Add dot gamma processing

README.md

@@ -37,6 +37,7 @@ Options
  * -f: Delete the mbtiles file if it already exists instead of giving an error
  * -r <i>rate</i>: Rate at which dots are dropped at lower zoom levels (default 2.5)
  * -b <i>pixels</i>: Buffer size where features are duplicated from adjacent tiles (default 5)
+ * -g <i>gamma</i>: Rate at which especially dense dots are dropped (default 1, for no effect). A gamma of 2 reduces the number of dots duplicating the same pixel to the square root of their original number.
 
 Example
 -------

geojson.c

@@ -893,7 +893,7 @@ int main(int argc, char **argv) {
 	pool_init(&include, 0);
 	int exclude_all = 0;
 
-	while ((i = getopt(argc, argv, "l:n:z:Z:d:D:o:x:y:r:b:fXt:")) != -1) {
+	while ((i = getopt(argc, argv, "l:n:z:Z:d:D:o:x:y:r:b:fXt:g:")) != -1) {
 		switch (i) {
 		case 'n':
 			name = optarg;
@@ -952,6 +952,10 @@ int main(int argc, char **argv) {
 			tmpdir = optarg;
 			break;
 
+		case 'g':
+			gamma = atof(optarg);
+			break;
+
 		default:
 			fprintf(stderr, "Usage: %s -o out.mbtiles [-n name] [-l layername] [-z maxzoom] [-Z minzoom] [-d detail] [-D lower-detail] [-x excluded-field ...] [-y included-field ...] [-X] [-r droprate] [-b buffer] [-t tmpdir] [file.json]\n", argv[0]);
 			exit(EXIT_FAILURE);

tile.cc

@@ -367,6 +367,10 @@ long long write_tile(char **geoms, char *metabase, unsigned *file_bbox, int z, u
 			interval = exp(log(droprate) * (basezoom - z));
 		}
 
+		unsigned long long previndex = 0;
+		double scale = (double) (1LL << (64 - 2 * (z + 8)));
+		double gap = 0;
+
 		std::vector<coalesce> features;
 
 		int within[4] = { 0 };
@@ -496,6 +500,33 @@ long long write_tile(char **geoms, char *metabase, unsigned *file_bbox, int z, u
 				} else {
 					continue;
 				}
+
+				unsigned long long index = encode(bbox[0] / 2 + bbox[2] / 2, bbox[1] / 2 + bbox[3] / 2);
+				if (gap > 0) {
+					if (index == previndex) {
+						continue; // Exact duplicate: can't fulfil the gap requirement
+					}
+
+					if (exp(log((index - previndex) / scale) * gamma) >= gap) {
+						// Dot is further from the previous than the nth root of the gap,
+						// so produce it, and choose a new gap at the next point.
+						gap = 0;
+					} else {
+						continue;
+					}
+				} else {
+					gap = (index - previndex) / scale;
+
+					if (gap == 0) {
+						continue; // Exact duplicate: skip
+					} else if (gap < 1) {
+						continue; // Narrow dot spacing: need to stretch out
+					} else {
+						gap = 0; // Wider spacing than minimum: so pass through unchanged
+					}
+				}
+
+				previndex = index;
 			}
 
 			bool reduced = false;