Make base zoom and drop rate guessing more testable, and test them

README.md

@@ -81,7 +81,7 @@ Options
  * -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 at most 50,000 features in the densest tile.
    You can also specify a marker-width with -Bg*width* to allow fewer features in the densest tile to
-   compensate for the larger marker.
+   compensate for the larger marker, or -Bf*number* to allow at most *number* 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)
@@ -97,6 +97,8 @@ Options
 
  * -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.
+   You can also specify a marker-width with -rg*width* to allow fewer features in the densest tile to
+   compensate for the larger marker, or -rf*number* to allow at most *number* 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

@@ -2105,12 +2105,15 @@ int main(int argc, char **argv) {
 		case 'B':
 			if (strcmp(optarg, "g") == 0) {
 				basezoom = -2;
-				basezoom_marker_width = 1;
-			} else if (optarg[0] == 'g') {
+			} else if (optarg[0] == 'g' || optarg[0] == 'f') {
 				basezoom = -2;
-				basezoom_marker_width = atof(optarg + 1);
-				if (basezoom_marker_width == 0) {
-					fprintf(stderr, "%s: Must specify marker width >0 with -Bg\n", argv[0]);
+				if (optarg[0] == 'g') {
+					basezoom_marker_width = atof(optarg + 1);
+				} else {
+					basezoom_marker_width = sqrt(50000 / atof(optarg + 1));
+				}
+				if (basezoom_marker_width == 0 || atof(optarg + 1) == 0) {
+					fprintf(stderr, "%s: Must specify value >0 with -B%c\n", argv[0], optarg[0]);
 					exit(EXIT_FAILURE);
 				}
 			} else {
@@ -2154,6 +2157,17 @@ int main(int argc, char **argv) {
 		case 'r':
 			if (strcmp(optarg, "g") == 0) {
 				droprate = -2;
+			} else if (optarg[0] == 'g' || optarg[0] == 'f') {
+				droprate = -2;
+				if (optarg[0] == 'g') {
+					basezoom_marker_width = atof(optarg + 1);
+				} else {
+					basezoom_marker_width = sqrt(50000 / atof(optarg + 1));
+				}
+				if (basezoom_marker_width == 0 || atof(optarg + 1) == 0) {
+					fprintf(stderr, "%s: Must specify value >0 with -r%c\n", argv[0], optarg[0]);
+					exit(EXIT_FAILURE);
+				}
 			} else {
 				droprate = atof(optarg);
 			}

man/tippecanoe.1

@@ -92,7 +92,7 @@ rather than a stream that can only be read sequentially.
 \-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 at most 50,000 features in the densest tile.
 You can also specify a marker\-width with \-Bg\fIwidth\fP to allow fewer features in the densest tile to
-compensate for the larger marker.
+compensate for the larger marker, or \-Bf\fInumber\fP to allow at most \fInumber\fP 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
@@ -116,6 +116,8 @@ compensate for the larger marker.
 .IP \(bu 2
 \-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.
+You can also specify a marker\-width with \-rg\fIwidth\fP to allow fewer features in the densest tile to
+compensate for the larger marker, or \-rf\fInumber\fP to allow at most \fInumber\fP 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