Add an option to keep intersection nodes from being simplified away

Author: e-n-f

README.md

@@ -470,6 +470,7 @@ the same layer, enclose them in an `all` expression so they will all be evaluate
    the line or polygon within one tile unit of its proper location. You can probably go up to about 10 without too much visible difference.
  * `-ps` or `--no-line-simplification`: Don't simplify lines and polygons
  * `-pS` or `--simplify-only-low-zooms`: Don't simplify lines and polygons at maxzoom (but do simplify at lower zooms)
+ * `-pn` or `--no-simplification-of-shared-nodes`: Don't simplify away nodes that appear in more than one feature or are used multiple times within the same feature, so that the intersection node will not be lost from intersecting roads. (This will not be effective if you also use `--coalesce` or `--detect-shared-borders`.)
  * `-pt` or `--no-tiny-polygon-reduction`: Don't combine the area of very small polygons into small squares that represent their combined area.
 
 ### Attempts to improve shared polygon boundaries

geometry.cpp

@@ -802,7 +802,7 @@ 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, size_t retain) {
+drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds, double simplification, size_t retain, drawvec const &shared_nodes) {
 	int res = 1 << (32 - detail - z);
 	long long area = 1LL << (32 - z);
 
@@ -814,6 +814,13 @@ drawvec simplify_lines(drawvec &geom, int z, int detail, bool mark_tile_bounds,
 		} else {
 			geom[i].necessary = 1;
 		}
+
+		if (prevent[P_SIMPLIFY_SHARED_NODES]) {
+			auto pt = std::lower_bound(shared_nodes.begin(), shared_nodes.end(), geom[i]);
+			if (pt != shared_nodes.end() && *pt == geom[i]) {
+				geom[i].necessary = true;
+			}
+		}
 	}
 
 	if (mark_tile_bounds) {

geometry.hpp

@@ -68,7 +68,7 @@ drawvec clip_lines(drawvec &geom, int z, long long buffer);
 drawvec stairstep(drawvec &geom, int z, int detail);
 bool point_within_tile(long long x, long long y, int z);
 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 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);

main.cpp

@@ -2569,6 +2569,7 @@ int main(int argc, char **argv) {
 		{"no-line-simplification", no_argument, &prevent[P_SIMPLIFY], 1},
 		{"simplify-only-low-zooms", no_argument, &prevent[P_SIMPLIFY_LOW], 1},
 		{"no-tiny-polygon-reduction", no_argument, &prevent[P_TINY_POLYGON_REDUCTION], 1},
+		{"no-simplification-of-shared-nodes", no_argument, &prevent[P_SIMPLIFY_SHARED_NODES], 1},
 
 		{"Attempts to improve shared polygon boundaries", 0, 0, 0},
 		{"detect-shared-borders", no_argument, &additional[A_DETECT_SHARED_BORDERS], 1},

options.hpp

@@ -28,6 +28,7 @@
 
 #define P_SIMPLIFY ((int) 's')
 #define P_SIMPLIFY_LOW ((int) 'S')
+#define P_SIMPLIFY_SHARED_NODES ((int) 'n')
 #define P_FEATURE_LIMIT ((int) 'f')
 #define P_KILOBYTE_LIMIT ((int) 'k')
 #define P_DYNAMIC_DROP ((int) 'd')

tests/tl_2018_51685_roads/in.json

@@ -381,6 +381,7 @@ struct partial_arg {
 	std::vector<struct partial> *partials = NULL;
 	int task = 0;
 	int tasks = 0;
+	drawvec *shared_nodes;
 };
 
 drawvec revive_polygon(drawvec &geom, double area, int z, int detail) {
@@ -464,7 +465,7 @@ void *partial_feature_worker(void *v) {
 				}
 
 				if (!already_marked) {
-					drawvec ngeom = simplify_lines(geom, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), (*partials)[i].simplification, t == VT_POLYGON ? 4 : 0);
+					drawvec ngeom = simplify_lines(geom, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), (*partials)[i].simplification, t == VT_POLYGON ? 4 : 0, *(a->shared_nodes));
 
 					if (t != VT_POLYGON || ngeom.size() >= 3) {
 						geom = ngeom;
@@ -898,7 +899,7 @@ bool find_common_edges(std::vector<partial> &partials, int z, int line_detail, d
 			}
 		}
 		if (!(prevent[P_SIMPLIFY] || (z == maxzoom && prevent[P_SIMPLIFY_LOW]) || (z < maxzoom && additional[A_GRID_LOW_ZOOMS]))) {
-			simplified_arcs[ai->second] = simplify_lines(dv, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification, 4);
+			simplified_arcs[ai->second] = simplify_lines(dv, z, line_detail, !(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification, 4, drawvec());
 		} else {
 			simplified_arcs[ai->second] = dv;
 		}
@@ -1763,6 +1764,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 		std::vector<long long> extents;
 		std::map<std::string, accum_state> attribute_accum_state;
 		double coalesced_area = 0;
+		drawvec shared_nodes;
 
 		int within[child_shards];
 		std::atomic<long long> geompos[child_shards];
@@ -1962,6 +1964,12 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 			}
 
 			if (sf.geometry.size() > 0) {
+				if (prevent[P_SIMPLIFY_SHARED_NODES]) {
+					for (auto &g : sf.geometry) {
+						shared_nodes.push_back(g);
+					}
+				}
+
 				partial p;
 				p.geoms.push_back(sf.geometry);
 				p.layer = sf.layer;
@@ -1991,6 +1999,25 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 			coalesced_area = 0;
 		}
 
+		{
+			drawvec just_shared_nodes;
+			std::sort(shared_nodes.begin(), shared_nodes.end());
+
+			for (size_t i = 0; i + 1 < shared_nodes.size(); i++) {
+				if (shared_nodes[i] == shared_nodes[i + 1]) {
+					just_shared_nodes.push_back(shared_nodes[i]);
+
+					draw d = shared_nodes[i];
+					i++;
+					while (i + 1 < shared_nodes.size() && shared_nodes[i + 1] == d) {
+						i++;
+					}
+				}
+			}
+
+			shared_nodes = just_shared_nodes;
+		}
+
 		for (size_t i = 0; i < partials.size(); i++) {
 			partial &p = partials[i];
 
@@ -2073,6 +2100,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 			args[i].task = i;
 			args[i].tasks = tasks;
 			args[i].partials = &partials;
+			args[i].shared_nodes = &shared_nodes;
 
 			if (tasks > 1) {
 				if (pthread_create(&pthreads[i], NULL, partial_feature_worker, &args[i]) != 0) {
@@ -2186,7 +2214,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 				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, layer_features[x].type == VT_POLYGON ? 4 : 0);
+										!(prevent[P_CLIPPING] || prevent[P_DUPLICATION]), simplification, layer_features[x].type == VT_POLYGON ? 4 : 0, shared_nodes);
 				}
 
 				if (layer_features[x].type == VT_POLYGON) {