Add an option to generate label points in place of polygons

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.9.0
+
+* Add an option to generate label points in place of polygons
+
 ## 2.8.1
 
 * Improve precision of polygon ring area calculations

README.md

@@ -518,6 +518,7 @@ 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.
+ * `-aP` or `--convert-polygons-to-label-points`: Replace polygon geometries with a label point or points for the polygon in each tile it intersects.
 
 ### Setting or disabling tile size limits
 

geometry.cpp

@@ -1364,3 +1364,65 @@ drawvec stairstep(drawvec &geom, int z, int detail) {
 
 	return out;
 }
+
+struct polygon_label {
+	long long size;
+	draw point;
+
+	bool operator<(const polygon_label &o) const {
+		return size > o.size; // reverse sort, largest first
+	}
+};
+
+// Generate a label point for a polygon feature, currently at the centroid of its largest-area
+// outer ring within the tile.
+drawvec polygon_to_anchor(const drawvec &geom) {
+	std::vector<polygon_label> labels;
+	drawvec dv;
+
+	for (size_t i = 0; i < geom.size(); i++) {
+		if (geom[i].op == VT_MOVETO) {
+			size_t j;
+			for (j = i + 1; j < geom.size(); j++) {
+				if (geom[j].op != VT_LINETO) {
+					break;
+				}
+			}
+
+			long long xsum = 0;
+			long long ysum = 0;
+			size_t count = 0;
+
+			double area = get_area(geom, i, j);
+			if (area > 0) {  // don't generate anchors for holes
+				// i + 1 to exclude the first point, which is duplicated as the last
+				for (size_t k = i + 1; k < j; k++) {
+					xsum += geom[k].x;
+					ysum += geom[k].y;
+					count++;
+				}
+
+				if (count > 0) {
+					draw d(VT_MOVETO, xsum / count, ysum / count);
+
+					polygon_label pl;
+					pl.size = area;
+					pl.point = d;
+
+					labels.push_back(pl);
+				}
+			}
+
+			i = j - 1;
+		}
+	}
+
+	std::sort(labels.begin(), labels.end());
+
+	if (labels.size() > 0) {
+		dv.push_back(labels[0].point);
+	}
+
+	return dv;
+}
+

geometry.hpp

@@ -76,6 +76,7 @@ std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms);
 void check_polygon(drawvec &geom);
 double get_area(const drawvec &geom, size_t i, size_t j);
 double get_mp_area(drawvec &geom);
+drawvec polygon_to_anchor(const drawvec &geom);
 
 drawvec simple_clip_poly(drawvec &geom, long long x1, long long y1, long long x2, long long y2);
 drawvec clip_lines(drawvec &geom, long long x1, long long y1, long long x2, long long y2);

main.cpp

@@ -2761,6 +2761,7 @@ 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, '~'},
+		{"convert-polygons-to-label-points", no_argument, &additional[A_GENERATE_POLYGON_LABEL_POINTS], 1},
 
 		{"Filtering tile contents", 0, 0, 0},
 		{"prefilter", required_argument, 0, 'C'},

man/tippecanoe.1

@@ -671,6 +671,8 @@ the line or polygon within one tile unit of its proper location. You can probabl
 \fB\fC\-pW\fR or \fB\fC\-\-reverse\-source\-polygon\-winding\fR: 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.
 .IP \(bu 2
 \fB\fC\-\-clip\-bounding\-box=\fR\fIminlon\fP\fB\fC,\fR\fIminlat\fP\fB\fC,\fR\fImaxlon\fP\fB\fC,\fR\fImaxlat\fP: Clip all features to the specified bounding box.
+.IP \(bu 2
+\fB\fC\-aP\fR or \fB\fC\-\-convert\-polygons\-to\-label\-points\fR: Replace polygon geometries with a label point or points for the polygon in each tile it intersects.
 .RE
 .SS Setting or disabling tile size limits
 .RS

options.hpp

@@ -26,6 +26,7 @@
 #define A_CONVERT_NUMERIC_IDS ((int) 'I')
 #define A_HILBERT ((int) 'h')
 #define A_VISVALINGAM ((int) 'v')
+#define A_GENERATE_POLYGON_LABEL_POINTS ((int) 'P')
 
 #define P_SIMPLIFY ((int) 's')
 #define P_SIMPLIFY_LOW ((int) 'S')

tile.cpp

@@ -2076,6 +2076,11 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 				}
 			}
 
+			if (sf.t == VT_POLYGON && additional[A_GENERATE_POLYGON_LABEL_POINTS]) {
+				sf.t = VT_POINT;
+				sf.geometry = polygon_to_anchor(sf.geometry);
+			}
+
 			if (sf.geometry.size() > 0) {
 				if (partials.size() > max_tile_size) {
 					// Even being maximally conservative, each feature is still going to be

version.hpp

@@ -1,6 +1,6 @@
 #ifndef VERSION_HPP
 #define VERSION_HPP
 
-#define VERSION "v2.8.1"
+#define VERSION "v2.9.0"
 
 #endif