Grow coverage polygons by the simplification threshold

This ensures that the result actually contains the original polygon in
all cases and we aren't creating gaps along the border.

CMakeLists.txt

@@ -41,7 +41,9 @@ endif()
 
 find_package(ZLIB REQUIRED)
 find_package(OsmTools)
-set_package_properties(OsmTools PROPERTIES TYPE OPTIONAL PURPOSE "Needed only for regenereating line metadata tables (ie. you most likely don't need this)")
+set_package_properties(OsmTools PROPERTIES TYPE OPTIONAL PURPOSE "Needed only for regenereating line metadata tables (you don't need this for distribution)")
+find_package(PolyClipping)
+set_package_properties(PolyClipping PROPERTIES TYPE OPTIONAL PURPOSE "Needed only for processing coverage area data from the Transport API Repository (you don't need this for distribution)")
 
 if (NOT WIN32)
     find_package(Protobuf)

cmake/FindPolyClipping.cmake

+# SPDX-FileCopyrightText: 2021 Volker Krause <vkrause@kde.org>
+# SPDX-License-Identifier: BSD-3-Clause
+
+find_package(PkgConfig QUIET)
+pkg_check_modules(PolyClipping polyclipping QUIET)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(PolyClipping
+    FOUND_VAR
+        PolyClipping_FOUND
+    REQUIRED_VARS
+        PolyClipping_LIBRARIES
+        PolyClipping_INCLUDE_DIRS
+    VERSION_VAR
+        PolyClipping_VERSION
+)
+
+if (PolyClipping_FOUND AND NOT TARGET PolyClipping::PolyClipping)
+    add_library(PolyClipping::PolyClipping UNKNOWN IMPORTED)
+    set_target_properties(PolyClipping::PolyClipping PROPERTIES
+        IMPORTED_LOCATION "${PolyClipping_LINK_LIBRARIES}"
+        INTERFACE_COMPILE_OPTIONS "${PolyClipping_CFLAGS}"
+        INTERFACE_INCLUDE_DIRECTORIES "${PolyClipping_INCLUDE_DIRS}"
+    )
+endif()
+
+mark_as_advanced(PolyClipping_LIBRARIES PolyClipping_INCLUDE_DIRS PolyClipping_VERSION)
+
+include(FeatureSummary)
+set_package_properties(PolyClipping PROPERTIES
+    DESCRIPTION "A library for clipping and offsetting lines and polygons"
+    URL "http://www.angusj.com/delphi/clipper.php"
+)

src/tools/CMakeLists.txt

@@ -16,10 +16,12 @@ add_executable(gbfsprobe gbfsprobe.cpp)
 target_link_libraries(gbfsprobe Qt5::Network KPublicTransport)
 add_custom_target(run-gbfs-probe COMMAND gbfsprobe ${CMAKE_SOURCE_DIR}/src/lib/gbfs/gbfs-feeds.json)
 
-add_executable(transport-api-sync
-    transport-api-sync.cpp
-    polygonsimplifier.cpp
-    ../lib/geo/geojson.cpp
-)
-target_link_libraries(transport-api-sync Qt5::Gui KOSM)
-add_custom_target(run-transport-api-sync COMMAND transport-api-sync --config-path ${CMAKE_SOURCE_DIR}/src/lib/networks --transport-apis ${CMAKE_CURRENT_BINARY_DIR}/transport-apis)
+if (TARGET PolyClipping::PolyClipping)
+    add_executable(transport-api-sync
+        transport-api-sync.cpp
+        polygonsimplifier.cpp
+        ../lib/geo/geojson.cpp
+    )
+    target_link_libraries(transport-api-sync Qt5::Gui KOSM PolyClipping::PolyClipping)
+    add_custom_target(run-transport-api-sync COMMAND transport-api-sync --config-path ${CMAKE_SOURCE_DIR}/src/lib/networks --transport-apis ${CMAKE_CURRENT_BINARY_DIR}/transport-apis)
+endif()

src/tools/polygonsimplifier.cpp

@@ -11,6 +11,8 @@
 #include <QDebug>
 #include <QPolygonF>
 
+#include <clipper.hpp>
+
 static QPolygonF douglasPeucker(const QPolygonF::const_iterator &begin, const QPolygonF::const_iterator &end, double threshold)
 {
     QPolygonF result;
@@ -54,3 +56,44 @@ QPolygonF PolygonSimplifier::douglasPeucker(const QPolygonF &poly, double thresh
 
     return result;
 }
+
+static ClipperLib::Path qPolygonToClipperPath(const QPolygonF &poly)
+{
+    ClipperLib::Path result;
+    result.reserve(poly.size());
+    std::transform(poly.begin(), poly.end(), std::back_inserter(result), [](auto p) {
+        return ClipperLib::IntPoint(p.x() * 10'000'000.0, p.y() * 10'000'000);
+    });
+    return result;
+}
+
+static QPolygonF clipperPathToQPolygon(const ClipperLib::Path &path)
+{
+    QPolygonF result;
+    result.reserve(path.size() + 1);
+    std::transform(path.begin(), path.end(), std::back_inserter(result), [](auto p) {
+        return QPointF(p.X / 10'000'000.0, p.Y / 10'000'000.0);
+    });
+    if (!result.empty() && !result.isClosed()) {
+        result.push_back(result.front());
+    }
+    return result;
+}
+
+QPolygonF PolygonSimplifier::offset(const QPolygonF &poly, double distance)
+{
+    if (poly.empty()) {
+        return {};
+    }
+
+    // convert meter distance to 100 nano-degree, the unit used by the polygon offset algorithm
+    const auto bbox = poly.boundingRect();
+    const auto bboxWidth = OSM::distance(bbox.center().y(), bbox.left(), bbox.center().y(), bbox.right());
+    const auto delta = bbox.width() / bboxWidth * distance * 10'000'000.0;
+
+    ClipperLib::ClipperOffset co;
+    co.AddPath(qPolygonToClipperPath(poly), ClipperLib::jtMiter, ClipperLib::etClosedPolygon);
+    ClipperLib::Paths result;
+    co.Execute(result, delta);
+    return clipperPathToQPolygon(result[0]);
+}

src/tools/polygonsimplifier.h

@@ -17,6 +17,9 @@ namespace PolygonSimplifier
      *  @see https://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm
      */
     QPolygonF douglasPeucker(const QPolygonF &poly, double distance);
+
+    /** Offset ("grow") a polygon by @p distance in meters. */
+    QPolygonF offset(const QPolygonF &poly, double distance);
 }
 
 #endif // POLYGONSIMPLIFIER_H

src/tools/transport-api-sync.cpp

@@ -122,7 +122,12 @@ static void preProcessCoverage(QJsonObject &obj)
     using namespace KPublicTransport;
     auto polys = GeoJson::readOuterPolygons(obj.take(QLatin1String("area")).toObject());
     for (auto &poly : polys) {
+        const auto originalPolySize = poly.size();
         poly = PolygonSimplifier::douglasPeucker(poly, 10'000.0);
+        if (originalPolySize > poly.size()) {
+            // only apply offsetting if we actually simplified the polygon
+            poly = PolygonSimplifier::offset(poly, 10'000.0);
+        }
     }
     if (!polys.empty()) {
         obj.insert(QLatin1String("area"), GeoJson::writePolygons(polys));