[FEATURE] Expose GEOS single sided buffer through QgsGeometry

Makes it easy for PyQGIS code to perform a single sided buffer operation
qgis · Aug 16, 2016 · e3f0d3d · e3f0d3d
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Showing 7 changed files with 218 additions and 22 deletions.
diff --git a/python/core/geometry/qgsgeometry.sip b/python/core/geometry/qgsgeometry.sip
@@ -447,27 +447,67 @@ class QgsGeometry
      *  @note added in 1.5 */
     bool crosses( const QgsGeometry& geometry ) const;
 
+    //! Side of line to buffer
+    enum BufferSide
+    {
+      SideLeft, //!< Buffer to left of line
+      SideRight, //!< Buffer to right of line
+    };
+
+    //! End cap styles for buffers
+    enum EndCapStyle
+    {
+      CapRound, //!< Round cap
+      CapFlat, //!< Flat cap (in line with start/end of line)
+      CapSquare, //!< Square cap (extends past start/end of line by buffer distance)
+    };
+
+    //! Join styles for buffers
+    enum JoinStyle
+    {
+      JoinStyleRound, //!< Use rounded joins
+      JoinStyleMitre, //!< Use mitred joins
+      JoinStyleBevel, //!< Use beveled joins
+    };
+
     /** Returns a buffer region around this geometry having the given width and with a specified number
         of segments used to approximate curves */
     QgsGeometry buffer( double distance, int segments ) const;
 
     /** Returns a buffer region around the geometry, with additional style options.
      * @param distance    buffer distance
-     * @param segments    For round joins, number of segments to approximate quarter-circle
-     * @param endCapStyle Round (1) / Flat (2) / Square (3) end cap style
-     * @param joinStyle   Round (1) / Mitre (2) / Bevel (3) join style
-     * @param mitreLimit  Limit on the mitre ratio used for very sharp corners
+     * @param segments    for round joins, number of segments to approximate quarter-circle
+     * @param endCapStyle end cap style
+     * @param joinStyle   join style for corners in geometry
+     * @param mitreLimit  limit on the mitre ratio used for very sharp corners (JoinStyleMitre only)
      * @note added in 2.4
-     * @note needs GEOS >= 3.3 - otherwise always returns 0
      */
-    QgsGeometry buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit ) const;
+    QgsGeometry buffer( double distance, int segments, EndCapStyle endCapStyle, JoinStyle joinStyle, double mitreLimit ) const;
 
     /** Returns an offset line at a given distance and side from an input line.
-     * See buffer() method for details on parameters.
+     * @param distance    buffer distance
+     * @param segments    for round joins, number of segments to approximate quarter-circle
+     * @param joinStyle   join style for corners in geometry
+     * @param mitreLimit  limit on the mitre ratio used for very sharp corners (JoinStyleMitre only)
      * @note added in 2.4
-     * @note needs GEOS >= 3.3 - otherwise always returns 0
      */
-    QgsGeometry offsetCurve( double distance, int segments, int joinStyle, double mitreLimit ) const;
+    QgsGeometry offsetCurve( double distance, int segments, JoinStyle joinStyle, double mitreLimit ) const;
+
+    /**
+     * Returns a single sided buffer for a (multi)line geometry. The buffer is only
+     * applied to one side of the line.
+     * @param distance buffer distance
+     * @param segments for round joins, number of segments to approximate quarter-circle
+     * @param side side of geometry to buffer
+     * @param joinStyle join style for corners
+     * @param mitreLimit limit on the mitre ratio used for very sharp corners
+     * @return buffered geometry, or an empty geometry if buffer could not be
+     * calculated
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry singleSidedBuffer( double distance, int segments, BufferSide side,
+                                   JoinStyle joinStyle = JoinStyleRound,
+                                   double mitreLimit = 2.0 ) const;
 
     /** Returns a simplified version of this geometry using a specified tolerance value */
     QgsGeometry simplify( double tolerance )  const;

diff --git a/src/core/geometry/qgsgeometry.cpp b/src/core/geometry/qgsgeometry.cpp
@@ -1296,7 +1296,7 @@ QgsGeometry QgsGeometry::buffer( double distance, int segments ) const
   return QgsGeometry( geom );
 }
 
-QgsGeometry QgsGeometry::buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit ) const
+QgsGeometry QgsGeometry::buffer( double distance, int segments, EndCapStyle endCapStyle, JoinStyle joinStyle, double mitreLimit ) const
 {
   if ( !d->geometry )
   {
@@ -1312,7 +1312,7 @@ QgsGeometry QgsGeometry::buffer( double distance, int segments, int endCapStyle,
   return QgsGeometry( geom );
 }
 
-QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, int joinStyle, double mitreLimit ) const
+QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, JoinStyle joinStyle, double mitreLimit ) const
 {
   if ( !d->geometry )
   {
@@ -1351,6 +1351,46 @@ QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, int joinSty
   }
 }
 
+QgsGeometry QgsGeometry::singleSidedBuffer( double distance, int segments, BufferSide side , JoinStyle joinStyle, double mitreLimit ) const
+{
+  if ( !d->geometry )
+  {
+    return QgsGeometry();
+  }
+
+  if ( QgsWkbTypes::isMultiType( d->geometry->wkbType() ) )
+  {
+    QList<QgsGeometry> parts = asGeometryCollection();
+    QList<QgsGeometry> results;
+    Q_FOREACH ( const QgsGeometry& part, parts )
+    {
+      QgsGeometry result = part.singleSidedBuffer( distance, segments, side, joinStyle, mitreLimit );
+      if ( result )
+        results << result;
+    }
+    if ( results.isEmpty() )
+      return QgsGeometry();
+
+    QgsGeometry first = results.takeAt( 0 );
+    Q_FOREACH ( const QgsGeometry& result, results )
+    {
+      first.addPart( & result );
+    }
+    return first;
+  }
+  else
+  {
+    QgsGeos geos( d->geometry );
+    QgsAbstractGeometry* bufferGeom = geos.singleSidedBuffer( distance, segments, side,
+                                      joinStyle, mitreLimit );
+    if ( !bufferGeom )
+    {
+      return QgsGeometry();
+    }
+    return QgsGeometry( bufferGeom );
+  }
+}
+
 QgsGeometry QgsGeometry::simplify( double tolerance ) const
 {
   if ( !d->geometry )

diff --git a/src/core/geometry/qgsgeometry.h b/src/core/geometry/qgsgeometry.h
@@ -484,27 +484,67 @@ class CORE_EXPORT QgsGeometry
      *  @note added in 1.5 */
     bool crosses( const QgsGeometry& geometry ) const;
 
+    //! Side of line to buffer
+    enum BufferSide
+    {
+      SideLeft = 0, //!< Buffer to left of line
+      SideRight, //!< Buffer to right of line
+    };
+
+    //! End cap styles for buffers
+    enum EndCapStyle
+    {
+      CapRound = 1, //!< Round cap
+      CapFlat, //!< Flat cap (in line with start/end of line)
+      CapSquare, //!< Square cap (extends past start/end of line by buffer distance)
+    };
+
+    //! Join styles for buffers
+    enum JoinStyle
+    {
+      JoinStyleRound = 1, //!< Use rounded joins
+      JoinStyleMitre, //!< Use mitred joins
+      JoinStyleBevel, //!< Use beveled joins
+    };
+
     /** Returns a buffer region around this geometry having the given width and with a specified number
         of segments used to approximate curves */
     QgsGeometry buffer( double distance, int segments ) const;
 
     /** Returns a buffer region around the geometry, with additional style options.
      * @param distance    buffer distance
-     * @param segments    For round joins, number of segments to approximate quarter-circle
-     * @param endCapStyle Round (1) / Flat (2) / Square (3) end cap style
-     * @param joinStyle   Round (1) / Mitre (2) / Bevel (3) join style
-     * @param mitreLimit  Limit on the mitre ratio used for very sharp corners
+     * @param segments    for round joins, number of segments to approximate quarter-circle
+     * @param endCapStyle end cap style
+     * @param joinStyle   join style for corners in geometry
+     * @param mitreLimit  limit on the mitre ratio used for very sharp corners (JoinStyleMitre only)
      * @note added in 2.4
-     * @note needs GEOS >= 3.3 - otherwise always returns 0
      */
-    QgsGeometry buffer( double distance, int segments, int endCapStyle, int joinStyle, double mitreLimit ) const;
+    QgsGeometry buffer( double distance, int segments, EndCapStyle endCapStyle, JoinStyle joinStyle, double mitreLimit ) const;
 
     /** Returns an offset line at a given distance and side from an input line.
-     * See buffer() method for details on parameters.
+     * @param distance    buffer distance
+     * @param segments    for round joins, number of segments to approximate quarter-circle
+     * @param joinStyle   join style for corners in geometry
+     * @param mitreLimit  limit on the mitre ratio used for very sharp corners (JoinStyleMitre only)
      * @note added in 2.4
-     * @note needs GEOS >= 3.3 - otherwise always returns 0
      */
-    QgsGeometry offsetCurve( double distance, int segments, int joinStyle, double mitreLimit ) const;
+    QgsGeometry offsetCurve( double distance, int segments, JoinStyle joinStyle, double mitreLimit ) const;
+
+    /**
+     * Returns a single sided buffer for a (multi)line geometry. The buffer is only
+     * applied to one side of the line.
+     * @param distance buffer distance
+     * @param segments for round joins, number of segments to approximate quarter-circle
+     * @param side side of geometry to buffer
+     * @param joinStyle join style for corners
+     * @param mitreLimit limit on the mitre ratio used for very sharp corners
+     * @return buffered geometry, or an empty geometry if buffer could not be
+     * calculated
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry singleSidedBuffer( double distance, int segments, BufferSide side,
+                                   JoinStyle joinStyle = JoinStyleRound,
+                                   double mitreLimit = 2.0 ) const;
 
     /** Returns a simplified version of this geometry using a specified tolerance value */
     QgsGeometry simplify( double tolerance ) const;

diff --git a/src/core/geometry/qgsgeos.cpp b/src/core/geometry/qgsgeos.cpp
@@ -1679,6 +1679,33 @@ QgsAbstractGeometry* QgsGeos::offsetCurve( double distance, int segments, int jo
   return offsetGeom;
 }
 
+QgsAbstractGeometry* QgsGeos::singleSidedBuffer( double distance, int segments, int side, int joinStyle, double mitreLimit, QString* errorMsg ) const
+{
+  if ( !mGeos )
+  {
+    return nullptr;
+  }
+
+  GEOSGeomScopedPtr geos;
+  try
+  {
+    GEOSBufferParams* bp  = GEOSBufferParams_create_r( geosinit.ctxt );
+    GEOSBufferParams_setSingleSided_r( geosinit.ctxt, bp, 1 );
+    GEOSBufferParams_setQuadrantSegments_r( geosinit.ctxt, bp, segments );
+    GEOSBufferParams_setJoinStyle_r( geosinit.ctxt, bp, joinStyle );
+    GEOSBufferParams_setMitreLimit_r( geosinit.ctxt, bp, mitreLimit );
+
+    if ( side == 1 )
+    {
+      distance = -distance;
+    }
+    geos.reset( GEOSBufferWithParams_r( geosinit.ctxt, mGeos, bp, distance ) );
+    GEOSBufferParams_destroy_r( geosinit.ctxt, bp );
+  }
+  CATCH_GEOS_WITH_ERRMSG( nullptr );
+  return fromGeos( geos.get() );
+}
+
 QgsAbstractGeometry* QgsGeos::reshapeGeometry( const QgsLineString& reshapeWithLine, int* errorCode, QString* errorMsg ) const
 {
   if ( !mGeos || reshapeWithLine.numPoints() < 2 || mGeometry->dimension() == 0 )

diff --git a/src/core/geometry/qgsgeos.h b/src/core/geometry/qgsgeos.h
@@ -85,6 +85,24 @@ class CORE_EXPORT QgsGeos: public QgsGeometryEngine
                        QString* errorMsg = nullptr ) const override;
 
     QgsAbstractGeometry* offsetCurve( double distance, int segments, int joinStyle, double mitreLimit, QString* errorMsg = nullptr ) const override;
+
+    /**
+     * Returns a single sided buffer for a geometry. The buffer is only
+     * applied to one side of the geometry.
+     * @param distance buffer distance
+     * @param segments for round joins, number of segments to approximate quarter-circle
+     * @param side side of geometry to buffer (0 = left, 1 = right)
+     * @param joinStyle join style for corners ( Round (1) / Mitre (2) / Bevel (3) )
+     * @param mitreLimit limit on the mitre ratio used for very sharp corners
+     * @return buffered geometry, or an nullptr if buffer could not be
+     * calculated
+     * @note added in QGIS 3.0
+     */
+    QgsAbstractGeometry* singleSidedBuffer( double distance, int segments, int side,
+                                            int joinStyle, double mitreLimit,
+                                            QString* errorMsg = nullptr ) const;
+
+
     QgsAbstractGeometry* reshapeGeometry( const QgsLineString& reshapeWithLine, int* errorCode, QString* errorMsg = nullptr ) const;
 
     /** Merges any connected lines in a LineString/MultiLineString geometry and

diff --git a/src/core/symbology-ng/qgssymbollayerutils.cpp b/src/core/symbology-ng/qgssymbollayerutils.cpp
@@ -718,9 +718,10 @@ QList<QPolygonF> offsetLine( QPolygonF polyline, double dist, QgsWkbTypes::Geome
     double mitreLimit = 2.0; // the default value in GEOS (5.0) allows for fairly sharp endings
     QgsGeometry offsetGeom;
     if ( geometryType == QgsWkbTypes::PolygonGeometry )
-      offsetGeom = tempGeometry.buffer( -dist, quadSegments, GEOSBUF_CAP_FLAT, GEOSBUF_JOIN_MITRE, mitreLimit );
+      offsetGeom = tempGeometry.buffer( -dist, quadSegments, QgsGeometry::CapFlat,
+                                        QgsGeometry::JoinStyleMitre, mitreLimit );
     else
-      offsetGeom = tempGeometry.offsetCurve( dist, quadSegments, GEOSBUF_JOIN_MITRE, mitreLimit );
+      offsetGeom = tempGeometry.offsetCurve( dist, quadSegments, QgsGeometry::JoinStyleMitre, mitreLimit );
 
     if ( !offsetGeom.isEmpty() )
     {

diff --git a/tests/src/python/test_qgsgeometry.py b/tests/src/python/test_qgsgeometry.py
@@ -3314,6 +3314,36 @@ def testDeleteVertexCurvePolygon(self):
         expected_wkt = "CurvePolygon (CompoundCurve (CircularString (0 0, 1 1, 2 0),(2 0, 0 0)))"
         self.assertEqual(geom.exportToWkt(), QgsGeometry.fromWkt(expected_wkt).exportToWkt())
 
+    def testSingleSidedBuffer(self):
+
+        wkt = "LineString( 0 0, 10 0)"
+        geom = QgsGeometry.fromWkt(wkt)
+        out = geom.singleSidedBuffer(1, 8, QgsGeometry.SideLeft)
+        result = out.exportToWkt()
+        expected_wkt = "Polygon ((10 0, 0 0, 0 1, 10 1, 10 0))"
+        self.assertTrue(compareWkt(result, expected_wkt, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(expected_wkt, result))
+
+        wkt = "LineString( 0 0, 10 0)"
+        geom = QgsGeometry.fromWkt(wkt)
+        out = geom.singleSidedBuffer(1, 8, QgsGeometry.SideRight)
+        result = out.exportToWkt()
+        expected_wkt = "Polygon ((0 0, 10 0, 10 -1, 0 -1, 0 0))"
+        self.assertTrue(compareWkt(result, expected_wkt, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(expected_wkt, result))
+
+        wkt = "LineString( 0 0, 10 0, 10 10)"
+        geom = QgsGeometry.fromWkt(wkt)
+        out = geom.singleSidedBuffer(1, 8, QgsGeometry.SideRight, QgsGeometry.JoinStyleMitre)
+        result = out.exportToWkt()
+        expected_wkt = "Polygon ((0 0, 10 0, 10 10, 11 10, 11 -1, 0 -1, 0 0))"
+        self.assertTrue(compareWkt(result, expected_wkt, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(expected_wkt, result))
+
+        wkt = "LineString( 0 0, 10 0, 10 10)"
+        geom = QgsGeometry.fromWkt(wkt)
+        out = geom.singleSidedBuffer(1, 8, QgsGeometry.SideRight, QgsGeometry.JoinStyleBevel)
+        result = out.exportToWkt()
+        expected_wkt = "Polygon ((0 0, 10 0, 10 10, 11 10, 11 0, 10 -1, 0 -1, 0 0))"
+        self.assertTrue(compareWkt(result, expected_wkt, 0.00001), "Merge lines: mismatch Expected:\n{}\nGot:\n{}\n".format(expected_wkt, result))
+
     def testMisc(self):
 
         # Test that we cannot add a CurvePolygon in a MultiPolygon