Add an option to specify the clustering distance directly

README.md

@@ -204,6 +204,7 @@ tippecanoe -z5 -o filtered.mbtiles -j '{ "ne_10m_admin_0_countries": [ "all", [
    compensate for the larger marker, or `-Bf`*number* to allow at most *number* features in the densest tile.
  * `-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
+ * `-K` _distance_ or `--cluster-distance=`_distance_: Cluster points (as with `--cluster-densest-as-needed`, but without the experimental discovery process) that are approximately within _distance_ of each other. The units are tile coordinates within a nominally 256-pixel tile, so the maximum value of 255 allows only one feature per tile. Values around 20 are probably appropriate for typical marker sizes.
 
 ### Dropping a fraction of features to keep under tile size limits
 

main.cpp

@@ -69,6 +69,7 @@ int geometry_scale = 0;
 double simplification = 1;
 size_t max_tile_size = 500000;
 size_t max_tile_features = 200000;
+int cluster_distance = 0;
 
 int prevent[256];
 int additional[256];
@@ -2271,6 +2272,7 @@ int main(int argc, char **argv) {
 		{"base-zoom", required_argument, 0, 'B'},
 		{"drop-lines", no_argument, &additional[A_LINE_DROP], 1},
 		{"drop-polygons", no_argument, &additional[A_POLYGON_DROP], 1},
+		{"cluster-distance", required_argument, 0, 'K'},
 
 		{"Dropping a fraction of features to keep under tile size limits", 0, 0, 0},
 		{"drop-densest-as-needed", no_argument, &additional[A_DROP_DENSEST_AS_NEEDED], 1},
@@ -2449,6 +2451,14 @@ int main(int argc, char **argv) {
 			}
 			break;
 
+		case 'K':
+			cluster_distance = atoi(optarg);
+			if (cluster_distance > 255) {
+				fprintf(stderr, "%s: --cluster-distance %d is too big; limit is 255\n", argv[0], cluster_distance);
+				exit(EXIT_FAILURE);
+			}
+			break;
+
 		case 'd':
 			full_detail = atoi(optarg);
 			break;

main.hpp

@@ -28,6 +28,7 @@ extern size_t TEMP_FILES;
 
 extern size_t max_tile_size;
 extern size_t max_tile_features;
+extern int cluster_distance;
 
 int mkstemp_cloexec(char *name);
 FILE *fopen_oflag(const char *name, const char *mode, int oflag);

man/tippecanoe.1

@@ -233,6 +233,8 @@ compensate for the larger marker, or \fB\fC\-Bf\fR\fInumber\fP to allow at most
 \fB\fC\-al\fR or \fB\fC\-\-drop\-lines\fR: Let "dot" dropping at lower zooms apply to lines too
 .IP \(bu 2
 \fB\fC\-ap\fR or \fB\fC\-\-drop\-polygons\fR: Let "dot" dropping at lower zooms apply to polygons too
+.IP \(bu 2
+\fB\fC\-K\fR \fIdistance\fP or \fB\fC\-\-cluster\-distance=\fR\fIdistance\fP: Cluster points (as with \fB\fC\-\-cluster\-densest\-as\-needed\fR, but without the experimental discovery process) that are approximately within \fIdistance\fP of each other. The units are tile coordinates within a nominally 256\-pixel tile, so the maximum value of 255 allows only one feature per tile. Values around 20 are probably appropriate for typical marker sizes.
 .RE
 .SS Dropping a fraction of features to keep under tile size limits
 .RS

serial.cpp

@@ -487,7 +487,7 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf) {
 	long long midy = (sf.bbox[1] / 2 + sf.bbox[3] / 2) & ((1LL << 32) - 1);
 	bbox_index = encode(midx, midy);
 
-	if (additional[A_DROP_DENSEST_AS_NEEDED] || additional[A_CLUSTER_DENSEST_AS_NEEDED] || additional[A_CALCULATE_FEATURE_DENSITY] || additional[A_INCREASE_GAMMA_AS_NEEDED] || sst->uses_gamma) {
+	if (additional[A_DROP_DENSEST_AS_NEEDED] || additional[A_CLUSTER_DENSEST_AS_NEEDED] || additional[A_CALCULATE_FEATURE_DENSITY] || additional[A_INCREASE_GAMMA_AS_NEEDED] || sst->uses_gamma || cluster_distance != 0) {
 		sf.index = bbox_index;
 	} else {
 		sf.index = 0;

tile.cpp

@@ -1569,7 +1569,7 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 				}
 			}
 
-			if (additional[A_CLUSTER_DENSEST_AS_NEEDED]) {
+			if (additional[A_CLUSTER_DENSEST_AS_NEEDED] || cluster_distance != 0) {
 				indices.push_back(sf.index);
 				if (sf.index - merge_previndex < mingap) {
 					clustered++;
@@ -2420,7 +2420,7 @@ int traverse_zooms(int *geomfd, off_t *geom_size, char *metabase, char *stringpo
 		}
 
 		double zoom_gamma = gamma;
-		unsigned long long zoom_mingap = 0;
+		unsigned long long zoom_mingap = ((1LL << (32 - i)) / 256 * cluster_distance) * ((1LL << (32 - i)) / 256 * cluster_distance);
 		long long zoom_minextent = 0;
 		double zoom_fraction = 1;