Merge pull request #302 from mapbox/simplify-polygons-together

Find edges shared between polygons and simplify them individually

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.14.3
+
+* Add --detect-shared-borders option for better polygon simplification
+
 ## 1.14.2
 
 * Enforce that string feature attributes must be encoded as UTF-8

README.md

@@ -125,6 +125,7 @@ resolution is obtained than by using a smaller _maxzoom_ or _detail_.
  * -al or --drop-lines: Let "dot" dropping at lower zooms apply to lines too
  * -ap or --drop-polygons: Let "dot" dropping at lower zooms apply to polygons too
  * -ag or --calculate-feature-density: Add a new attribute, `tippecanoe_feature_density`, to each feature, to record how densely features are spaced in that area of the tile. You can use this attribute in the style to produce a glowing effect where points are densely packed. It can range from 0 in the sparsest areas to 255 in the densest.
+ * -ab or --detect-shared-borders: In the manner of [TopoJSON](https://github.com/mbostock/topojson/wiki/Introduction), detect borders that are shared between multiple polygons and simplify them identically in each polygon. This takes more time and memory than considering each polygon individually.
 
 ### Doing less
 
@@ -253,12 +254,12 @@ lower resolutions before failing if it still doesn't fit.
 Development
 -----------
 
-Requires sqlite3 (should already be installed on MacOS). Rebuilding the manpage
+Requires sqlite3 and zlib (should already be installed on MacOS). Rebuilding the manpage
 uses md2man (`gem install md2man`).
 
 Linux:
 
-    sudo apt-get install libsqlite3-dev
+    sudo apt-get install libsqlite3-dev zlib1g-dev
 
 Then build:
 

geometry.cpp

@@ -1042,10 +1042,11 @@ drawvec impose_tile_boundaries(drawvec &geom, long long extent) {
 	return out;
 }
 
-drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification) {
+drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification, bool already_marked) {
 	int res = 1 << (32 - detail - z);
 	long long area = 1LL << (32 - z);
 
+	if (!already_marked) {
 		for (size_t i = 0; i < geom.size(); i++) {
 			if (geom[i].op == VT_MOVETO) {
 				geom[i].necessary = 1;
@@ -1055,6 +1056,7 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 				geom[i].necessary = 1;
 			}
 		}
+	}
 
 	if (mark_tile_bounds) {
 		geom = impose_tile_boundaries(geom, area);
@@ -1073,8 +1075,20 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 			geom[j - 1].necessary = 1;
 
 			if (j - i > 1) {
+				if (already_marked && geom[j - 1] < geom[i]) {
+					drawvec dv;
+					for (size_t k = j; k > i; k--) {
+						dv.push_back(geom[k - 1]);
+					}
+					douglas_peucker(dv, 0, j - i, res * simplification);
+					size_t l = 0;
+					for (size_t k = j; k > i; k--) {
+						geom[k - 1] = dv[l++];
+					}
+				} else {
 					douglas_peucker(geom, i, j - i, res * simplification);
 				}
+			}
 			i = j - 1;
 		}
 	}

geometry.hpp

@@ -24,9 +24,30 @@ struct draw {
 		this->op = nop;
 		this->x = nx;
 		this->y = ny;
+		this->necessary = 0;
 	}
 
 	draw() {
+		this->op = 0;
+		this->x = 0;
+		this->y = 0;
+		this->necessary = 0;
+	}
+
+	bool operator<(draw const &s) const {
+		if (y < s.y || (y == s.y && x < s.x)) {
+			return true;
+		} else {
+			return false;
+		}
+	}
+
+	bool operator==(draw const &s) const {
+		return y == s.y && x == s.x;
+	}
+
+	bool operator!=(draw const &s) const {
+		return y != s.y || x != s.x;
 	}
 };
 
@@ -43,7 +64,7 @@ drawvec reduce_tiny_poly(drawvec &geom, int z, int detail, bool *reduced, double
 drawvec clip_lines(drawvec &geom, int z, int detail, long long buffer);
 bool point_within_tile(long long x, long long y, int z, int detail, long long buffer);
 int quick_check(long long *bbox, int z, int detail, long long buffer);
-drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification);
+drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification, bool already_marked);
 drawvec reorder_lines(drawvec &geom);
 drawvec fix_polygon(drawvec &geom);
 std::vector<drawvec> chop_polygon(std::vector<drawvec> &geoms);

main.cpp

@@ -1801,6 +1801,7 @@ int main(int argc, char **argv) {
 		{"drop-polygons", no_argument, &additional[A_POLYGON_DROP], 1},
 		{"prefer-radix-sort", no_argument, &additional[A_PREFER_RADIX_SORT], 1},
 		{"calculate-feature-density", no_argument, &additional[A_CALCULATE_FEATURE_DENSITY], 1},
+		{"detect-shared-borders", no_argument, &additional[A_DETECT_SHARED_BORDERS], 1},
 
 		{"no-line-simplification", no_argument, &prevent[P_SIMPLIFY], 1},
 		{"simplify-only-low-zooms", no_argument, &prevent[P_SIMPLIFY_LOW], 1},

man/tippecanoe.1

@@ -148,6 +148,8 @@ which may not be what you want.
 \-ap or \-\-drop\-polygons: Let "dot" dropping at lower zooms apply to polygons too
 .IP \(bu 2
 \-ag or \-\-calculate\-feature\-density: Add a new attribute, \fB\fCtippecanoe_feature_density\fR, to each feature, to record how densely features are spaced in that area of the tile. You can use this attribute in the style to produce a glowing effect where points are densely packed. It can range from 0 in the sparsest areas to 255 in the densest.
+.IP \(bu 2
+\-ab or \-\-detect\-shared\-borders: In the manner of TopoJSON \[la]https://github.com/mbostock/topojson/wiki/Introduction\[ra], detect borders that are shared between multiple polygons and simplify them identically in each polygon. This takes more time and memory than considering each polygon individually.
 .RE
 .SS Doing less
 .RS
@@ -289,14 +291,14 @@ If a tile is larger than 500K, it will try encoding that tile at progressively
 lower resolutions before failing if it still doesn't fit.
 .SH Development
 .PP
-Requires sqlite3 (should already be installed on MacOS). Rebuilding the manpage
+Requires sqlite3 and zlib (should already be installed on MacOS). Rebuilding the manpage
 uses md2man (\fB\fCgem install md2man\fR).
 .PP
 Linux:
 .PP
 .RS
 .nf
-sudo apt\-get install libsqlite3\-dev
+sudo apt\-get install libsqlite3\-dev zlib1g\-dev
 .fi
 .RE
 .PP

options.hpp

@@ -4,6 +4,7 @@
 #define A_LINE_DROP ((int) 'l')
 #define A_DEBUG_POLYGON ((int) 'd')
 #define A_POLYGON_DROP ((int) 'p')
+#define A_DETECT_SHARED_BORDERS ((int) 'b')
 #define A_PREFER_RADIX_SORT ((int) 'R')
 #define A_CALCULATE_FEATURE_DENSITY ((int) 'g')
 

tile.cpp

@@ -19,6 +19,7 @@
 #include <sqlite3.h>
 #include <pthread.h>
 #include <errno.h>
+#include <time.h>
 #include "mvt.hpp"
 #include "mbtiles.hpp"
 #include "geometry.hpp"
@@ -372,6 +373,7 @@ struct partial {
 	std::vector<drawvec> geoms;
 	std::vector<long long> keys;
 	std::vector<long long> values;
+	std::vector<ssize_t> arc_polygon;
 	char *meta;
 	long long layer;
 	long long original_seq;
@@ -460,13 +462,20 @@ void *partial_feature_worker(void *v) {
 					geom = remove_noop(geom, t, 32 - z - line_detail);
 				}
 
-				drawvec ngeom = simplify_lines(geom, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), (*partials)[i].simplification);
+				bool already_marked = false;
+				if (additional[A_DETECT_SHARED_BORDERS] && t == VT_POLYGON) {
+					already_marked = true;
+				}
+
+				if (!already_marked) {
+					drawvec ngeom = simplify_lines(geom, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), (*partials)[i].simplification, already_marked);
 
 					if (t != VT_POLYGON || ngeom.size() >= 3) {
 						geom = ngeom;
 					}
 				}
 			}
+		}
 
 #if 0
 		if (t == VT_LINE && z != basezoom) {
@@ -561,6 +570,398 @@ int manage_gap(unsigned long long index, unsigned long long *previndex, double s
 	return 0;
 }
 
+// Does not fix up moveto/lineto
+static drawvec reverse_subring(drawvec const &dv) {
+	drawvec out;
+
+	for (size_t i = dv.size(); i > 0; i--) {
+		out.push_back(dv[i - 1]);
+	}
+
+	return out;
+}
+
+struct edge {
+	unsigned x1;
+	unsigned y1;
+	unsigned x2;
+	unsigned y2;
+	unsigned ring;
+
+	edge(unsigned _x1, unsigned _y1, unsigned _x2, unsigned _y2, unsigned _ring) {
+		x1 = _x1;
+		y1 = _y1;
+		x2 = _x2;
+		y2 = _y2;
+		ring = _ring;
+	}
+
+	bool operator<(const edge &s) const {
+		long long cmp = (long long) y1 - s.y1;
+		if (cmp == 0) {
+			cmp = (long long) x1 - s.x1;
+		}
+		if (cmp == 0) {
+			cmp = (long long) y2 - s.y2;
+		}
+		if (cmp == 0) {
+			cmp = (long long) x2 - s.x2;
+		}
+		return cmp < 0;
+	}
+};
+
+struct edgecmp_ring {
+	bool operator()(const edge &a, const edge &b) {
+		long long cmp = (long long) a.y1 - b.y1;
+		if (cmp == 0) {
+			cmp = (long long) a.x1 - b.x1;
+		}
+		if (cmp == 0) {
+			cmp = (long long) a.y2 - b.y2;
+		}
+		if (cmp == 0) {
+			cmp = (long long) a.x2 - b.x2;
+		}
+		if (cmp == 0) {
+			cmp = (long long) a.ring - b.ring;
+		}
+		return cmp < 0;
+	}
+} edgecmp_ring;
+
+bool edges_same(std::pair<std::vector<edge>::iterator, std::vector<edge>::iterator> e1, std::pair<std::vector<edge>::iterator, std::vector<edge>::iterator> e2) {
+	if ((e2.second - e2.first) != (e1.second - e1.first)) {
+		return false;
+	}
+
+	while (e1.first != e1.second) {
+		if (e1.first->ring != e2.first->ring) {
+			return false;
+		}
+
+		++e1.first;
+		++e2.first;
+	}
+
+	return true;
+}
+
+void find_common_edges(std::vector<partial> &partials, int z, int line_detail, double simplification, int maxzoom) {
+	for (size_t i = 0; i < partials.size(); i++) {
+		if (partials[i].t == VT_POLYGON) {
+			for (size_t j = 0; j < partials[i].geoms.size(); j++) {
+				drawvec &g = partials[i].geoms[j];
+				drawvec out;
+
+				for (size_t k = 0; k < g.size(); k++) {
+					if (g[k].op == VT_LINETO && k > 0 && g[k - 1] == g[k]) {
+						;
+					} else {
+						out.push_back(g[k]);
+					}
+				}
+
+				partials[i].geoms[j] = out;
+			}
+		}
+	}
+
+	// Construct a mapping from all polygon edges to the set of rings
+	// that each edge appears in. (The ring number is across all polygons;
+	// we don't need to look it back up, just to tell where it changes.)
+
+	std::vector<edge> edges;
+	size_t ring = 0;
+	for (size_t i = 0; i < partials.size(); i++) {
+		if (partials[i].t == VT_POLYGON) {
+			for (size_t j = 0; j < partials[i].geoms.size(); j++) {
+				for (size_t k = 0; k + 1 < partials[i].geoms[j].size(); k++) {
+					if (partials[i].geoms[j][k].op == VT_MOVETO) {
+						ring++;
+					}
+
+					if (partials[i].geoms[j][k + 1].op == VT_LINETO) {
+						drawvec dv;
+						if (partials[i].geoms[j][k] < partials[i].geoms[j][k + 1]) {
+							dv.push_back(partials[i].geoms[j][k]);
+							dv.push_back(partials[i].geoms[j][k + 1]);
+						} else {
+							dv.push_back(partials[i].geoms[j][k + 1]);
+							dv.push_back(partials[i].geoms[j][k]);
+						}
+
+						edges.push_back(edge(dv[0].x, dv[0].y, dv[1].x, dv[1].y, ring));
+					}
+				}
+			}
+		}
+	}
+
+	std::sort(edges.begin(), edges.end(), edgecmp_ring);
+	std::set<draw> necessaries;
+
+	// Now mark all the points where the set of rings using the edge on one side
+	// is not the same as the set of rings using the edge on the other side.
+
+	for (size_t i = 0; i < partials.size(); i++) {
+		if (partials[i].t == VT_POLYGON) {
+			for (size_t j = 0; j < partials[i].geoms.size(); j++) {
+				drawvec &g = partials[i].geoms[j];
+
+				for (size_t k = 0; k < g.size(); k++) {
+					g[k].necessary = 0;
+				}
+
+				for (size_t a = 0; a < g.size(); a++) {
+					if (g[a].op == VT_MOVETO) {
+						size_t b;
+
+						for (b = a + 1; b < g.size(); b++) {
+							if (g[b].op != VT_LINETO) {
+								break;
+							}
+						}
+
+						// -1 because of duplication at the end
+						size_t s = b - a - 1;
+
+						if (s > 0) {
+							drawvec left;
+							if (g[a + (0 - 1 + s) % s] < g[a + 0]) {
+								left.push_back(g[a + (0 - 1 + s) % s]);
+								left.push_back(g[a + 0]);
+							} else {
+								left.push_back(g[a + 0]);
+								left.push_back(g[a + (0 - 1 + s) % s]);
+							}
+							if (left[1] < left[0]) {
+								fprintf(stderr, "left misordered\n");
+							}
+							std::pair<std::vector<edge>::iterator, std::vector<edge>::iterator> e1 = std::equal_range(edges.begin(), edges.end(), edge(left[0].x, left[0].y, left[1].x, left[1].y, 0));
+
+							for (size_t k = 0; k < s; k++) {
+								drawvec right;
+
+								if (g[a + k] < g[a + k + 1]) {
+									right.push_back(g[a + k]);
+									right.push_back(g[a + k + 1]);
+								} else {
+									right.push_back(g[a + k + 1]);
+									right.push_back(g[a + k]);
+								}
+
+								std::pair<std::vector<edge>::iterator, std::vector<edge>::iterator> e2 = std::equal_range(edges.begin(), edges.end(), edge(right[0].x, right[0].y, right[1].x, right[1].y, 0));
+
+								if (right[1] < right[0]) {
+									fprintf(stderr, "left misordered\n");
+								}
+
+								if (e1.first == e1.second || e2.first == e2.second) {
+									fprintf(stderr, "Internal error: polygon edge lookup failed for %lld,%lld to %lld,%lld or %lld,%lld to %lld,%lld\n", left[0].x, left[0].y, left[1].x, left[1].y, right[0].x, right[0].y, right[1].x, right[1].y);
+									exit(EXIT_FAILURE);
+								}
+
+								if (!edges_same(e1, e2)) {
+									g[a + k].necessary = 1;
+									necessaries.insert(g[a + k]);
+								}
+
+								e1 = e2;
+							}
+						}
+
+						a = b - 1;
+					}
+				}
+			}
+		}
+	}
+
+	edges.clear();
+	std::map<drawvec, size_t> arcs;
+
+	// Roll rings that include a necessary point around so they start at one
+
+	for (size_t i = 0; i < partials.size(); i++) {
+		if (partials[i].t == VT_POLYGON) {
+			for (size_t j = 0; j < partials[i].geoms.size(); j++) {
+				drawvec &g = partials[i].geoms[j];
+
+				for (size_t k = 0; k < g.size(); k++) {
+					if (necessaries.count(g[k]) != 0) {
+						g[k].necessary = 1;
+					}
+				}
+
+				for (size_t k = 0; k < g.size(); k++) {
+					if (g[k].op == VT_MOVETO) {
+						ssize_t necessary = -1;
+						ssize_t lowest = k;
+						size_t l;
+						for (l = k + 1; l < g.size(); l++) {
+							if (g[l].op != VT_LINETO) {
+								break;
+							}
+
+							if (g[l].necessary) {
+								necessary = l;
+							}
+							if (g[l] < g[lowest]) {
+								lowest = l;
+							}
+						}
+
+						if (necessary < 0) {
+							necessary = lowest;
+							// Add a necessary marker if there was none in the ring,
+							// so the arc code below can find it.
+							g[lowest].necessary = 1;
+						}
+
+						{
+							drawvec tmp;
+
+							// l - 1 because the endpoint is duplicated
+							for (size_t m = necessary; m < l - 1; m++) {
+								tmp.push_back(g[m]);
+							}
+							for (size_t m = k; m < necessary; m++) {
+								tmp.push_back(g[m]);
+							}
+
+							// replace the endpoint
+							tmp.push_back(g[necessary]);
+
+							if (tmp.size() != l - k) {
+								fprintf(stderr, "internal error shifting ring\n");
+								exit(EXIT_FAILURE);
+							}
+
+							for (size_t m = 0; m < tmp.size(); m++) {
+								if (m == 0) {
+									tmp[m].op = VT_MOVETO;
+								} else {
+									tmp[m].op = VT_LINETO;
+								}
+
+								g[k + m] = tmp[m];
+							}
+						}
+
+						// Now peel off each set of segments from one necessary point to the next
+						// into an "arc" as in TopoJSON
+
+						for (size_t m = k; m < l; m++) {
+							if (!g[m].necessary) {
+								fprintf(stderr, "internal error in arc building\n");
+								exit(EXIT_FAILURE);
+							}
+
+							drawvec arc;
+							size_t n;
+							for (n = m; n < l; n++) {
+								arc.push_back(g[n]);
+								if (n > m && g[n].necessary) {
+									break;
+								}
+							}
+
+							auto f = arcs.find(arc);
+							if (f == arcs.end()) {
+								drawvec arc2 = reverse_subring(arc);
+
+								auto f2 = arcs.find(arc2);
+								if (f2 == arcs.end()) {
+									// Add new arc
+									size_t added = arcs.size() + 1;
+									arcs.insert(std::pair<drawvec, size_t>(arc, added));
+									partials[i].arc_polygon.push_back(added);
+								} else {
+									partials[i].arc_polygon.push_back(-f2->second);
+								}
+							} else {
+								partials[i].arc_polygon.push_back(f->second);
+							}
+
+							m = n - 1;
+						}
+
+						partials[i].arc_polygon.push_back(0);
+
+						k = l - 1;
+					}
+				}
+			}
+		}
+	}
+
+	std::vector<drawvec> simplified_arcs;
+
+	size_t count = 0;
+	for (auto ai = arcs.begin(); ai != arcs.end(); ++ai) {
+		if (simplified_arcs.size() < ai->second + 1) {
+			simplified_arcs.resize(ai->second + 1);
+		}
+
+		drawvec dv = ai->first;
+		for (size_t i = 0; i < dv.size(); i++) {
+			if (i == 0) {
+				dv[i].op = VT_MOVETO;
+			} else {
+				dv[i].op = VT_LINETO;
+			}
+		}
+		if (!(prevent[P_SIMPLIFY] || (z == maxzoom && prevent[P_SIMPLIFY_LOW]))) {
+			simplified_arcs[ai->second] = simplify_lines(dv, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification, false);
+		} else {
+			simplified_arcs[ai->second] = dv;
+		}
+		count++;
+	}
+
+	for (size_t i = 0; i < partials.size(); i++) {
+		if (partials[i].t == VT_POLYGON) {
+			partials[i].geoms.resize(0);
+			partials[i].geoms.push_back(drawvec());
+			bool at_start = true;
+			draw first(-1, 0, 0);
+
+			for (size_t j = 0; j < partials[i].arc_polygon.size(); j++) {
+				ssize_t p = partials[i].arc_polygon[j];
+
+				if (p == 0) {
+					if (first.op >= 0) {
+						partials[i].geoms[0].push_back(first);
+						first = draw(-1, 0, 0);
+					}
+					at_start = true;
+				} else if (p > 0) {
+					for (size_t k = 0; k + 1 < simplified_arcs[p].size(); k++) {
+						if (at_start) {
+							partials[i].geoms[0].push_back(draw(VT_MOVETO, simplified_arcs[p][k].x, simplified_arcs[p][k].y));
+							first = draw(VT_LINETO, simplified_arcs[p][k].x, simplified_arcs[p][k].y);
+						} else {
+							partials[i].geoms[0].push_back(draw(VT_LINETO, simplified_arcs[p][k].x, simplified_arcs[p][k].y));
+						}
+						at_start = 0;
+					}
+				} else { /* p < 0 */
+					for (ssize_t k = simplified_arcs[-p].size() - 1; k > 0; k--) {
+						if (at_start) {
+							partials[i].geoms[0].push_back(draw(VT_MOVETO, simplified_arcs[-p][k].x, simplified_arcs[-p][k].y));
+							first = draw(VT_LINETO, simplified_arcs[-p][k].x, simplified_arcs[-p][k].y);
+						} else {
+							partials[i].geoms[0].push_back(draw(VT_LINETO, simplified_arcs[-p][k].x, simplified_arcs[-p][k].y));
+						}
+						at_start = 0;
+					}
+				}
+			}
+		}
+	}
+}
+
 long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *stringpool, int z, unsigned tx, unsigned ty, int detail, int min_detail, int basezoom, sqlite3 *outdb, double droprate, int buffer, const char *fname, FILE **geomfile, int minzoom, int maxzoom, double todo, volatile long long *along, long long alongminus, double gamma, int child_shards, long long *meta_off, long long *pool_off, unsigned *initial_x, unsigned *initial_y, volatile int *running, double simplification, std::vector<std::map<std::string, layermap_entry>> *layermaps, std::vector<std::vector<std::string>> *layer_unmaps) {
 	int line_detail;
 	double fraction = 1;
@@ -868,6 +1269,10 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 			}
 		}
 
+		if (additional[A_DETECT_SHARED_BORDERS]) {
+			find_common_edges(partials, z, line_detail, simplification, maxzoom);
+		}
+
 		int tasks = ceil((double) CPUS / *running);
 		if (tasks < 1) {
 			tasks = 1;
@@ -987,7 +1392,7 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 				if (layer_features[x].coalesced && layer_features[x].type == VT_LINE) {
 					layer_features[x].geom = remove_noop(layer_features[x].geom, layer_features[x].type, 0);
 					layer_features[x].geom = simplify_lines(layer_features[x].geom, 32, 0,
-										!(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification);
+										!(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification, false);
 				}
 
 				if (layer_features[x].type == VT_POLYGON) {

version.hpp

@@ -1 +1 @@
-#define VERSION "tippecanoe v1.14.2\n"
+#define VERSION "tippecanoe v1.14.3\n"