r.to.vect: new flag to re-center centroids (#4690)

* r.to.vect: new flag to re-center centroids

raster/r.to.vect/areas_io.c

@@ -290,6 +290,10 @@ int write_area(
 
     catNum = 1;
 
+    if (centroid_flag) {
+        Vect_build_partial(&Map, GV_BUILD_ATTACH_ISLES);
+    }
+
     G_important_message(_("Writing areas..."));
     for (i = 0, p = a_list; i < n_areas; i++, p++) {
         G_percent(i, n_areas, 3);
@@ -327,6 +331,21 @@ int write_area(
                 break;
             }
 
+            if (centroid_flag) {
+                int area, ret;
+
+                area = Vect_find_area(&Map, x, y);
+                if (area == 0) {
+                    G_warning(_("No area for centroid %d"), i);
+                }
+                else {
+                    ret = Vect_get_point_in_area(&Map, area, &x, &y);
+                    if (ret < 0) {
+                        G_warning(_("Unable to calculate area centroid"));
+                    }
+                }
+            }
+
             Vect_reset_line(points);
             Vect_append_point(points, x, y, 0.0);
 

raster/r.to.vect/global.h

@@ -48,7 +48,8 @@ extern int n_alloced_ptrs;
 
 extern int
     smooth_flag; /* this is 0 for no smoothing, 1 for smoothing of lines */
-extern int value_flag; /* use raster values as categories */
+extern int value_flag;    /* use raster values as categories */
+extern int centroid_flag; /* re-center centroids */
 
 extern struct Categories RastCats;
 extern int has_cats; /* Category labels available */

raster/r.to.vect/main.c

@@ -48,8 +48,10 @@ int row_length, row_count, n_rows;
 int total_areas;
 int n_alloced_ptrs;
 
-int smooth_flag; /* this is 0 for no smoothing, 1 for smoothing of lines */
-int value_flag;  /* use raster values as categories */
+int smooth_flag;   /* this is 0 for no smoothing, 1 for smoothing of lines */
+int value_flag;    /* use raster values as categories */
+int centroid_flag; /* re-center centroids */
+
 struct Categories RastCats;
 int has_cats; /* Category labels available */
 struct field_info *Fi;
@@ -66,6 +68,7 @@ int main(int argc, char *argv[])
     struct GModule *module;
     struct Option *in_opt, *out_opt, *feature_opt, *column_name;
     struct Flag *smooth_flg, *value_flg, *z_flg, *no_topol, *notab_flg;
+    struct Flag *centroid_flg;
     int feature, notab_flag;
 
     G_gisinit(argv[0]);
@@ -115,6 +118,13 @@ int main(int argc, char *argv[])
     no_topol->label = _("Do not build vector topology");
     no_topol->description = _("Recommended for massive point conversion");
 
+    centroid_flg = G_define_flag();
+    centroid_flg->key = 'c';
+    centroid_flg->label =
+        _("Move centroids to more central locations within areas");
+    centroid_flg->description = _("Default: centroids are located anywhere in "
+                                  "areas, often close to boundaries");
+
     notab_flg = G_define_standard_flag(G_FLG_V_TABLE);
 
     if (G_parser(argc, argv))
@@ -123,6 +133,7 @@ int main(int argc, char *argv[])
     feature = Vect_option_to_types(feature_opt);
     smooth_flag = (smooth_flg->answer) ? SMOOTH : NO_SMOOTH;
     value_flag = value_flg->answer;
+    centroid_flag = centroid_flg->answer;
     notab_flag = notab_flg->answer;
 
     if (z_flg->answer && (feature != GV_POINT))

raster/r.to.vect/r.to.vect.html

@@ -57,6 +57,11 @@ <h3>Area conversion</h3>
 input file. If the raster map contains other data (i.e., line edges,
 or point data) the output may be wrong.
 
+<p>
+By default, area centroids are often located close to boundaries and not
+in the middle of an area. Centroids can be more centrally located with
+the <em>-c</em> flag.
+
 <h2>EXAMPLES</h2>
 
 The examples are based on the North Carolina sample dataset: