def getGeomPolygonRing(rings, idRing):
     # rings = [ QgsPointXY ]
     ringPoints = [ QgsPoint( p ) for p in rings[ idRing  ] ]  # [ QgsPoint ]
     line = QgsLineString( ringPoints )
     del ringPoints[:]
     polygon = QgsPolygon()
     polygon.setExteriorRing( line )
     del line
     return QgsGeometry( polygon )

    def getSurfaces(self, geometry):
        surfaces = []
        if isinstance(geometry, QgsGeometryCollection):
            # collection
            for i in range(geometry.numGeometries()):
                surfaces.extend(self.getSurfaces(geometry.geometryN(i)))
        else:
            # not collection
            if geometry.vertexCount() > 2:
                surface = QgsPolygon()
                surface.setExteriorRing(geometry.clone())
                surfaces.append(surface)

        return surfaces

 def convertToPolygon(self, geom):
     if QgsWkbTypes.geometryType(geom.wkbType()) == QgsWkbTypes.PointGeometry and geom.constGet().nCoordinates() < 3:
         raise QgsProcessingException(
             self.tr('Cannot convert from Point to Polygon').format(QgsWkbTypes.displayString(geom.wkbType())))
     elif QgsWkbTypes.geometryType(geom.wkbType()) == QgsWkbTypes.PointGeometry:
         # multipoint with at least 3 points
         # TODO: mega inefficient - needs rework when geometry iterators land
         # (but at least it doesn't lose Z/M values)
         points = []
         for g in geom.constGet().coordinateSequence():
             for r in g:
                 for p in r:
                     points.append(p)
         linestring = QgsLineString(points)
         linestring.close()
         p = QgsPolygon()
         p.setExteriorRing(linestring)
         return [QgsGeometry(p)]
     elif QgsWkbTypes.geometryType(geom.wkbType()) == QgsWkbTypes.LineGeometry:
         if QgsWkbTypes.isMultiType(geom):
             parts = []
             for i in range(geom.constGet().numGeometries()):
                 p = QgsPolygon()
                 linestring = geom.constGet().geometryN(i).clone()
                 linestring.close()
                 p.setExteriorRing(linestring)
                 parts.append(QgsGeometry(p))
             return QgsGeometry.collectGeometry(parts)
         else:
             # linestring to polygon
             p = QgsPolygon()
             linestring = geom.constGet().clone()
             linestring.close()
             p.setExteriorRing(linestring)
             return [QgsGeometry(p)]
     else:
         #polygon
         if QgsWkbTypes.isMultiType(geom):
             return geom.asGeometryCollection()
         else:
             return [geom]

 def get_mesh_geometry(self, mesh, index):
     face = mesh.face(index)
     points = [mesh.vertex(v) for v in face]
     polygon = QgsPolygon()
     polygon.setExteriorRing(QgsLineString(points))
     return QgsGeometry(polygon)