From ffe4c953760d8fd74d1564f5ba4a6194ee78fcac Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Fri, 14 Nov 2014 12:42:50 -0800
Subject: [PATCH] Calculate point dropping during encoding instead of tiling

so it is consistent across tile boundaries, and so every point
that exists at zoom x always also exists at zoom x+1.
---
 geojson.c | 16 ++++++++++++++++
 tile.cc   | 22 ++++------------------
 2 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/geojson.c b/geojson.c
index f985e166..c26ca7c3 100644
--- a/geojson.c
+++ b/geojson.c
@@ -445,6 +445,16 @@ void read_json(FILE *f, char *fname, char *layername, int maxzoom, int minzoom,
 			unsigned cx = bbox[0] / 2 + bbox[2] / 2;
 			unsigned cy = bbox[1] / 2 + bbox[3] / 2;
 
+			/*
+			 * Note that minzoom for lines is the dimension
+			 * of the geometry in world coordinates, but
+			 * for points is the lowest zoom level (in tiles,
+			 * not in pixels) at which it should be drawn.
+			 *
+			 * So a line that is too small for, say, z8
+			 * will have minzoom of 18 (if tile detail is 10),
+			 * not 8.
+			 */
 			int minzoom = 0;
 			if (mb_geometry[t] == VT_LINE) {
 				for (minzoom = 0; minzoom < 31; minzoom++) {
@@ -455,6 +465,12 @@ void read_json(FILE *f, char *fname, char *layername, int maxzoom, int minzoom,
 						break;
 					}
 				}
+			} else if (mb_geometry[t] == VT_POINT) {
+				double r = ((double) rand()) / RAND_MAX;
+				if (r == 0) {
+					r = .00000001;
+				}
+				minzoom = maxzoom - floor(log(r) / - log(droprate));
 			}
 
 			/* XXX do proper overlap instead of whole bounding box */
diff --git a/tile.cc b/tile.cc
index 27f69f28..cb0bcabc 100644
--- a/tile.cc
+++ b/tile.cc
@@ -354,24 +354,21 @@ long long write_tile(struct index *start, struct index *end, char *metabase, uns
 		pool_init(&values, 0);
 		std::set<long long> dup;
 
-		double interval = 1;
-		double seq = 0;
 		long long count = 0;
 		//long long along = 0;
 		double accum_area = 0;
 
-		if (z < basezoom) {
-			interval = exp(log(droprate) * (basezoom - z));
-		}
-
 		std::vector<coalesce> features;
 
 		struct index *i;
 		for (i = start; i < end; i++) {
+			int t = i->type;
+
 			if (z > i->maxzoom) {
 				continue;
 			}
-			if (z + line_detail <= i->minzoom) {
+			if ((t == VT_LINE && z + line_detail <= i->minzoom) ||
+			    (t == VT_POINT && z < i->minzoom)) {
 				continue;
 			}
 
@@ -382,17 +379,6 @@ long long write_tile(struct index *start, struct index *end, char *metabase, uns
 				dup.insert(i->fpos);
 			}
 
-			int t = i->type;
-			if (t == VT_POINT) {
-				seq++;
-
-				if (seq >= 0) {
-					seq -= interval;
-				} else {
-					continue;
-				}
-			}
-
 			char *meta = metabase + i->fpos;
 			drawvec geom = decode_geometry(&meta, z, tx, ty, line_detail);