# qgis / QGIS

[FEATURE] Expression function for hausdorff distance

```Allows calculation of the Hausdorff distance between two
geometries```
nyalldawson committed Aug 30, 2017
1 parent d860722 commit 0a9e9944b4f28254d71ccfcaa6e364fd2ffe38fb
 @@ -0,0 +1,12 @@ { "name": "hausdorff_distance", "type": "function", "description": "Returns the Hausdorff distance between two geometries. This is basically a measure of how similar or dissimilar 2 geometries are, with a lower distance indicating more similar geometries.
The function can be executed with an optional densify fraction argument. If not specified, an appoximation to the standard Hausdorff distance is used. This approximation is exact or close enough for a large subset of useful cases. Examples of these are:

• computing distance between Linestrings that are roughly parallel to each other, and roughly equal in length. This occurs in matching linear networks.
• Testing similarity of geometries.

• If the default approximate provided by this method is insufficient, specify the optional densify fraction argument. Specifying this argument performs a segment densification before computing the discrete Hausdorff distance. The parameter sets the fraction by which to densify each segment. Each segment will be split into a number of equal-length subsegments, whose fraction of the total length is closest to the given fraction. Decreasing the densify fraction parameter will make the distance returned approach the true Hausdorff distance for the geometries.", "arguments": [ {"arg":"geometry a","description":"a geometry"}, {"arg":"geometry b","description":"a geometry"}, {"arg":"densify_fraction","description":"densify fraction amount", "optional":true}], "examples": [ { "expression":"hausdorff_distance( geometry1:= geom_from_wkt('LINESTRING (0 0, 2 1)'),geometry2:=geom_from_wkt('LINESTRING (0 0, 2 0)'))", "returns":"2"}, { "expression":"hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)'))", "returns":"14.142135623"}, { "expression":"hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)'),0.5)", "returns":"70.0"} ] }
 @@ -2574,6 +2574,27 @@ static QVariant fcnDistance( const QVariantList &values, const QgsExpressionCont QgsGeometry sGeom = QgsExpressionUtils::getGeometry( values.at( 1 ), parent ); return QVariant( fGeom.distance( sGeom ) ); } static QVariant fcnHausdorffDistance( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent ) { QgsGeometry g1 = QgsExpressionUtils::getGeometry( values.at( 0 ), parent ); QgsGeometry g2 = QgsExpressionUtils::getGeometry( values.at( 1 ), parent ); double res = -1; if ( values.length() == 3 && values.at( 2 ).isValid() ) { double densify = QgsExpressionUtils::getDoubleValue( values.at( 2 ), parent ); densify = qBound( 0.0, densify, 1.0 ); res = g1.hausdorffDistanceDensify( g2, densify ); } else { res = g1.hausdorffDistance( g2 ); } return res > -1 ? QVariant( res ) : QVariant(); } static QVariant fcnIntersection( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent ) { QgsGeometry fGeom = QgsExpressionUtils::getGeometry( values.at( 0 ), parent ); @@ -4113,6 +4134,8 @@ const QList &QgsExpression::Functions() << new QgsStaticExpressionFunction( QStringLiteral( "convex_hull" ), 1, fcnConvexHull, QStringLiteral( "GeometryGroup" ), QString(), false, QSet(), false, QStringList() << QStringLiteral( "convexHull" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "difference" ), 2, fcnDifference, QStringLiteral( "GeometryGroup" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "distance" ), 2, fcnDistance, QStringLiteral( "GeometryGroup" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "hausdorff_distance" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "geometry1" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "geometry2" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "densify_fraction" ), true ), fcnHausdorffDistance, QStringLiteral( "GeometryGroup" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "intersection" ), 2, fcnIntersection, QStringLiteral( "GeometryGroup" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "sym_difference" ), 2, fcnSymDifference, QStringLiteral( "GeometryGroup" ), QString(), false, QSet(), false, QStringList() << QStringLiteral( "symDifference" ) ) << new QgsStaticExpressionFunction( QStringLiteral( "combine" ), 2, fcnCombine, QStringLiteral( "GeometryGroup" ) )
 @@ -872,6 +872,11 @@ class TestQgsExpression: public QObject QTest::newRow( "smooth point" ) << "geom_to_wkt(smooth(geom_from_wkt('POINT(1 2)'),10))" << false << QVariant( "Point (1 2)" ); QTest::newRow( "smooth line" ) << "geom_to_wkt(smooth(geometry:=geom_from_wkt('LineString(0 0, 5 0, 5 5)'),iterations:=1,offset:=0.2,min_length:=-1,max_angle:=180))" << false << QVariant( "LineString (0 0, 4 0, 5 1, 5 5)" ); QTest::newRow( "transform invalid" ) << "transform(make_point(500,500),'EPSG:4326','EPSG:28356')" << false << QVariant(); QTest::newRow( "hausdorff line to line" ) << " hausdorff_distance( geometry1:= geom_from_wkt('LINESTRING (0 0, 2 1)'),geometry2:=geom_from_wkt('LINESTRING (0 0, 2 0)'))" << false << QVariant( 1.0 ); QTest::newRow( "hausdorff line to line default" ) << " round(hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)')))" << false << QVariant( 14 ); QTest::newRow( "hausdorff line to line densify" ) << " round(hausdorff_distance( geom_from_wkt('LINESTRING (130 0, 0 0, 0 150)'),geom_from_wkt('LINESTRING (10 10, 10 150, 130 10)'),0.5))" << false << QVariant( 70 ); QTest::newRow( "hausdorff not geom 1" ) << " hausdorff_distance( 'a',geom_from_wkt('LINESTRING (0 0, 2 0)'))" << true << QVariant(); QTest::newRow( "hausdorff not geom 2" ) << " hausdorff_distance( geom_from_wkt('LINESTRING (0 0, 2 0)'), 'b')" << true << QVariant(); // string functions QTest::newRow( "lower" ) << "lower('HeLLo')" << false << QVariant( "hello" );