From 7aa27b7f4c6688f18891ef4171519c1e3b9c182a Mon Sep 17 00:00:00 2001
From: Eric Fischer <enf@pobox.com>
Date: Wed, 20 Jan 2016 13:58:17 -0800
Subject: [PATCH] Make calculation of child tile overlap match clipping
 behavior.

Coordinates that are right on the tile boundary appear in both
adjacent tiles.

Make tippecanoe-decode also use exact edge coordinates instead of
half-pixel locations.
---
 decode.cc | 4 ++--
 tile.cc   | 6 ++++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/decode.cc b/decode.cc
index b5c77b3..d744f25 100644
--- a/decode.cc
+++ b/decode.cc
@@ -194,8 +194,8 @@ void handle(std::string message, int z, unsigned x, unsigned y, int describe) {
 						g += 2;
 
 						long long scale = 1LL << (32 - z);
-						long long wx = scale * x + (scale / extent) * (px + .5);
-						long long wy = scale * y + (scale / extent) * (py + .5);
+						long long wx = scale * x + (scale / extent) * px;
+						long long wy = scale * y + (scale / extent) * py;
 
 						double lat, lon;
 						tile2latlon(wx, wy, 32, &lat, &lon);
diff --git a/tile.cc b/tile.cc
index 0e19cb7..8c3f255 100644
--- a/tile.cc
+++ b/tile.cc
@@ -376,8 +376,10 @@ void rewrite(drawvec &geom, int z, int nextzoom, int maxzoom, long long *bbox, u
 			// Division instead of right-shift because coordinates can be negative
 			bbox2[k] = bbox[k] / (1 << (32 - nextzoom - 8));
 		}
-		bbox2[0] -= buffer;
-		bbox2[1] -= buffer;
+		// Decrement the top and left edges so that any features that are
+		// touching the edge can potentially be included in the adjacent tiles too.
+		bbox2[0] -= buffer + 1;
+		bbox2[1] -= buffer + 1;
 		bbox2[2] += buffer;
 		bbox2[3] += buffer;