Add method to QgsGeometry to extend the start/end of a linestring

python/core/geometry/qgsgeometry.sip

@@ -514,6 +514,14 @@ class QgsGeometry
                                    JoinStyle joinStyle = JoinStyleRound,
                                    double mitreLimit = 2.0 ) const;
 
+    /**
+     * Extends a (multi)line geometry by extrapolating out the start or end of the line
+     * by a specified distance. Lines are extended using the bearing of the first or last
+     * segment in the line.
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry extendLine( double startDistance, double endDistance ) const;
+
     /** Returns a simplified version of this geometry using a specified tolerance value */
     QgsGeometry simplify( double tolerance )  const;
 

python/core/geometry/qgslinestring.sip

@@ -92,6 +92,14 @@ class QgsLineString: public QgsCurve
         @return the converted geometry. Caller takes ownership*/
     QgsAbstractGeometry* toCurveType() const /Factory/;
 
+    /**
+     * Extends the line geometry by extrapolating out the start or end of the line
+     * by a specified distance. Lines are extended using the bearing of the first or last
+     * segment in the line.
+     * @note added in QGIS 3.0
+     */
+    void extend( double startDistance, double endDistance );
+
     //reimplemented methods
 
     virtual QString geometryType() const;

src/core/geometry/qgsgeometry.cpp

@@ -1452,6 +1452,45 @@ QgsGeometry QgsGeometry::singleSidedBuffer( double distance, int segments, Buffe
   }
 }
 
+QgsGeometry QgsGeometry::extendLine( double startDistance, double endDistance ) const
+{
+  if ( !d->geometry || type() != QgsWkbTypes::LineGeometry )
+  {
+    return QgsGeometry();
+  }
+
+  if ( QgsWkbTypes::isMultiType( d->geometry->wkbType() ) )
+  {
+    QList<QgsGeometry> parts = asGeometryCollection();
+    QList<QgsGeometry> results;
+    Q_FOREACH ( const QgsGeometry& part, parts )
+    {
+      QgsGeometry result = part.extendLine( startDistance, endDistance );
+      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
+  {
+    QgsLineString* line = dynamic_cast< QgsLineString* >( d->geometry );
+    if ( !line )
+      return QgsGeometry();
+
+    QgsLineString* newLine = line->clone();
+    newLine->extend( startDistance, endDistance );
+    return QgsGeometry( newLine );
+  }
+}
+
 QgsGeometry QgsGeometry::simplify( double tolerance ) const
 {
   if ( !d->geometry )

src/core/geometry/qgsgeometry.h

@@ -558,6 +558,14 @@ class CORE_EXPORT QgsGeometry
                                    JoinStyle joinStyle = JoinStyleRound,
                                    double mitreLimit = 2.0 ) const;
 
+    /**
+     * Extends a (multi)line geometry by extrapolating out the start or end of the line
+     * by a specified distance. Lines are extended using the bearing of the first or last
+     * segment in the line.
+     * @note added in QGIS 3.0
+     */
+    QgsGeometry extendLine( double startDistance, double endDistance ) const;
+
     //! Returns a simplified version of this geometry using a specified tolerance value
     QgsGeometry simplify( double tolerance ) const;
 

src/core/geometry/qgslinestring.cpp

@@ -583,6 +583,32 @@ QgsAbstractGeometry* QgsLineString::toCurveType() const
   return compoundCurve;
 }
 
+void QgsLineString::extend( double startDistance, double endDistance )
+{
+  if ( mX.size() < 2 || mY.size() < 2 )
+    return;
+
+  // start of line
+  if ( startDistance > 0 )
+  {
+    double currentLen = sqrt( qPow( mX.at( 0 ) - mX.at( 1 ), 2 ) +
+                              qPow( mY.at( 0 ) - mY.at( 1 ), 2 ) );
+    double newLen = currentLen + startDistance;
+    mX[ 0 ] = mX.at( 1 ) + ( mX.at( 0 ) - mX.at( 1 ) ) / currentLen * newLen;
+    mY[ 0 ] = mY.at( 1 ) + ( mY.at( 0 ) - mY.at( 1 ) ) / currentLen * newLen;
+  }
+  // end of line
+  if ( endDistance > 0 )
+  {
+    int last = mX.size() - 1;
+    double currentLen = sqrt( qPow( mX.at( last ) - mX.at( last - 1 ), 2 ) +
+                              qPow( mY.at( last ) - mY.at( last - 1 ), 2 ) );
+    double newLen = currentLen + endDistance;
+    mX[ last ] = mX.at( last - 1 ) + ( mX.at( last ) - mX.at( last - 1 ) ) / currentLen * newLen;
+    mY[ last ] = mY.at( last - 1 ) + ( mY.at( last ) - mY.at( last - 1 ) ) / currentLen * newLen;
+  }
+}
+
 /***************************************************************************
  * This class is considered CRITICAL and any change MUST be accompanied with
  * full unit tests.

src/core/geometry/qgslinestring.h

@@ -120,6 +120,14 @@ class CORE_EXPORT QgsLineString: public QgsCurve
         @return the converted geometry. Caller takes ownership*/
     QgsAbstractGeometry* toCurveType() const override;
 
+    /**
+     * Extends the line geometry by extrapolating out the start or end of the line
+     * by a specified distance. Lines are extended using the bearing of the first or last
+     * segment in the line.
+     * @note added in QGIS 3.0
+     */
+    void extend( double startDistance, double endDistance );
+
     //reimplemented methods
 
     virtual QString geometryType() const override { return QStringLiteral( "LineString" ); }

tests/src/core/testqgsgeometry.cpp

@@ -2208,6 +2208,16 @@ void TestQgsGeometry::lineString()
   QCOMPARE( static_cast< QgsPointV2*>( mpBoundary->geometryN( 1 ) )->x(), 1.0 );
   QCOMPARE( static_cast< QgsPointV2*>( mpBoundary->geometryN( 1 ) )->y(), 1.0 );
   QCOMPARE( static_cast< QgsPointV2*>( mpBoundary->geometryN( 1 ) )->z(), 20.0 );
+
+
+  //extend
+  QgsLineString extend1;
+  extend1.extend( 10, 10 ); //test no crash
+  extend1.setPoints( QList<QgsPointV2>() << QgsPointV2( 0, 0 ) << QgsPointV2( 1, 0 ) << QgsPointV2( 1, 1 ) );
+  extend1.extend( 1, 2 );
+  QCOMPARE( extend1.pointN( 0 ), QgsPointV2( QgsWkbTypes::Point, -1, 0 ) );
+  QCOMPARE( extend1.pointN( 1 ), QgsPointV2( QgsWkbTypes::Point, 1, 0 ) );
+  QCOMPARE( extend1.pointN( 2 ), QgsPointV2( QgsWkbTypes::Point, 1, 3 ) );
 }
 
 void TestQgsGeometry::polygon()

tests/src/python/test_qgsgeometry.py

@@ -3594,5 +3594,31 @@ class TestQgsGeometry(unittest.TestCase):
         self.assertAlmostEqual(polygon.angleAtVertex(3), math.radians(225.0), places=3)
         self.assertAlmostEqual(polygon.angleAtVertex(4), math.radians(135.0), places=3)
 
+    def testExtendLine(self):
+        """ test QgsGeometry.extendLine """
+
+        empty = QgsGeometry()
+        self.assertFalse(empty.extendLine(1, 2))
+
+        # not a linestring
+        point = QgsGeometry.fromWkt('Point(1 2)')
+        self.assertFalse(point.extendLine(1, 2))
+
+        # linestring
+        linestring = QgsGeometry.fromWkt('LineString(0 0, 1 0, 1 1)')
+        extended = linestring.extendLine(1, 2)
+        exp = 'LineString(-1 0, 1 0, 1 3)'
+        result = extended.exportToWkt()
+        self.assertTrue(compareWkt(result, exp, 0.00001),
+                        "Extend line: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
+        # multilinestring
+        multilinestring = QgsGeometry.fromWkt('MultiLineString((0 0, 1 0, 1 1),(11 11, 11 10, 10 10))')
+        extended = multilinestring.extendLine(1, 2)
+        exp = 'MultiLineString((-1 0, 1 0, 1 3),(11 12, 11 10, 8 10))'
+        result = extended.exportToWkt()
+        self.assertTrue(compareWkt(result, exp, 0.00001),
+                        "Extend line: mismatch Expected:\n{}\nGot:\n{}\n".format(exp, result))
+
 if __name__ == '__main__':
     unittest.main()