diff --git a/README.md b/README.md
index a308df8d..9dbbe7d2 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,7 @@ Options
* -f: Delete the mbtiles file if it already exists instead of giving an error
* -r rate: Rate at which dots are dropped at lower zoom levels (default 2.5)
* -b pixels: Buffer size where features are duplicated from adjacent tiles (default 5)
+ * -g gamma: 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
-------
diff --git a/geojson.c b/geojson.c
index ba11b217..0a0dbd43 100644
--- a/geojson.c
+++ b/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);
diff --git a/tile.cc b/tile.cc
index 0eb4fef1..a0c9b851 100644
--- a/tile.cc
+++ b/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 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;