Am 20.01.2016 um 07:35 schrieb Kai Muehlbauer:
Am 19.01.2016 um 13:09 schrieb Even Rouault:
Le mardi 19 janvier 2016 08:40:30, Ari Jolma a écrit :
18.01.2016, 11:56, Kai Muehlbauer kirjoitti:
Hi,
at the moment I calculate the intersection of two OGR Layers (A, B) by
iterating over features/geometries (GA) of layer A and then calling
GA.Intersection(GB) for every Feature/Geometry (GB) of B. I set a
spatial filter for layer B with every GA. The intersections
features/geometries are written to a result layer (C).
In my use case the features/geometries of layer B are much smaller
than the features of layer A. Means that quite a big number of B's
geometries are fully contained in A's geometries.
To speed things up, I check for containment GA.contains(GB). If GB is
contained in GA, I just copy GB to C, otherwise I calculate
GA.Intersection(GB) and write this to C.
I found that I also could use the layer intersection function
(OGRLayer::Intersection). Unfortunately the intersection is calculated
for every feature/geometry.
Would it be possible to extend the layer intersection function to
allow for containment check (eg. as an option) and just copy the
contained geometries without calculation of expensive intersection?
Suggestions, opinions?
There's the Options argument, which could be used. Something like
"PRETEST=CONTAINMENT" could indicate a wish to test containment before
intersection.
The use of GEOS prepared geometry operations could also potentially
help. OGR
has already mapped GEOSPreparedIntersects as
OGRPreparedGeometryIntersects and
that could probably be unconditionnaly used, with the geometry of the
outer
loop being the prepared geometry with OGRCreatePreparedGeometry()
And perhaps GEOSPreparedContains could be used for containement (if
the above
suggestion desn't speed up things enough).
Thank you Ari and Even for the comments and suggestions. I'll try to
test the "prepared" stuff first, hoping that those functions are also
available within the python bindings (which I use).
I'll give an update here, when I have results on this.
OK, here we go.
JFTR, I'am using the SWIG python bindings for ogr.
To quickly check if there is an improvement I used the python shapely
package. Converting the outer loop geometry to an shapely prepared
geometry and calling 'contains()' with the inner loop geometries to find
those fully contained geometries I get an improvement of nearly 40% to
the former ogr only code.
As Even pointed out, there is only OGRPreparedGeometryIntersects()
(GEOSPreparedIntersects) exposed within ogrgeometry.cpp. In my use case
I would need GEOSPreparedContains to mimic the shapely behaviour.
Next thing is that the ogr SWIG python bindings do not wrap those
available OGRPrepared-functions.
I'm no expert when it comes to c/c++-python interaction to create a
patch to have this functionality within python ogr/gdal. So hoping for
help in this matter, I'm asking for suggestions how to get the
GEOSPreparedGeometry stuff called from OGR.
Cheers,
Kai
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