From 91eefcbd168d7128c5f75c0485e9c16c3c994d8a Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Tue, 23 Sep 2014 14:42:17 -0700
Subject: [PATCH] Drop more points at each lower zoom level

---
 geojson.c |  6 ++++--
 tile.cc   | 20 +++++++++++++++++++-
 tile.h    |  2 +-
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/geojson.c b/geojson.c
index d8cf465..8d02bc3 100644
--- a/geojson.c
+++ b/geojson.c
@@ -14,6 +14,8 @@
 #include "jsonpull.h"
 #include "tile.h"
 
+#define BASE_ZOOM 14
+
 #define GEOM_POINT 0                /* array of positions */
 #define GEOM_MULTIPOINT 1           /* array of arrays of positions */
 #define GEOM_LINESTRING 2           /* array of arrays of positions */
@@ -303,7 +305,7 @@ void check(struct index *ix, long long n, char *metabase, unsigned *file_bbox) {
 	fprintf(stderr, "\n");
 
 	int z;
-	for (z = 14; z >= 0; z--) {
+	for (z = BASE_ZOOM; z >= 0; z--) {
 		struct index *i, *j = NULL;
 		for (i = ix; i < ix + n && i != NULL; i = j) {
 			unsigned wx, wy;
@@ -334,7 +336,7 @@ void check(struct index *ix, long long n, char *metabase, unsigned *file_bbox) {
 
 			printf("%d/%u/%u    %x %x   %lld to %lld\n", z, tx, ty, wx, wy, (long long)(i - ix), (long long)(j - ix));
 
-			write_tile(i, j, metabase, file_bbox, z, tx, ty, z == 14 ? 12 : 10);
+			write_tile(i, j, metabase, file_bbox, z, tx, ty, z == BASE_ZOOM ? 12 : 10, BASE_ZOOM);
 		}
 	}
 }
diff --git a/tile.cc b/tile.cc
index 3a30cb0..9d28229 100644
--- a/tile.cc
+++ b/tile.cc
@@ -6,6 +6,7 @@
 #include <zlib.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <math.h>
 #include "vector_tile.pb.h"
 
 extern "C" {
@@ -121,7 +122,7 @@ int draw(char **meta, mapnik::vector::tile_feature *feature, int z, unsigned tx,
 }
 
 
-void write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned tx, unsigned ty, int detail) {
+void write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned tx, unsigned ty, int detail, int basezoom) {
 	GOOGLE_PROTOBUF_VERIFY_VERSION;
 
 	mapnik::vector::tile tile;
@@ -144,6 +145,13 @@ void write_tile(struct index *start, struct index *end, char *metabase, unsigned
 	values.head = NULL;
 	values.tail = NULL;
 
+	double interval = 1;
+	double seq = 0;
+
+	if (z < basezoom) {
+		interval = exp(log(2.5) * (basezoom - z));
+	}
+
 	struct index *i;
 	for (i = start; i < end; i++) {
 		int t;
@@ -151,6 +159,16 @@ void write_tile(struct index *start, struct index *end, char *metabase, unsigned
 		char *meta = metabase + i->fpos;
 		deserialize_int(&meta, &t);
 
+		if (t == VT_POINT) {
+			seq++;
+
+			if (seq >= 0) {
+				seq -= interval;
+			} else {
+				continue;
+			}
+		}
+
 		if (t == VT_POINT || draw(&meta, NULL, z, tx, ty, detail)) {
 			meta = metabase + i->fpos;
 			deserialize_int(&meta, &t);
diff --git a/tile.h b/tile.h
index 7c8945d..5ecd231 100644
--- a/tile.h
+++ b/tile.h
@@ -47,4 +47,4 @@ struct index {
 };
 
 
-void write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned x, unsigned y, int detail);
+void write_tile(struct index *start, struct index *end, char *metabase, unsigned *file_bbox, int z, unsigned x, unsigned y, int detail, int basezoom);