[Qgis-user] QgsGeometry - how to detect nr of rings and vertices

[Carson Farmer]

Hi Andreas,
> I am using QgsGeometry from python. I would like to find out the nr of
> rings and vertices per ring but could not find a function for this purpose
> in http://doc.qgis.org/classQgsGeometry.html
>   
I don't think there is one...
> Is there something hidden/undocumented to find out this information?
>
> The idea is to step through all rings and vertices of a QgsGeometry object
> and process the coordinates.
>   
The fTools function extract nodes does something like this (only it's 
not especially efficient...)

What about something like:

ring_count = 0
vertices_count = 0
geom = QgsGeometry() # doesn't matter how you get this ;-)
# figure out the geometry type() here
if geom.type() == QGis.Polygon:
    if geom.isMultipart():
        polygons = geom.fromMultiPolygon()
    else:
        polygons = [ geom.fromPolygon() ]
    for polygon in polygons:
        for line in polygon: # these are your rings
            print ring_count
            for vertices in line: # these are your vertices
                print vertices_count

Obviously you wouldn't really loop through all of the vertices and rings 
this way, but you can use this concept to figure out how many of each 
there are:

...
num_rings = len( polygon )
num_vetices = len( line )

I don't know if this is what you were looking for, but hopefully it helps,


Carson

-- 
Carson J. Q. Farmer
ISSP Doctoral Fellow
National Centre for Geocomputation (NCG),
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National University of Ireland, Maynooth,
Co. Kildare,
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