mapnik::util::is_clockwise - translate coordinates relative to the or…

…igin (0,0) to avoid numeric precision issues while using double precision. (ref #3402)
mapnik · Apr 18, 2016 · 66160e9 · springmeyer · Apr 18, 2016 · flippmoke
1 parent c8902ac
commit 66160e9
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diff --git a/include/mapnik/util/is_clockwise.hpp b/include/mapnik/util/is_clockwise.hpp
@@ -23,18 +23,27 @@
 #ifndef MAPNIK_UTIL_IS_CLOCKWISE_HPP
 #define MAPNIK_UTIL_IS_CLOCKWISE_HPP
 
+#include <cassert>
+
 namespace mapnik { namespace util {
 
 template <typename T>
 bool is_clockwise(T const& ring)
 {
     double area = 0.0;
     std::size_t num_points = ring.size();
+    assert(num_point > 2);
+    double orig_x = ring[0].x;
+    double orig_y = ring[0].y;
     for (std::size_t i = 0; i < num_points; ++i)
     {
         auto const& p0 = ring[i];
         auto const& p1 = ring[(i + 1) % num_points];
-        area += p0.x * p1.y - p0.y * p1.x;
+        double x0 = p0.x - orig_x;
+        double y0 = p0.y - orig_y;
+        double x1 = p1.x - orig_x;
+        double y1 = p1.y - orig_y;
+        area += x0 * y1 - x1 * y0;
     }
     return (area < 0.0) ? true : false;
 }

diff --git a/test/unit/geometry/is_clockwise.cpp b/test/unit/geometry/is_clockwise.cpp
@@ -0,0 +1,27 @@
+#include "catch.hpp"
+
+#include <mapnik/geometry.hpp>
+#include <mapnik/util/is_clockwise.hpp>
+
+TEST_CASE("Ring is_clockwise") {
+
+    // Input is rather thin triangle to test precision issues aren't getting in the way.
+    SECTION("Clockwise")
+    {
+        mapnik::geometry::linear_ring<double> ring;
+        ring.emplace_back(-13499697.0366658326, 4698431.85179749783);
+        ring.emplace_back(-13499697.1113113686, 4698431.85179749783);
+        ring.emplace_back(-13499697.0366658326, 4698431.92644303292);
+        ring.emplace_back(-13499697.0366658326, 4698431.85179749783);
+        REQUIRE(mapnik::util::is_clockwise(ring) == true);
+    }
+    SECTION("Anti-Clockwise")
+    {
+        mapnik::geometry::linear_ring<double> ring;
+        ring.emplace_back(-13499697.0366658326, 4698431.85179749783);
+        ring.emplace_back(-13499697.0366658326, 4698431.92644303292);
+        ring.emplace_back(-13499697.1113113686, 4698431.85179749783);
+        ring.emplace_back(-13499697.0366658326, 4698431.85179749783);
+        REQUIRE(mapnik::util::is_clockwise(ring) == false);
+    }
+}