Split complex polygons into multiple features

README.md

@@ -114,6 +114,7 @@ Options
  * -pk: Don't limit tiles to 500K bytes
  * -pd: Dynamically drop some fraction of features from large tiles to keep them under the 500K size limit. It will probably look ugly at the tile boundaries.
  * -pi: Preserve the original input order of features as the drawing order instead of ordering geographically. (This is implemented as a restoration of the original order at the end, so that dot-dropping is still geographic, which means it also undoes -ao).
+ * -pp: Don't split complex polygons (over 700 vertices after simplification) into multiple features.
  * -q: Work quietly instead of reporting progress
 
 Example
@@ -202,6 +203,9 @@ have their probability diffused, so that some of them will be drawn as a square
 this minimum size and others will not be drawn at all, preserving the total area that
 all of them should have had together.
 
+Any polygons that have over 700 vertices after line simplification will be split into
+multiple features so they can be rendered efficiently, unless you use -pp to prevent this.
+
 Features in the same tile that share the same type and attributes are coalesced
 together into a single geometry. You are strongly encouraged to use -x to exclude
 any unnecessary properties to reduce wasted file size.

geometry.cc

@@ -436,7 +436,9 @@ static drawvec clip_poly1(drawvec &geom, long long minx, long long miny, long lo
 		}
 
 		if (out.size() < 3) {
-			fprintf(stderr, "Polygon degenerated to a line segment\n");
+			// fprintf(stderr, "Polygon degenerated to a line segment\n");
+			out.clear();
+			return out;
 		}
 
 		out[0].op = VT_MOVETO;
@@ -943,16 +945,12 @@ drawvec fix_polygon(drawvec &geom) {
 }
 
 std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms) {
-	return geoms;
-
 	while (1) {
 		bool again = false;
 		std::vector<drawvec> out;
 
 		for (unsigned i = 0; i < geoms.size(); i++) {
 			if (geoms[i].size() > 700) {
-				again = true;
-
 				long long midx = 0, midy = 0, count = 0;
 				long long maxx = LONG_LONG_MIN, maxy = LONG_LONG_MIN, minx = LONG_LONG_MAX, miny = LONG_LONG_MAX;
 
@@ -976,6 +974,23 @@ std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms) {
 						}
 					}
 				}
+
+				midx /= count;
+				midy /= count;
+
+				if (maxy - miny > maxx - minx) {
+					// printf("clipping y to %lld %lld %lld %lld\n", minx, miny, maxx, midy);
+					out.push_back(simple_clip_poly(geoms[i], minx, miny, maxx, midy));
+					// printf("          and %lld %lld %lld %lld\n", minx, midy, maxx, maxy);
+					out.push_back(simple_clip_poly(geoms[i], minx, midy, maxx, maxy));
+				} else {
+					// printf("clipping x to %lld %lld %lld %lld\n", minx, miny, midx, maxy);
+					out.push_back(simple_clip_poly(geoms[i], minx, miny, midx, maxy));
+					// printf("          and %lld %lld %lld %lld\n", midx, midy, maxx, maxy);
+					out.push_back(simple_clip_poly(geoms[i], midx, miny, maxx, maxy));
+				}
+
+				again = true;
 			} else {
 				out.push_back(geoms[i]);
 			}

geometry.hh

@@ -29,3 +29,4 @@ int quick_check(long long *bbox, int z, int detail, long long buffer);
 drawvec simplify_lines(drawvec &geom, int z, int detail);
 drawvec reorder_lines(drawvec &geom);
 drawvec fix_polygon(drawvec &geom);
+std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms);

man/tippecanoe.1

@@ -8,6 +8,8 @@ features. This is a tool for making maps from huge datasets
 .PP
 [Build Status](https://travis\-ci.org/mapbox/tippecanoe.svg)
 \[la]https://travis-ci.org/mapbox/tippecanoe\[ra]
+[Coverage Status](https://coveralls.io/repos/mapbox/tippecanoe/badge.svg?branch=master&service=github)
+\[la]https://coveralls.io/github/mapbox/tippecanoe?branch=master\[ra]
 .SH Intent
 .PP
 The goal of Tippecanoe is to enable making a scale\-independent view of your data,
@@ -143,6 +145,8 @@ If you use \-rg, it will guess a drop rate that will keep at most 50,000 feature
 .IP \(bu 2
 \-pi: Preserve the original input order of features as the drawing order instead of ordering geographically. (This is implemented as a restoration of the original order at the end, so that dot\-dropping is still geographic, which means it also undoes \-ao).
 .IP \(bu 2
+\-pp: Don't split complex polygons (over 700 vertices after simplification) into multiple features.
+.IP \(bu 2
 \-q: Work quietly instead of reporting progress
 .RE
 .SH Example
@@ -232,6 +236,9 @@ have their probability diffused, so that some of them will be drawn as a square
 this minimum size and others will not be drawn at all, preserving the total area that
 all of them should have had together.
 .PP
+Any polygons that have over 700 vertices after line simplification will be split into
+multiple features so they can be rendered efficiently, unless you use \-pp to prevent this.
+.PP
 Features in the same tile that share the same type and attributes are coalesced
 together into a single geometry. You are strongly encouraged to use \-x to exclude
 any unnecessary properties to reduce wasted file size.

tile.cc

@@ -529,11 +529,9 @@ void *partial_feature_worker(void *v) {
 		std::vector<drawvec> geoms;
 		geoms.push_back(geom);
 
-#if 0
-		if (t == VT_POLYGON) {
-			geoms = chop_geometry(geoms); // XXX
+		if (t == VT_POLYGON && !prevent[P_POLYGON_SPLIT]) {
+			geoms = chop_polygon(geoms);
 		}
-#endif
 
 		if (t == VT_POLYGON) {
 			// Scaling may have made the polygon degenerate.