Fixes after review

python/core/geometry/qgsgeometry.sip.in

@@ -1339,6 +1339,18 @@ If it is already a single part geometry, it will return true and
 not change the geometry.
 
 :return: true in case of success and false else
+%End
+
+    bool convertGeometryCollectionToSubclass( QgsWkbTypes::GeometryType geomType );
+%Docstring
+Converts geometry collection to a the desired geometry type subclass (multi-point,
+multi-linestring or multi-polygon). Child geometries of different type are filtered out.
+Does nothing the geometry is not a geometry collection. May leave the geometry
+empty if none of the child geometries match the desired type.
+
+:return: true in case of success and false else
+
+.. versionadded:: 3.2
 %End
 
     int avoidIntersections( const QList<QgsVectorLayer *> &avoidIntersectionsLayers );

python/plugins/processing/algs/help/qgis.yaml

@@ -226,11 +226,6 @@ qgis:importintopostgis: >
 
   Prior to this a connection between QGIS and the PostGIS database has to be created (for example with the DB Manager).
 
-qgis:intersection: >
-  This algorithm extracts the overlapping portions of features in the Input and Intersect layers. Features in the Intersection layer are assigned the attributes of the overlapping features from both the Input and Intersect layers.
-
-  Attributes are not modified.
-
 qgis:joinattributesbylocation: >
   This algorithm takes an input vector layer and creates a new vector layer that is an extended version of the input one, with additional attributes in its attribute table.
 

python/plugins/processing/algs/qgis/QgisAlgorithmProvider.py

@@ -79,7 +79,6 @@
 from .IdwInterpolation import IdwInterpolation
 from .ImportIntoPostGIS import ImportIntoPostGIS
 from .ImportIntoSpatialite import ImportIntoSpatialite
-from .Intersection import Intersection
 from .KeepNBiggestParts import KeepNBiggestParts
 from .LinesToPolygons import LinesToPolygons
 from .MinimumBoundingGeometry import MinimumBoundingGeometry
@@ -198,7 +197,6 @@ def getAlgs(self):
                 IdwInterpolation(),
                 ImportIntoPostGIS(),
                 ImportIntoSpatialite(),
-                Intersection(),
                 KeepNBiggestParts(),
                 LinesToPolygons(),
                 MinimumBoundingGeometry(),

src/analysis/processing/qgsalgorithmintersection.cpp

@@ -18,45 +18,8 @@
 #include "qgsgeometrycollection.h"
 #include "qgsgeometryengine.h"
 
-#include "qgsmultipoint.h"
-#include "qgsmultipolygon.h"
-#include "qgsmultilinestring.h"
-
 ///@cond PRIVATE
 
-static QgsGeometry geometryCollectionToSingleGeometryType( const QgsGeometry &geom, QgsWkbTypes::GeometryType geomType )
-{
-  const QgsGeometryCollection *origGeom = qgsgeometry_cast<const QgsGeometryCollection *>( geom.constGet() );
-  if ( !origGeom )
-    return QgsGeometry();
-
-  QgsGeometryCollection *resGeom = nullptr;
-  switch ( geomType )
-  {
-    case QgsWkbTypes::PointGeometry:
-      resGeom = new QgsMultiPoint;
-      break;
-    case QgsWkbTypes::LineGeometry:
-      resGeom = new QgsMultiLineString;
-      break;
-    case QgsWkbTypes::PolygonGeometry:
-      resGeom = new QgsMultiPolygon;
-      break;
-    default:
-      break;
-  }
-  if ( !resGeom )
-    return QgsGeometry();
-
-  for ( int i = 0; i < origGeom->numGeometries(); ++i )
-  {
-    const QgsAbstractGeometry *g = origGeom->geometryN( i );
-    if ( QgsWkbTypes::geometryType( g->wkbType() ) == geomType )
-      resGeom->addGeometry( g->clone() );
-  }
-  return QgsGeometry( resGeom );
-}
-
 
 QString QgsIntersectionAlgorithm::name() const
 {
@@ -78,6 +41,12 @@ QString QgsIntersectionAlgorithm::groupId() const
   return QStringLiteral( "vectoroverlay" );
 }
 
+QString QgsIntersectionAlgorithm::shortHelpString() const
+{
+  return QObject::tr( "This algorithm extracts the overlapping portions of features in the Input and Overlay layers. Features in the Overlay layer are assigned the attributes of the overlapping features from both the Input and Overlay layers." )
+         + QStringLiteral( "\n\n" )
+         + QObject::tr( "Optionally, the rotation can occur around a preset point. If not set the rotation occurs around each feature's centroid." );
+}
 
 QgsProcessingAlgorithm *QgsIntersectionAlgorithm::createInstance() const
 {
@@ -225,27 +194,25 @@ QVariantMap QgsIntersectionAlgorithm::processAlgorithm( const QVariantMap &param
 
       if ( intGeom.isNull() )
       {
-        // TODO: not sure if this ever happens - if it does, that means GEOS failed badly and we should try to provide an error from it
-        throw QgsProcessingException( QObject::tr( "GEOS geoprocessing error: intersection failed." ) );
+        // TODO: not sure if this ever happens - if it does, that means GEOS failed badly - would be good to have a test for such situation
+        throw QgsProcessingException( QStringLiteral( "%1\n\n%2" ).arg( QObject::tr( "GEOS geoprocessing error: intersection failed." ), intGeom.lastError() ) );
       }
 
       // Intersection of geometries may give use also geometries we do not want in our results.
       // For example, two square polygons touching at the corner have a point as the intersection, but no area.
       // In other cases we may get a mixture of geometries in the output - we want to keep only the expected types.
-      if ( QgsWkbTypes::geometryType( intGeom.wkbType() ) != QgsWkbTypes::geometryType( geomType ) )
+      if ( QgsWkbTypes::flatType( intGeom.wkbType() ) == QgsWkbTypes::GeometryCollection )
       {
-        if ( QgsWkbTypes::flatType( intGeom.wkbType() ) == QgsWkbTypes::GeometryCollection )
-        {
-          // try to filter out irrelevant parts with different geometry type than what we want
-          intGeom = geometryCollectionToSingleGeometryType( intGeom, QgsWkbTypes::geometryType( geomType ) );
-          if ( intGeom.isEmpty() )
-            continue;
-        }
-        else
-        {
-          // we can't make use of this resulting geometry -
+        // try to filter out irrelevant parts with different geometry type than what we want
+        intGeom.convertGeometryCollectionToSubclass( QgsWkbTypes::geometryType( geomType ) );
+        if ( intGeom.isEmpty() )
           continue;
-        }
+      }
+
+      if ( QgsWkbTypes::geometryType( intGeom.wkbType() ) != QgsWkbTypes::geometryType( geomType ) )
+      {
+        // we can't make use of this resulting geometry
+        continue;
       }
 
       const QgsAttributes attrsB( featB.attributes() );

src/analysis/processing/qgsalgorithmintersection.h

@@ -31,6 +31,7 @@ class QgsIntersectionAlgorithm : public QgsProcessingAlgorithm
     virtual QString displayName() const override;
     virtual QString group() const override;
     virtual QString groupId() const override;
+    QString shortHelpString() const override;
 
   protected:
     virtual QgsProcessingAlgorithm *createInstance() const override;

src/core/geometry/qgsgeometry.cpp

@@ -1315,6 +1315,43 @@ bool QgsGeometry::convertToSingleType()
   return true;
 }
 
+
+bool QgsGeometry::convertGeometryCollectionToSubclass( QgsWkbTypes::GeometryType geomType )
+{
+  const QgsGeometryCollection *origGeom = qgsgeometry_cast<const QgsGeometryCollection *>( constGet() );
+  if ( !origGeom )
+    return false;
+
+  std::unique_ptr<QgsGeometryCollection> resGeom;
+  switch ( geomType )
+  {
+    case QgsWkbTypes::PointGeometry:
+      resGeom = qgis::make_unique<QgsMultiPoint>();
+      break;
+    case QgsWkbTypes::LineGeometry:
+      resGeom = qgis::make_unique<QgsMultiLineString>();
+      break;
+    case QgsWkbTypes::PolygonGeometry:
+      resGeom = qgis::make_unique<QgsMultiPolygon>();
+      break;
+    default:
+      break;
+  }
+  if ( !resGeom )
+    return false;
+
+  for ( int i = 0; i < origGeom->numGeometries(); ++i )
+  {
+    const QgsAbstractGeometry *g = origGeom->geometryN( i );
+    if ( QgsWkbTypes::geometryType( g->wkbType() ) == geomType )
+      resGeom->addGeometry( g->clone() );
+  }
+
+  set( resGeom.release() );
+  return true;
+}
+
+
 QgsPointXY QgsGeometry::asPoint() const
 {
   if ( !d->geometry || QgsWkbTypes::flatType( d->geometry->wkbType() ) != QgsWkbTypes::Point )