On 02/10/2011 10:28 AM, Frank Broniewski wrote:
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 ;-)
In this case the GetPoint method exists already, so to maintain
backwards compatibility, its behavior should not be changed. However,
the behavior is only really defined in the context where it is expected
to return a list (the coordinates). Thus, it would be possible to
extend/define its behavior to scalar context, i.e., where it is expected
to return a single value. For example in Perl, @list is a variable that
is a list and $scalar is a variable that is a scalar (one thing):
@list = $geometry_object->GetPoint(0);
$scalar = $geometry_object->GetPoint(0);
now @list would be array (x,y) and $scalar would be a Geometry object of
type Point.
This might be confusing and I don't know if Python supports this kind of
play with the syntax.
Anyway, as PointN is the standard name, which is not used in GDAL so
far, we could use that.
Ari
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
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