[gdal-dev] [python-ogr] GetNumPoints from geometry

[Frank Broniewski]

Hi Ari,

thanks for your answer. I am not sure what you mean with return a 
geometry in scalar context, and I don't have a sound understanding of 
perl, SWIG and the impacts of it on the objects created by the bindings.

A very quick google search didn't reveal a good explanation of what a 
scalar in perl is, but from what I found it seems to have something to 
do with lists in python [1]
It would be possible to let an object (e.g. OGRPoint) inherit from list 
and use that as a container, but this is just speculation on my side ;-)

Thanks for the tipp with the documentation. I will use the C API in the 
future then ...


Thanks,

Frank


[1] http://docs.python.org/library/functions.html#list

Am 09.02.2011 18:15, schrieb Ari Jolma:
> On 02/09/2011 05:36 PM, Frank Broniewski wrote:
>> Yes, thank you. GetPointCount() works. Another thing I noticed:
>>
>> point = geometry.GetPoint(point_index)
>>
>> point is of type tuple(x, y, z), not as one would guess an OGRPoint()
>> geometry. At least for me this was unexpected. And the python GetPoint
>> method does not take a OGRPoint geometry, like it is mentioned here [1]
>
> The OGC name of GetPoint is PointN and it should return a Point object
> according to the standard.
>
> The GDAL Perl (which I care for) works the same way, GetPoint returns a
> list of coordinates. It would be possible to make it return a geometry
> object in scalar context, but I'm not sure if Python has that possibility.
>
> The GDAL Perl documentation for Geometry object is at
> http://geoinformatics.tkk.fi/doc/Geo-GDAL/html/class_geo_1_1_o_g_r_1_1_geometry.html
> I'm not aware of GDAL Python documentation in the web. There are
> differences between GDAL C++ API and GDAL C API. For the bindings the C
> API documentation is usually better:
> http://www.gdal.org/ogr/ogr__api_8h.html#67b897ffcd88a83832964c3735fa1dcb
>
> Best regards,
>
> Ari
>
>>
>> Frank
>>
>>
>> [1]
>> http://www.gdal.org/ogr/classOGRLineString.html#ab63760022e49a795477f2fd31da059f
>>
>>
>> Am 09.02.2011 15:44, schrieb Ari Jolma:
>>> On 02/09/2011 04:19 PM, Frank Broniewski wrote:
>>>> Hi,
>>>>
>>>> just a quick question: isn't GetNumPoints implemented on a geometry in
>>>> ogr python?
>>>
>>> There are OGR_G_GetPointCount in ogr_api.h and GetPointCount method in
>>> the Geometry class in Swig bindings.
>>>
>>> Hm. It seems that NumPoints is the name of the method in the OGC simple
>>> features standard (it is defined for LineStrings).
>>>
>>> Ari
>>>
>>>> Neither GetNumPoints() nor getNumPoints() (small g) work.
>>>> The passed in geometry is a geometry object from a
>>>>
>>>> geometry = feature.GetGeometryRef()
>>>>
>>>> call and the geometry is of type 'LINESTRING' taken from
>>>>
>>>> geometry.GetGeometryName() ...
>>>>
>>>>
>>>> code:
>>>> def line_nodes(geometry):
>>>> nodes = list()
>>>>
>>>> for point_index in xrange(geometry.GetNumPoints()):
>>>> ogr_point = ogr.OGRPoint()
>>>> point = geometry.GetPoint(point_index, ogr_point)
>>>> dimenion = point.GetCoordinateDimension()
>>>> if dimension == 3:
>>>> list.append(point_index, point.GetX(), point.GetY(), point.GetZ())
>>>> else:
>>>> list.append(point_index, point.GetX(), point.GetY())
>>>>
>>>> return nodes
>>>>
>>>>
>>>> raises an AttributeError
>>>> ...
>>>> for point_index in xrange(geometry.getNumPoints()):
>>>> File "C:\Python26\lib\site-packages\osgeo\ogr.py", line 2672, in
>>>> <lambda>
>>>> __getattr__ = lambda self, name: _swig_getattr(self, Geometry, name)
>>>> File "C:\Python26\lib\site-packages\osgeo\ogr.py", line 34, in
>>>> _swig_getattr
>>>> raise AttributeError,name
>>>> AttributeError: GetNumPoints
>>>>
>>>>
>>>> Many thanks
>>>>
>>>> Frank
>>>>
>>>
>>> _______________________________________________
>>> gdal-dev mailing list
>>> gdal-dev at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>>>
>>
>>
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>


-- 
Frank BRONIEWSKI

METRICO s.à r.l.
géomètres
technologies d'information géographique
rue des Romains 36
L-5433 NIEDERDONVEN

tél.: +352 26 74 94 - 28
fax.: +352 26 74 94 99
http://www.metrico.lu