[FEATURE] Expression functions for offset_curve and single_sided_buffer

Especially useful with geometry generators!

Author: nyalldawson

resources/function_help/json/offset_curve

@@ -0,0 +1,15 @@
+{
+  "name": "offset_curve",
+  "type": "function",
+  "description": "Returns a geometry formed by offseting a linestring geometry to the side. Distances are in the Spatial Reference System of this geometry.",
+  "arguments": [ {"arg":"geometry","description":"a (multi)linestring geometry"},
+                 {"arg":"distance","description":"offset distance. Positive values will be buffered to the left of lines, negative values to the right"},
+                 {"arg":"segments","optional":true,"default":"8","description":"number of segments to use to represent a quarter circle when a round join style is used. A larger number results in a smoother line with more nodes."},
+                 {"arg":"join","optional":true,"default":"1","description":"join style for corners, where 1 = round, 2 = mitre and 3 = bevel"},
+                 {"arg":"mitre_limit","optional":true,"default":"2.0","description":"limit on the mitre ratio used for very sharp corners (when using mitre joins only)"}],
+  "examples": [ { "expression":"offset_curve($geometry, 10.5)", "returns":"line offset to the left by 10.5 units"},
+  { "expression":"offset_curve($geometry, -10.5)", "returns":"line offset to the right by 10.5 units"},
+  { "expression":"offset_curve($geometry, 10.5, segments=16, join=1)", "returns":"line offset to the left by 10.5 units, using more segments to result in a smoother curve"},
+  { "expression":"offset_curve($geometry, 10.5, join=3)", "returns":"line offset to the left by 10.5 units, using a beveled join"}]
+}
+

resources/function_help/json/single_sided_buffer

@@ -0,0 +1,15 @@
+{
+  "name": "single_sided_buffer",
+  "type": "function",
+  "description": "Returns a geometry formed by buffering out just one side of a linestring geometry. Distances are in the Spatial Reference System of this geometry.",
+  "arguments": [ {"arg":"geometry","description":"a (multi)linestring geometry"},
+                 {"arg":"distance","description":"buffer distance. Positive values will be buffered to the left of lines, negative values to the right"},
+                 {"arg":"segments","optional":true,"default":"8","description":"number of segments to use to represent a quarter circle when a round join style is used. A larger number results in a smoother buffer with more nodes."},
+                 {"arg":"join","optional":true,"default":"1","description":"join style for corners, where 1 = round, 2 = mitre and 3 = bevel"},
+                 {"arg":"mitre_limit","optional":true,"default":"2.0","description":"limit on the mitre ratio used for very sharp corners (when using mitre joins only)"}],
+  "examples": [ { "expression":"single_sided_buffer($geometry, 10.5)", "returns":"line buffered to the left by 10.5 units"},
+  { "expression":"single_sided_buffer($geometry, -10.5)", "returns":"line buffered to the right by 10.5 units"},
+  { "expression":"single_sided_buffer($geometry, 10.5, segments=16, join=1)", "returns":"line buffered to the left by 10.5 units, using more segments to result in a smoother buffer"},
+  { "expression":"single_sided_buffer($geometry, 10.5, join=3)", "returns":"line buffered to the left by 10.5 units, using a beveled join"}]
+}
+

src/core/geometry/qgsgeometry.cpp

@@ -1314,7 +1314,7 @@ QgsGeometry QgsGeometry::buffer( double distance, int segments, EndCapStyle endC
 
 QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, JoinStyle joinStyle, double mitreLimit ) const
 {
-  if ( !d->geometry )
+  if ( !d->geometry || type() != QgsWkbTypes::LineGeometry )
   {
     return QgsGeometry();
   }
@@ -1353,7 +1353,7 @@ QgsGeometry QgsGeometry::offsetCurve( double distance, int segments, JoinStyle j
 
 QgsGeometry QgsGeometry::singleSidedBuffer( double distance, int segments, BufferSide side , JoinStyle joinStyle, double mitreLimit ) const
 {
-  if ( !d->geometry )
+  if ( !d->geometry || type() != QgsWkbTypes::LineGeometry )
   {
     return QgsGeometry();
   }

src/core/qgsexpression.cpp

@@ -2212,6 +2212,37 @@ static QVariant fcnBuffer( const QVariantList& values, const QgsExpressionContex
   QVariant result = !geom.isEmpty() ? QVariant::fromValue( geom ) : QVariant();
   return result;
 }
+
+static QVariant fcnOffsetCurve( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry fGeom = getGeometry( values.at( 0 ), parent );
+  double dist = getDoubleValue( values.at( 1 ), parent );
+  int segments = getIntValue( values.at( 2 ), parent );
+  QgsGeometry::JoinStyle join = static_cast< QgsGeometry::JoinStyle >( getIntValue( values.at( 3 ), parent ) );
+  if ( join < QgsGeometry::JoinStyleRound || join > QgsGeometry::JoinStyleBevel )
+    return QVariant();
+  double mitreLimit = getDoubleValue( values.at( 3 ), parent );
+
+  QgsGeometry geom = fGeom.offsetCurve( dist, segments, join, mitreLimit );
+  QVariant result = !geom.isEmpty() ? QVariant::fromValue( geom ) : QVariant();
+  return result;
+}
+
+static QVariant fcnSingleSidedBuffer( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
+{
+  QgsGeometry fGeom = getGeometry( values.at( 0 ), parent );
+  double dist = getDoubleValue( values.at( 1 ), parent );
+  int segments = getIntValue( values.at( 2 ), parent );
+  QgsGeometry::JoinStyle join = static_cast< QgsGeometry::JoinStyle >( getIntValue( values.at( 3 ), parent ) );
+  if ( join < QgsGeometry::JoinStyleRound || join > QgsGeometry::JoinStyleBevel )
+    return QVariant();
+  double mitreLimit = getDoubleValue( values.at( 3 ), parent );
+
+  QgsGeometry geom = fGeom.singleSidedBuffer( dist, segments, QgsGeometry::SideLeft, join, mitreLimit );
+  QVariant result = !geom.isEmpty() ? QVariant::fromValue( geom ) : QVariant();
+  return result;
+}
+
 static QVariant fcnTranslate( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   QgsGeometry fGeom = getGeometry( values.at( 0 ), parent );
@@ -3084,7 +3115,8 @@ const QStringList& QgsExpression::BuiltinFunctions()
     << "geom_from_gml" << "geomFromGML" << "intersects_bbox" << "bbox"
     << "disjoint" << "intersects" << "touches" << "crosses" << "contains"
     << "relate"
-    << "overlaps" << "within" << "buffer" << "centroid" << "bounds" << "reverse" << "exterior_ring"
+    << "overlaps" << "within" << "buffer" << "offset_curve" << "single_sided_buffer"
+    << "centroid" << "bounds" << "reverse" << "exterior_ring"
     << "boundary" << "line_merge"
     << "bounds_width" << "bounds_height" << "is_closed" << "convex_hull" << "difference"
     << "distance" << "intersection" << "sym_difference" << "combine"
@@ -3271,6 +3303,18 @@ const QList<QgsExpression::Function*>& QgsExpression::Functions()
     << new StaticFunction( "within", 2, fcnWithin, "GeometryGroup" )
     << new StaticFunction( "translate", 3, fcnTranslate, "GeometryGroup" )
     << new StaticFunction( "buffer", -1, fcnBuffer, "GeometryGroup" )
+    << new StaticFunction( "offset_curve", ParameterList() << Parameter( "geometry" )
+                           << Parameter( "distance" )
+                           << Parameter( "segments", true, 8.0 )
+                           << Parameter( "join", true, QgsGeometry::JoinStyleRound )
+                           << Parameter( "mitre_limit", true, 2.0 ),
+                           fcnOffsetCurve, "GeometryGroup" )
+    << new StaticFunction( "single_sided_buffer", ParameterList() << Parameter( "geometry" )
+                           << Parameter( "distance" )
+                           << Parameter( "segments", true, 8.0 )
+                           << Parameter( "join", true, QgsGeometry::JoinStyleRound )
+                           << Parameter( "mitre_limit", true, 2.0 ),
+                           fcnSingleSidedBuffer, "GeometryGroup" )
     << new StaticFunction( "centroid", 1, fcnCentroid, "GeometryGroup" )
     << new StaticFunction( "point_on_surface", 1, fcnPointOnSurface, "GeometryGroup" )
     << new StaticFunction( "reverse", 1, fcnReverse, "GeometryGroup" )

tests/src/core/testqgsexpression.cpp

@@ -677,6 +677,18 @@ class TestQgsExpression: public QObject
       QTest::newRow( "line_merge point" ) << "line_merge(geom_from_wkt('POINT(1 2)'))" << false << QVariant();
       QTest::newRow( "line_merge line" ) << "geom_to_wkt(line_merge(geometry:=geom_from_wkt('LineString(0 0, 10 10)')))" << false << QVariant( "LineString (0 0, 10 10)" );
       QTest::newRow( "line_merge multiline" ) << "geom_to_wkt(line_merge(geom_from_wkt('MultiLineString((0 0, 10 10),(10 10, 20 20))')))" << false << QVariant( "LineString (0 0, 10 10, 20 20)" );
+      QTest::newRow( "offset_curve not geom" ) << "offset_curve('g', 5)" << true << QVariant();
+      QTest::newRow( "offset_curve null" ) << "offset_curve(NULL, 5)" << false << QVariant();
+      QTest::newRow( "offset_curve point" ) << "offset_curve(geom_from_wkt('POINT(1 2)'),5)" << false << QVariant();
+      QTest::newRow( "offset_curve line" ) << "geom_to_wkt(offset_curve(geom_from_wkt('LineString(0 0, 10 0)'),1,segments:=4))" << false << QVariant( "LineString (0 1, 10 1)" );
+      QTest::newRow( "offset_curve line mitre" ) << "geom_to_wkt(offset_curve(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),distance:=-1,join:=2,mitre_limit:=1))" << false << QVariant( "LineString (10 -1, 0 -1)" );
+      QTest::newRow( "offset_curve line bevel" ) << "geom_to_wkt(offset_curve(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),distance:=1,join:=3))" << false << QVariant( "LineString (0 1, 9 1, 9 10)" );
+      QTest::newRow( "single_sided_buffer not geom" ) << "single_sided_buffer('g', 5)" << true << QVariant();
+      QTest::newRow( "single_sided_buffer null" ) << "single_sided_buffer(NULL, 5)" << false << QVariant();
+      QTest::newRow( "single_sided_buffer point" ) << "single_sided_buffer(geom_from_wkt('POINT(1 2)'),5)" << false << QVariant();
+      QTest::newRow( "single_sided_buffer line" ) << "geom_to_wkt(single_sided_buffer(geom_from_wkt('LineString(0 0, 10 0)'),1,segments:=4))" << false << QVariant( "Polygon ((10 0, 0 0, 0 1, 10 1, 10 0))" );
+      QTest::newRow( "single_sided_buffer line mitre" ) << "geom_to_wkt(single_sided_buffer(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),distance:=-1,join:=2,mitre_limit:=1))" << false << QVariant( "Polygon ((0 0, 10 0, 10 -1, 0 -1, 0 0))" );
+      QTest::newRow( "single_sided_buffer line bevel" ) << "geom_to_wkt(single_sided_buffer(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),distance:=1,join:=3))" << false << QVariant( "Polygon ((10 10, 10 0, 0 0, 0 1, 9 1, 9 10, 10 10))" );
       QTest::newRow( "start_point point" ) << "geom_to_wkt(start_point(geom_from_wkt('POINT(2 0)')))" << false << QVariant( "Point (2 0)" );
       QTest::newRow( "start_point multipoint" ) << "geom_to_wkt(start_point(geom_from_wkt('MULTIPOINT((3 3), (1 1), (2 2))')))" << false << QVariant( "Point (3 3)" );
       QTest::newRow( "start_point line" ) << "geom_to_wkt(start_point(geom_from_wkt('LINESTRING(4 1, 1 1, 2 2)')))" << false << QVariant( "Point (4 1)" );