From 57c769595b1ce3d8e21ce95bfcacc8bc04d28e08 Mon Sep 17 00:00:00 2001
From: Wouter van Kleunen <wouter.van@kleunen.nl>
Date: Mon, 3 May 2021 08:27:55 +0000
Subject: [PATCH] Fix polygon pruning

---
 src/output_object.cpp | 66 ++++++++++++++++++++++++++-----------------
 1 file changed, 40 insertions(+), 26 deletions(-)

diff --git a/src/output_object.cpp b/src/output_object.cpp
index e70f87a2..21ff9f5e 100644
--- a/src/output_object.cpp
+++ b/src/output_object.cpp
@@ -108,12 +108,17 @@ Geometry buildWayGeometry(OSMStore &osmStore, OutputObject const &oo, const Tile
 
 		case OutputGeometryType::POLYGON:
 		{
-			auto const &mp = osmStore.retrieve<mmap::multi_polygon_t>(oo.handle);
+			auto const &input = osmStore.retrieve<mmap::multi_polygon_t>(oo.handle);
 
 			Polygon clippingPolygon;
 			geom::convert(bbox.clippingBox, clippingPolygon);
-			if (!geom::intersects(mp, clippingPolygon)) { return MultiPolygon(); }
+			if (!geom::intersects(input, clippingPolygon)) { return MultiPolygon(); }
 
+			MultiPolygon mp;
+			Polygon extendPolygon;
+			geom::convert(bbox.getExtendBox(), extendPolygon);
+			geom::intersection(input, extendPolygon, mp);
+			
 			MultiPolygon out;
 			for(auto const &p: mp) {
 				Polygon newp;
@@ -125,40 +130,49 @@ Geometry buildWayGeometry(OSMStore &osmStore, OutputObject const &oo, const Tile
 				}
 
 				std::size_t start_index = 0;
+				for(std::size_t j = 0; j < p.outer().size() - 1; ++j) {
+					if (geom::within(p.outer()[start_index], bbox.clippingBox)) {
+						start_index = j;
+						break;
+					}
+				} 
 
-				geom::append(newp.outer(), p.outer[start_index]);
-				for(std::size_t j = 1; j < p.outer.size() - 1; ++j) {
-					std::size_t i = (start_index + j) % p.outer.size();
-					std::size_t next_i = (i + 1) % p.outer.size();
-
-					 bool keep_point = 
-						geom::within(newp.outer().back(), bbox.clippingBox) || 
-					   	geom::within(p.outer[i], bbox.clippingBox) || 
-					   	geom::within(p.outer[next_i], bbox.clippingBox);
-
-					 if(!keep_point) {
+				geom::append(newp.outer(), p.outer()[start_index]);
+				geom::append(newp.outer(), p.outer()[start_index + 1]);
+				for(std::size_t j = 2; j < p.outer().size() - 1; ++j) {
+					std::size_t i = (start_index + j) % (p.outer().size() - 1);
+					auto prev_2 = newp.outer()[newp.outer().size() - 2];
+					auto prev_1 = newp.outer()[newp.outer().size() - 1];
+					
+					bool remove_point = 
+						!geom::within(prev_2, bbox.clippingBox) && !geom::within(prev_1, bbox.clippingBox) && !geom::within(p.outer()[i], bbox.clippingBox);
+
+				 	if(remove_point) {
+						// Check if 3 points are locally convex
 						double det = 
-							(p.outer[i].x() - newp.outer().back().x()) * (p.outer[next_i].y() * newp.outer().back().y()) - 
-							(p.outer[next_i].x() - newp.outer().back().x()) * (p.outer[i].y() * newp.outer().back().y());
-							
-						if(det < 0) { 
-							Linestring ls({ newp.outer().back(), p.outer[next_i] });
+							(p.outer()[i].x() - prev_2.x()) * (prev_1.y() - prev_2.y()) - 
+							(prev_1.x() - prev_2.x()) * (p.outer()[i].y() - prev_2.y());
+
+						if(det <= 0) { 
+							Linestring ls({ prev_2, p.outer()[i] });
 							bool intersects = geom::intersects(ls, bbox.clippingBox);
-							for(auto const &inner: p.inners) {
+							for(auto const &inner: p.inners()) {
 								intersects |= geom::intersects(ls, inner);
 							} 
 
-							if(!intersects) 
+							if(!intersects) {
+								newp.outer()[newp.outer().size() - 1] = p.outer()[i];
 								continue;
-						}
+							}
+						}	
 					} 
 
-					geom::append(newp.outer(), p.outer[i]);
-				} 
-				geom::append(newp.outer(), p.outer[start_index]);
+					geom::append(newp.outer(), p.outer()[i]); 
+				}
+				geom::append(newp.outer(), p.outer()[start_index]); 
 
-				newp.inners().reserve(p.inners.size());
-				for(auto const &inner: p.inners) {
+				newp.inners().reserve(p.inners().size());
+				for(auto const &inner: p.inners()) {
 					if(geom::within(inner, newp.outer())) { 
 						newp.inners().resize(newp.inners().size() + 1);
 						boost::geometry::assign(newp.inners().back(), inner);