Add an option to retain N times as many points as usual at each zoom

main.cpp

@@ -93,6 +93,7 @@ unsigned int drop_denser = 0;
 std::map<std::string, serial_val> set_attributes;
 unsigned long long preserve_point_density_threshold = 0;
 long long extend_zooms_max = 0;
+double retain_points_multiplier = 1;
 
 std::vector<order_field> order_by;
 bool order_reverse;
@@ -1058,7 +1059,10 @@ void prep_drop_states(struct drop_state *ds, int maxzoom, int basezoom, double d
 		ds[i].interval = 0;
 
 		if (i < basezoom) {
-			ds[i].interval = std::exp(std::log(droprate) * (basezoom - i));
+			ds[i].interval = std::exp(std::log(droprate) * (basezoom - i)) / retain_points_multiplier;
+			if (ds[i].interval < 1) {
+				ds[i].interval = 1;
+			}
 		}
 
 		ds[i].seq = 0;
@@ -2705,7 +2709,10 @@ std::pair<int, metadata> read_input(std::vector<source> &sources, char *fname, i
 
 			size_t i = 0;
 			for (int z = 0; z <= basezoom; z++) {
-				double keep_fraction = 1.0 / std::exp(std::log(droprate) * (basezoom - z));
+				double keep_fraction = 1.0 / std::exp(std::log(droprate) * (basezoom - z)) / retain_points_multiplier;
+				if (keep_fraction > 1) {
+					keep_fraction = 1;
+				}
 				size_t keep_count = ddv.size() * keep_fraction;
 
 				for (; i < keep_count && i < ddv.size(); i++) {
@@ -3150,6 +3157,7 @@ int main(int argc, char **argv) {
 
 		{"Dropping a fixed fraction of features by zoom level", 0, 0, 0},
 		{"drop-rate", required_argument, 0, 'r'},
+		{"retain-points-multiplier", required_argument, 0, '~'},
 		{"base-zoom", required_argument, 0, 'B'},
 		{"drop-denser", required_argument, 0, '~'},
 		{"limit-base-zoom-to-maximum-zoom", no_argument, &prevent[P_BASEZOOM_ABOVE_MAXZOOM], 1},
@@ -3370,6 +3378,8 @@ int main(int argc, char **argv) {
 				preserve_point_density_threshold = atoll_require(optarg, "Preserve point density threshold");
 			} else if (strcmp(opt, "extend-zooms-if-still-dropping-maximum") == 0) {
 				extend_zooms_max = atoll_require(optarg, "Maximum number by which to extend zooms");
+			} else if (strcmp(opt, "retain-points-multiplier") == 0) {
+				retain_points_multiplier = atof_require(optarg, "Multiply the fraction of points retained by zoom level");
 			} else {
 				fprintf(stderr, "%s: Unrecognized option --%s\n", argv[0], opt);
 				exit(EXIT_ARGS);