "   neighbour1 = %d", neighbour1);

	    for (j = 0; j < List->n_values; j++) {
		int line, left, right, neighbour2;

		line = List->value[j];
		Vect_get_line_areas(Map, abs(line), &left, &right);
		if (line > 0)
		    neighbour2 = left;
		else
		    neighbour2 = right;

		if (neighbour2 == neighbour1) {
		    Vect_read_line(Map, Points, NULL, abs(line));
		    l += Vect_line_length(Points);
		}
	    }
	    if (l > length) {
		length = l;
		dissolve_neighbour = neighbour1;
	    }
	}

	G_debug(3, "dissolve_neighbour = %d", dissolve_neighbour);

	/* Make list of boundaries to be removed */
	Vect_reset_list(AList);
	for (i = 0; i < List->n_values; i++) {
	    int line, left, right, neighbour;

	    line = List->value[i];
	    Vect_get_line_areas(Map, abs(line), &left, &right);
	    if (line > 0)
		neighbour = left;
	    else
		neighbour = right;

	    G_debug(3, "   neighbour = %d", neighbour);

	    if (neighbour == dissolve_neighbour) {
		Vect_list_append(AList, abs(line));
	    }
	}

	/* Remove boundaries */
	for (i = 0; i < AList->n_values; i++) {
	    int line;

	    line = AList->value[i];

	    if (Err) {
		Vect_read_line(Map, Points, Cats, line);
		Vect_write_line(Err, GV_BOUNDARY, Points, Cats);
	    }
	    Vect_delete_line(Map, line);
	}

	nremoved++;
    }

    if (removed_area)
	*removed_area = size_removed;

    G_verbose_message(_("%d areas of total size %g removed"), nremoved,
		size_removed);

    return (nremoved);
}