Add an option to guess the drop rate

README.md

@@ -69,7 +69,7 @@ Options
  * -z _zoom_: Maxzoom: the highest zoom level for which tiles are generated (default 14)
  * -Z _zoom_: Minzoom: the lowest zoom level for which tiles are generated (default 0)
  * -B _zoom_: Base zoom, the level at and above which all points are included in the tiles (default maxzoom).
-   If you use -Bg, it will guess a zoom level that will keep less than 50,000 features in the densest tile.
+   If you use -Bg, it will guess a zoom level that will keep at most 50,000 features in the densest tile.
  * -d _detail_: Detail at max zoom level (default 12, for tile resolution of 4096)
  * -D _detail_: Detail at lower zoom levels (default 12, for tile resolution of 4096)
  * -m _detail_: Minimum detail that it will try if tiles are too big at regular detail (default 7)
@@ -83,7 +83,8 @@ Options
 
 ### Point simplification
 
- * -r _rate_: Rate at which dots are dropped at zoom levels below basezoom (default 2.5)
+ * -r _rate_: Rate at which dots are dropped at zoom levels below basezoom (default 2.5).
+   If you use -rg, it will guess a drop rate that will keep at most 50,000 features in the densest tile.
  * -g _gamma_: Rate at which especially dense dots are dropped (default 0, for no effect). A gamma of 2 reduces the number of dots less than a pixel apart to the square root of their original number.
 
 ### Doing more

geojson.c

@@ -1046,7 +1046,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		exit(EXIT_FAILURE);
 	}
 
-	if (basezoom < 0) {
+	if (basezoom < 0 || droprate < 0) {
 		struct index *map = mmap(NULL, indexpos, PROT_READ, MAP_PRIVATE, indexfd, 0);
 		if (map == MAP_FAILED) {
 			perror("mmap");
@@ -1096,15 +1096,20 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 			}
 		}
 
-		basezoom = MAX_ZOOM;
-
 		int z;
 		for (z = MAX_ZOOM; z >= 0; z--) {
 			if (tile[z].count > max[z].count) {
 				max[z] = tile[z];
 			}
+		}
 
-			if (max[z].count < 50000) {
+#define MAX_FEATURES 50000
+
+		if (basezoom < 0) {
+			basezoom = MAX_ZOOM;
+
+			for (z = MAX_ZOOM; z >= 0; z--) {
+				if (max[z].count < MAX_FEATURES) {
 					basezoom = z;
 				}
 
@@ -1112,6 +1117,23 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 			}
 
 			fprintf(stderr, "Choosing a base zoom of -B%d to keep %lld features in tile %d/%u/%u.\n", basezoom, max[basezoom].count, basezoom, max[basezoom].x, max[basezoom].y);
+		}
+
+		if (droprate < 0) {
+			droprate = 1;
+
+			for (z = 0; z <= basezoom; z++) {
+				double interval = exp(log(droprate) * (basezoom - z));
+
+				if (max[z].count / interval >= MAX_FEATURES) {
+					interval = (long double) max[z].count / MAX_FEATURES;
+					droprate = exp(log(interval) / (basezoom - z));
+					interval = exp(log(droprate) * (basezoom - z));
+
+					fprintf(stderr, "Choosing a drop rate of -r%f to keep %f features in tile %d/%u/%u.\n", droprate, max[z].count / interval, z, max[z].x, max[z].y);
+				}
+			}
+		}
 
 		munmap(map, indexpos);
 	}
@@ -1375,7 +1397,11 @@ int main(int argc, char **argv) {
 			break;
 
 		case 'r':
+			if (strcmp(optarg, "g") == 0) {
+				droprate = -2;
+			} else {
 				droprate = atof(optarg);
+			}
 			break;
 
 		case 'b':

man/tippecanoe.1

@@ -76,7 +76,7 @@ specified, the files are all merged into the single named layer.
 \-Z \fIzoom\fP: Minzoom: the lowest zoom level for which tiles are generated (default 0)
 .IP \(bu 2
 \-B \fIzoom\fP: Base zoom, the level at and above which all points are included in the tiles (default maxzoom).
-If you use \-Bg, it will guess a zoom level that will keep less than 50,000 features in the densest tile.
+If you use \-Bg, it will guess a zoom level that will keep at most 50,000 features in the densest tile.
 .IP \(bu 2
 \-d \fIdetail\fP: Detail at max zoom level (default 12, for tile resolution of 4096)
 .IP \(bu 2
@@ -98,7 +98,8 @@ If you use \-Bg, it will guess a zoom level that will keep less than 50,000 feat
 .SS Point simplification
 .RS
 .IP \(bu 2
-\-r \fIrate\fP: Rate at which dots are dropped at zoom levels below basezoom (default 2.5)
+\-r \fIrate\fP: Rate at which dots are dropped at zoom levels below basezoom (default 2.5).
+If you use \-rg, it will guess a drop rate that will keep at most 50,000 features in the densest tile.
 .IP \(bu 2
 \-g \fIgamma\fP: Rate at which especially dense dots are dropped (default 0, for no effect). A gamma of 2 reduces the number of dots less than a pixel apart to the square root of their original number.
 .RE