Add an option to retain extra coordinate precision at maxzoom

Makefile

@@ -351,7 +351,7 @@ layer-json-test:
 prep-test: $(TESTS)
 
 tests/%.json: Makefile tippecanoe tippecanoe-decode
-	./tippecanoe -q -a@ -f -o $@.check.mbtiles $(subst @,:,$(subst %,/,$(subst _, ,$(patsubst %.json,%,$(word 4,$(subst /, ,$@)))))) $(foreach suffix,$(suffixes),$(sort $(wildcard $(subst $(SPACE),/,$(wordlist 1,2,$(subst /, ,$@)))/*.$(suffix))))
+	./tippecanoe -aE -q -a@ -f -o $@.check.mbtiles $(subst @,:,$(subst %,/,$(subst _, ,$(patsubst %.json,%,$(word 4,$(subst /, ,$@)))))) $(foreach suffix,$(suffixes),$(sort $(wildcard $(subst $(SPACE),/,$(wordlist 1,2,$(subst /, ,$@)))/*.$(suffix))))
 	./tippecanoe-decode -x generator $@.check.mbtiles > $@
 	cmp $(patsubst %.check,%,$@) $@
 	rm $@.check.mbtiles

README.md

@@ -383,6 +383,7 @@ zoom level | precision (ft) | precision (m) | map scale
  * `-d` _detail_ or `--full-detail=`_detail_: Detail at max zoom level (default 12, for tile resolution of 2^12=4096)
  * `-D` _detail_ or `--low-detail=`_detail_: Detail at lower zoom levels (default 12, for tile resolution of 2^12=4096)
  * `-m` _detail_ or `--minimum-detail=`_detail_: Minimum detail that it will try if tiles are too big at regular detail (default 7)
+ * `-aE` or `--extra-detail`: Generate tiles with even more detail than the "full" detail at the max zoom level, to maximize location precision. These tiles may not work with some rendering software that internally limits detail to 12 or 13. The extra detail does not affect the choice of maxzoom guessing, the amount of simplification, or the "tiny polygon" threshold as `--full-detail` does. The tiles should look the same as they did without it, except that they will be more precise when overzoomed.
 
 All internal math is done in terms of a 32-bit tile coordinate system, so 1/(2^32) of the size of Earth,
 or about 1cm, is the smallest distinguishable distance. If _maxzoom_ + _detail_ > 32, no additional

main.cpp

@@ -2653,6 +2653,7 @@ int main(int argc, char **argv) {
 		{"full-detail", required_argument, 0, 'd'},
 		{"low-detail", required_argument, 0, 'D'},
 		{"minimum-detail", required_argument, 0, 'm'},
+		{"extra-detail", no_argument, &additional[A_EXTRA_DETAIL], 1},
 
 		{"Filtering feature attributes", 0, 0, 0},
 		{"exclude", required_argument, 0, 'x'},

man/tippecanoe.1

@@ -475,6 +475,8 @@ zoom level	precision (ft)	precision (m)	map scale
 \fB\fC\-D\fR \fIdetail\fP or \fB\fC\-\-low\-detail=\fR\fIdetail\fP: Detail at lower zoom levels (default 12, for tile resolution of 2
 .IP \(bu 2
 \fB\fC\-m\fR \fIdetail\fP or \fB\fC\-\-minimum\-detail=\fR\fIdetail\fP: Minimum detail that it will try if tiles are too big at regular detail (default 7)
+.IP \(bu 2
+\fB\fC\-aE\fR or \fB\fC\-\-extra\-detail\fR: Generate tiles with even more detail than the "full" detail at the max zoom level, to maximize location precision. These tiles may not work with some rendering software that internally limits detail to 12 or 13. The extra detail does not affect the choice of maxzoom guessing, the amount of simplification, or the "tiny polygon" threshold as \fB\fC\-\-full\-detail\fR does. The tiles should look the same as they did without it, except that they will be more precise when overzoomed.
 .RE
 .PP
 All internal math is done in terms of a 32\-bit tile coordinate system, so 1/(2 of the size of Earth,

options.hpp

@@ -25,6 +25,7 @@
 #define A_GENERATE_IDS ((int) 'i')
 #define A_CONVERT_NUMERIC_IDS ((int) 'I')
 #define A_HILBERT ((int) 'h')
+#define A_EXTRA_DETAIL ((int) 'E')
 
 #define P_SIMPLIFY ((int) 's')
 #define P_SIMPLIFY_LOW ((int) 'S')

tile.cpp

@@ -374,6 +374,7 @@ struct partial {
 	bool reduced = 0;
 	int z = 0;
 	int line_detail = 0;
+	int extra_detail = 0;
 	int maxzoom = 0;
 	double spacing = 0;
 	double simplification = 0;
@@ -452,6 +453,7 @@ void *partial_feature_worker(void *v) {
 		signed char t = (*partials)[i].t;
 		int z = (*partials)[i].z;
 		int line_detail = (*partials)[i].line_detail;
+		int extra_detail = (*partials)[i].extra_detail;
 		int maxzoom = (*partials)[i].maxzoom;
 
 		if (additional[A_GRID_LOW_ZOOMS] && z < maxzoom) {
@@ -466,6 +468,7 @@ void *partial_feature_worker(void *v) {
 		if ((t == VT_LINE || t == VT_POLYGON) && !(prevent[P_SIMPLIFY] || (z == maxzoom && prevent[P_SIMPLIFY_LOW]) || (z < maxzoom && additional[A_GRID_LOW_ZOOMS]))) {
 			if (1 /* !reduced */) {	 // XXX why did this not simplify if reduced?
 				if (t == VT_LINE) {
+					// continues to deduplicate to line_detail even if we have extra detail
 					geom = remove_noop(geom, t, 32 - z - line_detail);
 				}
 
@@ -475,6 +478,7 @@ void *partial_feature_worker(void *v) {
 				}
 
 				if (!already_marked) {
+					// continues to simplify to line_detail even if we have extra detail
 					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) {
@@ -484,17 +488,11 @@ void *partial_feature_worker(void *v) {
 			}
 		}
 
-#if 0
-		if (t == VT_LINE && z != basezoom) {
-			geom = shrink_lines(geom, z, line_detail, basezoom, &along);
-		}
-#endif
-
 		if (t == VT_LINE && additional[A_REVERSE]) {
 			geom = reorder_lines(geom);
 		}
 
-		to_tile_scale(geom, z, line_detail);
+		to_tile_scale(geom, z, extra_detail);
 
 		std::vector<drawvec> geoms;
 		geoms.push_back(geom);
@@ -511,7 +509,8 @@ void *partial_feature_worker(void *v) {
 
 				if (geoms[g].size() < 3) {
 					if (area > 0) {
-						geoms[g] = revive_polygon(before, area / geoms.size(), z, line_detail);
+						// area is in world coordinates, calculated before scaling down
+						geoms[g] = revive_polygon(before, area / geoms.size(), z, extra_detail);
 					} else {
 						geoms[g].clear();
 					}
@@ -1721,8 +1720,7 @@ static bool line_is_too_small(drawvec const &geometry, int z, int detail) {
 	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, atomic_strategy *strategy) {
-	int line_detail;
+long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *metabase, char *stringpool, int z, unsigned tx, unsigned ty, const 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, atomic_strategy *strategy) {
 	double merge_fraction = 1;
 	double mingap_fraction = 1;
 	double minextent_fraction = 1;
@@ -1755,6 +1753,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 	bool first_time = true;
 	// This only loops if the tile data didn't fit, in which case the detail
 	// goes down and the progress indicator goes backward for the next try.
+	int line_detail;
 	for (line_detail = detail; line_detail >= min_detail || line_detail == detail; line_detail--, oprogress = 0) {
 		long long count = 0;
 		double accum_area = 0;
@@ -1777,6 +1776,8 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 		double coalesced_area = 0;
 		drawvec shared_nodes;
 
+		int tile_detail = line_detail;
+
 		int within[child_shards];
 		std::atomic<long long> geompos[child_shards];
 		for (size_t i = 0; i < (size_t) child_shards; i++) {
@@ -2007,6 +2008,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 				p.reduced = reduced;
 				p.z = z;
 				p.line_detail = line_detail;
+				p.extra_detail = line_detail;
 				p.maxzoom = maxzoom;
 				p.keys = sf.keys;
 				p.values = sf.values;
@@ -2020,6 +2022,14 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 				p.renamed = -1;
 				p.extent = sf.extent;
 				p.clustered = 0;
+
+				if (line_detail == detail && additional[A_EXTRA_DETAIL] && z == maxzoom) {
+					// maximum allowed coordinate delta in geometries is 2^31 - 1
+					// so we need to stay under that, including the buffer
+					p.extra_detail = 30 - z;
+					tile_detail = p.extra_detail;
+				}
+
 				partials.push_back(p);
 			}
 
@@ -2272,7 +2282,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 			mvt_layer layer;
 			layer.name = layer_iterator->first;
 			layer.version = 2;
-			layer.extent = 1 << line_detail;
+			layer.extent = 1 << tile_detail;
 
 			for (size_t x = 0; x < layer_features.size(); x++) {
 				mvt_feature feature;
@@ -2330,7 +2340,7 @@ long long write_tile(FILE *geoms, std::atomic<long long> *geompos_in, char *meta
 		}
 
 		if (postfilter != NULL) {
-			tile.layers = filter_layers(postfilter, tile.layers, z, tx, ty, layermaps, tiling_seg, layer_unmaps, 1 << line_detail);
+			tile.layers = filter_layers(postfilter, tile.layers, z, tx, ty, layermaps, tiling_seg, layer_unmaps, 1 << tile_detail);
 		}
 
 		if (z == 0 && unclipped_features < original_features / 2 && clipbboxes.size() == 0) {