Add --split-complex-polygons option

README.md

@@ -503,6 +503,7 @@ the same layer, enclose them in an `all` expression so they will all be evaluate
  * `-pw` or `--use-source-polygon-winding`: Instead of respecting GeoJSON polygon ring order, use the original polygon winding in the source data to distinguish inner (clockwise) and outer (counterclockwise) polygon rings.
  * `-pW` or `--reverse-source-polygon-winding`: Instead of respecting GeoJSON polygon ring order, use the opposite of the original polygon winding in the source data to distinguish inner (counterclockwise) and outer (clockwise) polygon rings.
  * `--clip-bounding-box=`*minlon*`,`*minlat*`,`*maxlon*`,`*maxlat*: Clip all features to the specified bounding box.
+ * `--split-complex-polygons=`*limit*`: Subdivide polygons with more than *limit* vertices into multiple features.
 
 ### Setting or disabling tile size limits
 

geometry.cpp

@@ -975,16 +975,16 @@ drawvec fix_polygon(drawvec &geom) {
 	return out;
 }
 
-std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms) {
+std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms, size_t n) {
 	while (1) {
 		bool again = false;
 		std::vector<drawvec> out;
 
 		for (size_t i = 0; i < geoms.size(); i++) {
-			if (geoms[i].size() > 700) {
+			if (geoms[i].size() > n) {
 				static bool warned = false;
 				if (!warned) {
-					fprintf(stderr, "Warning: splitting up polygon with more than 700 sides\n");
+					fprintf(stderr, "Warning: splitting up polygon with more than %zu sides\n", n);
 					warned = true;
 				}
 

geometry.hpp

@@ -71,7 +71,7 @@ int quick_check(long long *bbox, int z, long long buffer);
 drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification, size_t retain, drawvec const &shared_nodes);
 drawvec reorder_lines(drawvec &geom);
 drawvec fix_polygon(drawvec &geom);
-std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms);
+std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms, size_t n);
 void check_polygon(drawvec &geom);
 double get_area(drawvec &geom, size_t i, size_t j);
 double get_mp_area(drawvec &geom);

main.cpp

@@ -77,6 +77,7 @@ size_t max_tile_features = 200000;
 int cluster_distance = 0;
 long justx = -1, justy = -1;
 std::string attribute_for_id = "";
+size_t polygon_split = 0;
 
 int prevent[256];
 int additional[256];
@@ -2596,6 +2597,7 @@ int main(int argc, char **argv) {
 		{"use-source-polygon-winding", no_argument, &prevent[P_USE_SOURCE_POLYGON_WINDING], 1},
 		{"reverse-source-polygon-winding", no_argument, &prevent[P_REVERSE_SOURCE_POLYGON_WINDING], 1},
 		{"clip-bounding-box", required_argument, 0, '~'},
+		{"split-complex-polygons", required_argument, 0, '~'},
 
 		{"Filtering tile contents", 0, 0, 0},
 		{"prefilter", required_argument, 0, 'C'},
@@ -2695,6 +2697,8 @@ int main(int argc, char **argv) {
 					fprintf(stderr, "%s: Can't parse bounding box --%s=%s\n", argv[0], opt, optarg);
 					exit(EXIT_FAILURE);
 				}
+			} else if (strcmp(opt, "split-complex-polygons") == 0) {
+				polygon_split = atoi(optarg);
 			} else if (strcmp(opt, "use-attribute-for-id") == 0) {
 				attribute_for_id = optarg;
 			} else {

main.hpp

@@ -48,6 +48,7 @@ extern size_t max_tile_size;
 extern size_t max_tile_features;
 extern int cluster_distance;
 extern std::string attribute_for_id;
+extern size_t polygon_split;
 
 int mkstemp_cloexec(char *name);
 FILE *fopen_oflag(const char *name, const char *mode, int oflag);

tile.cpp

@@ -493,6 +493,10 @@ void *partial_feature_worker(void *v) {
 		std::vector<drawvec> geoms;
 		geoms.push_back(geom);
 
+		if (t == VT_POLYGON && polygon_split > 0) {
+			geoms = chop_polygon(geoms, polygon_split);
+		}
+
 		if (t == VT_POLYGON) {
 			// Scaling may have made the polygon degenerate.
 			// Give Clipper a chance to try to fix it.