[FEATURE] line_substring function

Returns the portion of a line (or curve) geometry which falls between the specified start and end distances (measured from the beginning of the line). Z and M values are linearly interpolated from existing values. Fixes #15611
qgis · Aug 15, 2018 · a4386d6 · a4386d6
1 parent 513bcb6
commit a4386d6
Show file tree

Hide file tree

Showing 3 changed files with 49 additions and 1 deletion.
diff --git a/resources/function_help/json/line_substring b/resources/function_help/json/line_substring
@@ -0,0 +1,9 @@
+{
+  "name": "line_substring",
+  "type": "function",
+  "description": "Returns the portion of a line (or curve) geometry which falls between the specified start and end distances (measured from the beginning of the line). Z and M values are linearly interpolated from existing values.",
+  "arguments": [ {"arg":"geometry","description":"a linestring or curve geometry"},
+                 {"arg":"start_distance","description":"distance to start of substring"},
+                 {"arg":"end_distance","description":"distance to end of substring"}],
+  "examples": [ { "expression":"geom_to_wkt(line_substring(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),start_distance:=2,end_distance=6))", "returns":"'LineString (2 0,6 0)'"}]
+}
diff --git a/src/core/expression/qgsexpressionfunction.cpp b/src/core/expression/qgsexpressionfunction.cpp
@@ -3047,6 +3047,39 @@ static QVariant fcnLineInterpolatePoint( const QVariantList &values, const QgsEx
   return result;
 }
 
+static QVariant fcnLineSubset( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  QgsGeometry lineGeom = QgsExpressionUtils::getGeometry( values.at( 0 ), parent );
+  if ( lineGeom.type() != QgsWkbTypes::LineGeometry )
+  {
+    parent->setEvalErrorString( QObject::tr( "line_substring requires a curve geometry input" ) );
+    return QVariant();
+  }
+
+  const QgsCurve *curve = nullptr;
+  if ( !lineGeom.isMultipart() )
+    curve = qgsgeometry_cast< const QgsCurve * >( lineGeom.constGet() );
+  else
+  {
+    if ( const QgsGeometryCollection *collection = qgsgeometry_cast< const QgsGeometryCollection * >( lineGeom.constGet() ) )
+    {
+      if ( collection->numGeometries() > 0 )
+      {
+        curve = qgsgeometry_cast< const QgsCurve * >( collection->geometryN( 0 ) );
+      }
+    }
+  }
+  if ( !curve )
+    return QVariant();
+
+  double startDistance = QgsExpressionUtils::getDoubleValue( values.at( 1 ), parent );
+  double endDistance = QgsExpressionUtils::getDoubleValue( values.at( 2 ), parent );
+
+  std::unique_ptr< QgsCurve > substring( curve->curveSubstring( startDistance, endDistance ) );
+  QgsGeometry result( std::move( substring ) );
+  return !result.isNull() ? QVariant::fromValue( result ) : QVariant();
+}
+
 static QVariant fcnLineInterpolateAngle( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QgsGeometry lineGeom = QgsExpressionUtils::getGeometry( values.at( 0 ), parent );
@@ -4527,7 +4560,9 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
         << new QgsStaticExpressionFunction( QStringLiteral( "angle_at_vertex" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "geometry" ) )
                                             << QgsExpressionFunction::Parameter( QStringLiteral( "vertex" ) ), fcnAngleAtVertex, QStringLiteral( "GeometryGroup" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "distance_to_vertex" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "geometry" ) )
-                                            << QgsExpressionFunction::Parameter( QStringLiteral( "vertex" ) ), fcnDistanceToVertex, QStringLiteral( "GeometryGroup" ) );
+                                            << QgsExpressionFunction::Parameter( QStringLiteral( "vertex" ) ), fcnDistanceToVertex, QStringLiteral( "GeometryGroup" ) )
+        << new QgsStaticExpressionFunction( QStringLiteral( "line_substring" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "geometry" ) )
+                                            << QgsExpressionFunction::Parameter( QStringLiteral( "start_distance" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "end_distance" ) ), fcnLineSubset, QStringLiteral( "GeometryGroup" ) );
 
 
     // **Record** functions

diff --git a/tests/src/core/testqgsexpression.cpp b/tests/src/core/testqgsexpression.cpp
@@ -988,6 +988,10 @@ class TestQgsExpression: public QObject
       QTest::newRow( "distance_to_vertex null" ) << "distance_to_vertex(NULL, 0)" << false << QVariant();
       QTest::newRow( "distance_to_vertex point" ) << "distance_to_vertex(geom_from_wkt('POINT(1 2)'),0)" << false << QVariant( 0.0 );
       QTest::newRow( "distance_to_vertex line" ) << "distance_to_vertex(geometry:=geom_from_wkt('LineString(0 0, 10 0, 10 10)'),vertex:=1)" << false << QVariant( 10.0 );
+      QTest::newRow( "line_substring not geom" ) << "line_substring('g', 5, 6)" << true << QVariant();
+      QTest::newRow( "line_substring null" ) << "line_substring(NULL, 5, 6)" << false << QVariant();
+      QTest::newRow( "line_substring point" ) << "line_substring(geom_from_wkt('POINT(1 2)'),5,6)" << true << QVariant();
+      QTest::newRow( "line_substring line" ) << "geom_to_wkt(line_substring(geometry:=geom_from_wkt('LineString(0 0, 10 0)'),start_distance:=5,end_distance:=6))" << false << QVariant( "LineString (5 0, 6 0)" );
       QTest::newRow( "simplify not geom" ) << "simplify('g',5)" << true << QVariant();
       QTest::newRow( "simplify null" ) << "simplify(NULL,5)" << false << QVariant();
       QTest::newRow( "simplify point" ) << "geom_to_wkt(simplify(geom_from_wkt('POINT(1 2)'),10))" << false << QVariant( "Point (1 2)" );