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

@@ -1042,17 +1042,19 @@ 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);
 
-	for (size_t i = 0; i < geom.size(); i++) {
-		if (geom[i].op == VT_MOVETO) {
-			geom[i].necessary = 1;
-		} else if (geom[i].op == VT_LINETO) {
-			geom[i].necessary = 0;
-		} else {
-			geom[i].necessary = 1;
+	if (!already_marked) {
+		for (size_t i = 0; i < geom.size(); i++) {
+			if (geom[i].op == VT_MOVETO) {
+				geom[i].necessary = 1;
+			} else if (geom[i].op == VT_LINETO) {
+				geom[i].necessary = 0;
+			} else {
+				geom[i].necessary = 1;
+			}
 		}
 	}
 
@@ -1073,7 +1075,19 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 			geom[j - 1].necessary = 1;
 
 			if (j - i > 1) {
-				douglas_peucker(geom, i, j - i, res * simplification);
+				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')