Improve OffsetCurve to handle mitre joins for polygons

libgeos · Jan 7, 2025 · f997d40 · f997d40
1 parent e1c9a7c
commit f997d40
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Showing 3 changed files with 38 additions and 10 deletions.
diff --git a/include/geos/operation/buffer/OffsetCurve.h b/include/geos/operation/buffer/OffsetCurve.h
@@ -101,6 +101,9 @@ class GEOS_DLL OffsetCurve {
 
     // Methods
 
+    std::unique_ptr<Geometry> computePolygonCurve(
+        const Polygon& polyGeom, double distance);
+
     std::unique_ptr<Geometry> computeCurve(
         const LineString& lineGeom, double distance);
 

diff --git a/src/operation/buffer/OffsetCurve.cpp b/src/operation/buffer/OffsetCurve.cpp
@@ -66,15 +66,7 @@ OffsetCurve::getCurve()
 
         if (geom.getGeometryTypeId() == GEOS_POINT) return nullptr;
         if (geom.getGeometryTypeId() == GEOS_POLYGON) {
-            auto boundary = geom.buffer(distance)->getBoundary();
-
-            if (boundary->getGeometryTypeId() == GEOS_LINEARRING) {
-                const LinearRing& ring = static_cast<const LinearRing&>(geom);
-                auto ringCs = ring.getCoordinatesRO();
-                std::unique_ptr<Geometry> ls(geom.getFactory()->createLineString(*ringCs));
-                return ls;
-            }
-            return boundary;
+            return computePolygonCurve(static_cast<const Polygon&>(geom), distance);
         }
         return computeCurve(static_cast<const LineString&>(geom), distance);
     };
@@ -141,6 +133,22 @@ OffsetCurve::rawOffset(const LineString& line, double dist)
     return rawOffsetCurve(line, dist, bufParams);
 }
 
+/* private */
+std::unique_ptr<Geometry>
+OffsetCurve::computePolygonCurve(const Polygon& polyGeom, double dist)
+{
+    auto buffer = BufferOp::bufferOp(&polyGeom, dist, bufferParams);
+    std::unique_ptr<Geometry> boundary = buffer->getBoundary();
+    //-- convert LinearRing to LineString if needed
+    if (boundary->getGeometryTypeId() == GEOS_LINEARRING) {
+        const LinearRing& ring = static_cast<LinearRing&>(*boundary);
+        auto ringCs = ring.getCoordinatesRO();
+        std::unique_ptr<Geometry> ls(polyGeom.getFactory()->createLineString(*ringCs));
+        return ls;
+    }
+    return boundary;
+}
+
 /* private */
 std::unique_ptr<Geometry>
 OffsetCurve::computeCurve(const LineString& lineGeom, double dist)

diff --git a/tests/unit/operation/buffer/OffsetCurveTest.cpp b/tests/unit/operation/buffer/OffsetCurveTest.cpp
@@ -588,6 +588,23 @@ void object::test<43> ()
     );
 }
 
-
+// testPolygonJoinMitre
+template<>
+template<>
+void object::test<44> ()
+{
+    checkOffsetCurve(
+        "POLYGON ((1 1, 1 7, 5 4, 8 8, 8 1, 1 1))",
+        1, 0, BufferParameters::JOIN_MITRE, 5,
+        "LINESTRING (0 0, 0 9, 4.8 5.4, 9 11, 9 0, 0 0)",
+        0.0001
+    );
+    checkOffsetCurve(
+        "POLYGON ((1 1, 1 7, 5 4, 8 8, 8 1, 1 1))",
+        -1, 0, BufferParameters::JOIN_MITRE, 5,
+        "LINESTRING (2 2, 2 5, 5.2 2.6, 7 5, 7 2, 2 2)",
+        0.0001
+    );
+}
 
 } // namespace tut2