Fix clipping bug when the clip region doesn't intersect the tile (#312)

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 2.73.0
+
+* Correctly clip features down to nothing when the clip region doesn't intersect the tile at all
+
 # 2.72.0
 
 * Add --clip-polygon-file and --feature-filter-file options to tippecanoe-overzoom

Makefile

@@ -439,6 +439,10 @@ overzoom-test: tippecanoe-overzoom
 	./tippecanoe-decode tests/pbf/countries-8-135-86-bigclip.pbf 8 135 86 > tests/pbf/countries-8-135-86-bigclip.json.check
 	cmp tests/pbf/countries-8-135-86-bigclip.json.check tests/pbf/countries-8-135-86-bigclip.json
 	rm tests/pbf/countries-8-135-86-bigclip.pbf tests/pbf/countries-8-135-86-bigclip.json.check
+	# Clip region that does not intersect with the tile
+	./tippecanoe-overzoom -o tests/pbf/squirrels-13-2413-3077-clip.pbf --clip-polygon-file tests/pbf/squirrels-clip.json tests/pbf/squirrels-13-2413-3077.pbf 13/2413/3077 13/2413/3077
+	cmp tests/pbf/squirrels-13-2413-3077-clip.pbf /dev/null  # clipped away
+	rm tests/pbf/squirrels-13-2413-3077-clip.pbf
 
 join-test: tippecanoe tippecanoe-decode tile-join
 	./tippecanoe -q -f -z12 -o tests/join-population/tabblock_06001420.mbtiles -YALAND10:'Land area' -L'{"file": "tests/join-population/tabblock_06001420.json", "description": "population"}'

overzoom.cpp

@@ -294,6 +294,7 @@ int main(int argc, char **argv) {
 	// clip the clip polygons, if any, to the tile bounds,
 	// to reduce their complexity
 
+	bool clipped_to_nothing = false;
 	if (clipbboxes.size() > 0) {
 		long long wx1 = (nx - buffer / 256.0) * (1LL << (32 - nz));
 		long long wy1 = (ny - buffer / 256.0) * (1LL << (32 - nz));
@@ -315,10 +316,18 @@ int main(int argc, char **argv) {
 
 			if (c.dv.size() > 0) {
 				c.dv = clip_poly_poly(c.dv, tile_bounds);
+
+				if (c.dv.size() == 0) {
+					clipped_to_nothing = true;
+					break;
+				}
 			}
 		}
 	}
 
+	std::string out;
+
+	if (!clipped_to_nothing) {
 		json_object *json_filter = NULL;
 		if (filter.size() > 0) {
 			json_filter = parse_filter(filter.c_str());
@@ -345,7 +354,8 @@ int main(int argc, char **argv) {
 			its.push_back(std::move(t));
 		}
 
-	std::string out = overzoom(its, nz, nx, ny, detail, buffer, keep, exclude, exclude_prefix, do_compress, NULL, demultiply, json_filter, preserve_input_order, attribute_accum, unidecode_data, simplification, tiny_polygon_size, bins, bin_by_id_list, accumulate_numeric, SIZE_MAX, clipbboxes);
+		out = overzoom(its, nz, nx, ny, detail, buffer, keep, exclude, exclude_prefix, do_compress, NULL, demultiply, json_filter, preserve_input_order, attribute_accum, unidecode_data, simplification, tiny_polygon_size, bins, bin_by_id_list, accumulate_numeric, SIZE_MAX, clipbboxes);
+	}
 
 	FILE *f = fopen(outfile, "wb");
 	if (f == NULL) {

platform.hpp

@@ -11,6 +11,8 @@ size_t calc_memsize();
 
 size_t get_max_open_files();
 
-constexpr const char *get_null_device() { return "/dev/null"; }
+constexpr const char *get_null_device() {
+	return "/dev/null";
+}
 
 #endif	// PLATFORM_HPP

tests/pbf/squirrels-clip.json

@@ -0,0 +1 @@
+{"coordinates":[[[-73.9722549,40.7904659],[-73.9755817,40.7674848],[-73.938783,40.767022],[-73.9457082,40.7791562],[-73.9628855,40.7760201],[-73.9629534,40.7914425],[-73.9722549,40.7904659]]],"crs":{"properties":{"name":"EPSG:4326"},"type":"name"},"type":"Polygon"}

version.hpp

@@ -1,6 +1,6 @@
 #ifndef VERSION_HPP
 #define VERSION_HPP
 
-#define VERSION "v2.72.0"
+#define VERSION "v2.73.0"
 
 #endif