Drop LineStrings and Polygons early in tiling if they are smaller than a pixel

CHANGELOG.md

@@ -1,3 +1,10 @@
+## 1.32.5
+
+* Use less memory on lines and polygons that are too small for the tile
+* Fix coordinate rounding problem that was causing --grid-low-zooms grids
+  to be lost at low zooms if the original polygons were not aligned to
+  tile boundaries
+
 ## 1.32.4
 
 * Ignore leading zeroes when converting string attributes to feature IDs

geometry.cpp

@@ -1135,8 +1135,8 @@ drawvec stairstep(drawvec &geom, int z, int detail) {
 	double scale = 1 << (32 - detail - z);
 
 	for (size_t i = 0; i < geom.size(); i++) {
-		geom[i].x = std::round(geom[i].x / scale);
-		geom[i].y = std::round(geom[i].y / scale);
+		geom[i].x = std::floor(geom[i].x / scale);
+		geom[i].y = std::floor(geom[i].y / scale);
 	}
 
 	for (size_t i = 0; i < geom.size(); i++) {

main.cpp

@@ -3050,9 +3050,14 @@ int main(int argc, char **argv) {
 		full_detail = 12;
 	}
 
-	if (full_detail < min_detail || low_detail < min_detail) {
-		fprintf(stderr, "%s: Full detail and low detail must be at least minimum detail\n", argv[0]);
-		exit(EXIT_FAILURE);
+	if (full_detail < min_detail) {
+		min_detail = full_detail;
+		fprintf(stderr, "%s: Reducing minimum detail to match full detail %d\n", argv[0], min_detail);
+	}
+
+	if (low_detail < min_detail) {
+		min_detail = low_detail;
+		fprintf(stderr, "%s: Reducing minimum detail to match low detail %d\n", argv[0], min_detail);
 	}
 
 	// Need two checks: one for geometry representation, the other for

serial.cpp

@@ -318,6 +318,7 @@ static long long scale_geometry(struct serialization_state *sst, long long *bbox
 	long long offset = 0;
 	long long prev = 0;
 	bool has_prev = false;
+	double scale = 1.0 / (1 << geometry_scale);
 
 	for (size_t i = 0; i < geom.size(); i++) {
 		if (geom[i].op == VT_MOVETO || geom[i].op == VT_LINETO) {
@@ -365,8 +366,17 @@ static long long scale_geometry(struct serialization_state *sst, long long *bbox
 				*(sst->initialized) = 1;
 			}
 
-			geom[i].x = x >> geometry_scale;
-			geom[i].y = y >> geometry_scale;
+			if (additional[A_GRID_LOW_ZOOMS]) {
+				// If we are gridding, snap to the maxzoom grid in case the incoming data
+				// is already supposed to be aligned to tile boundaries (but is not, exactly,
+				// because of rounding error during projection).
+
+				geom[i].x = std::round(x * scale);
+				geom[i].y = std::round(y * scale);
+			} else {
+				geom[i].x = x >> geometry_scale;
+				geom[i].y = y >> geometry_scale;
+			}
 		}
 	}
 

tile.cpp

@@ -1688,6 +1688,26 @@ bool find_partial(std::vector<partial> &partials, serial_feature &sf, ssize_t &o
 	return false;
 }
 
+static bool line_is_too_small(drawvec const &geometry, int z, int detail) {
+	if (geometry.size() == 0) {
+		return true;
+	}
+
+	long long x = geometry[0].x >> (32 - detail - z);
+	long long y = geometry[0].y >> (32 - detail - z);
+
+	for (auto &g : geometry) {
+		long long xx = g.x >> (32 - detail - z);
+		long long yy = g.y >> (32 - detail - z);
+
+		if (xx != x || yy != y) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
 long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *metabase, char *stringpool, int z, unsigned tx, unsigned ty, int detail, int min_detail, sqlite3 *outdb, const char *outdir, int buffer, const char *fname, FILE **geomfile, int minzoom, int maxzoom, double todo, std::atomic<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, std::atomic<int> *running, double simplification, std::vector<std::map<std::string, layermap_entry>> *layermaps, std::vector<std::vector<std::string>> *layer_unmaps, size_t tiling_seg, size_t pass, size_t passes, unsigned long long mingap, long long minextent, double fraction, const char *prefilter, const char *postfilter, struct json_object *filter, write_tile_args *arg) {
 	int line_detail;
 	double merge_fraction = 1;
@@ -1929,6 +1949,11 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 				}
 				has_polygons = true;
 			}
+			if (sf.t == VT_POLYGON || sf.t == VT_LINE) {
+				if (line_is_too_small(sf.geometry, z, line_detail)) {
+					continue;
+				}
+			}
 
 			if (sf.geometry.size() > 0) {
 				partial p;

version.hpp

@@ -1,6 +1,6 @@
 #ifndef VERSION_HPP
 #define VERSION_HPP
 
-#define VERSION "v1.32.4"
+#define VERSION "v1.32.5"
 
 #endif