Add method to QgsCurvePolygon to force RHR, ensuring standard ring or…

…ientation

python/core/auto_generated/geometry/qgscurvepolygon.sip.in

@@ -136,6 +136,17 @@ For example, this removes unclosed rings and rings with less than 4 vertices.
 .. versionadded:: 3.0
 %End
 
+
+    void forceRHR();
+%Docstring
+Forces the geometry to respect the Right-Hand-Rule, in which the area that is
+bounded by the polygon is to the right of the boundary. In particular, the exterior
+ring is oriented in a clockwise direction and the interior rings in a counter-clockwise
+direction.
+
+.. versionadded:: 3.6
+%End
+
     virtual void draw( QPainter &p ) const;
 
     virtual void transform( const QgsCoordinateTransform &ct, QgsCoordinateTransform::TransformDirection d = QgsCoordinateTransform::ForwardTransform, bool transformZ = false ) throw( QgsCsException );

src/core/geometry/qgscurvepolygon.cpp

@@ -705,6 +705,33 @@ void QgsCurvePolygon::removeInvalidRings()
   mInteriorRings = validRings;
 }
 
+void QgsCurvePolygon::forceRHR()
+{
+  if ( mExteriorRing && mExteriorRing->orientation() != QgsCurve::Clockwise )
+  {
+    // flip exterior ring orientation
+    std::unique_ptr< QgsCurve > flipped( mExteriorRing->reversed() );
+    mExteriorRing = std::move( flipped );
+  }
+
+  QVector<QgsCurve *> validRings;
+  for ( QgsCurve *curve : qgis::as_const( mInteriorRings ) )
+  {
+    if ( curve && curve->orientation() != QgsCurve::CounterClockwise )
+    {
+      // flip interior ring orientation
+      QgsCurve *flipped = curve->reversed();
+      validRings << flipped;
+      delete curve;
+    }
+    else
+    {
+      validRings << curve;
+    }
+  }
+  mInteriorRings = validRings;
+}
+
 void QgsCurvePolygon::draw( QPainter &p ) const
 {
   if ( !mExteriorRing )

src/core/geometry/qgscurvepolygon.h

@@ -133,6 +133,17 @@ class CORE_EXPORT QgsCurvePolygon: public QgsSurface
      */
     void removeInvalidRings();
 
+
+    /**
+     * Forces the geometry to respect the Right-Hand-Rule, in which the area that is
+     * bounded by the polygon is to the right of the boundary. In particular, the exterior
+     * ring is oriented in a clockwise direction and the interior rings in a counter-clockwise
+     * direction.
+     *
+     * \since QGIS 3.6
+     */
+    void forceRHR();
+
     void draw( QPainter &p ) const override;
     void transform( const QgsCoordinateTransform &ct, QgsCoordinateTransform::TransformDirection d = QgsCoordinateTransform::ForwardTransform, bool transformZ = false ) override SIP_THROW( QgsCsException );
     void transform( const QTransform &t, double zTranslate = 0.0, double zScale = 1.0, double mTranslate = 0.0, double mScale = 1.0 ) override;

tests/src/core/testqgsgeometry.cpp

@@ -6596,6 +6596,25 @@ void TestQgsGeometry::polygon()
   invalidRingPolygon.addInteriorRing( removeInvalidPolygonRing.clone() );
   invalidRingPolygon.removeInvalidRings();
   QCOMPARE( invalidRingPolygon.asWkt( 2 ), QStringLiteral( "PolygonZM ((11 2 3 4, 4 12 13 14, 11 12 13 14, 11 22 23 24, 11 2 3 4),(1 2 5 6, 11.01 2.01 15 16, 11 2.01 25 26, 1 2 5 6))" ) );
+
+  // force RHR
+  QgsPolygon rhr;
+  rhr.forceRHR(); // no crash
+  rhr.fromWkt( QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4))" ) );
+  rhr.forceRHR();
+  QCOMPARE( rhr.asWkt( 2 ), QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4))" ) );
+  rhr.fromWkt( QStringLiteral( "PolygonZM ((0 0 3 4, 100 0 13 14, 100 100 23 24, 0 100 23 24, 0 0 3 4))" ) );
+  rhr.forceRHR();
+  QCOMPARE( rhr.asWkt( 2 ), QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 23 24, 100 100 23 24, 100 0 13 14, 0 0 3 4))" ) );
+  rhr.fromWkt( QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4),(10 10 1 2, 20 10 3 4, 20 20 4, 5, 10 20 6 8, 10 10 1 2))" ) );
+  rhr.forceRHR();
+  QCOMPARE( rhr.asWkt( 2 ), QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4),(10 10 1 2, 20 10 3 4, 10 20 6 8, 10 10 1 2))" ) );
+  rhr.fromWkt( QStringLiteral( "PolygonZM ((0 0 3 4, 100 0 13 14, 100 100 13 14, 0 100 23 24, 0 0 3 4),(10 10 1 2, 20 10 3 4, 20 20 4, 5, 10 20 6 8, 10 10 1 2))" ) );
+  rhr.forceRHR();
+  QCOMPARE( rhr.asWkt( 2 ), QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 23 24, 100 100 13 14, 100 0 13 14, 0 0 3 4),(10 10 1 2, 20 10 3 4, 10 20 6 8, 10 10 1 2))" ) );
+  rhr.fromWkt( QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4),(10 10 1 2, 10 20 3 4, 20 10 6 8, 10 10 1 2))" ) );
+  rhr.forceRHR();
+  QCOMPARE( rhr.asWkt( 2 ), QStringLiteral( "PolygonZM ((0 0 3 4, 0 100 13 14, 100 100 13 14, 100 0 23 24, 0 0 3 4),(10 10 1 2, 20 10 6 8, 10 20 3 4, 10 10 1 2))" ) );
 }
 
 void TestQgsGeometry::triangle()