Folks,
I'll start a new thread for this RFC.
I've uploaded a new patch, which hopefully corrects the issues Even
brought up. Below are comments.
I've not added the Append or Buffer methods, which basically work on
just one layer. I believe such functionality is simple enough to create
using feature iteration.
Ari
the patch:
http://trac.osgeo.org/gdal/attachment/wiki/rfc39_ogr_layer_algebra/rfc39.patch
On 04/19/2012 02:04 PM, Even Rouault wrote:
http://trac.osgeo.org/gdal/wiki/rfc39_ogr_layer_algebra
Ari,
I have reviewed your proposal and it looks interesting. Here are my observations
:
1) Argument order. A.Operation(B, C) where C is the target layer doesn't seem
very intuitive, but I'm not sure I have something fundamentally better to
propose. An alternate solution would be to make the operations static methods,
and have OGRLayer::Operation(output_layer, input_layer, method_layer) ? I'd
suggest at least to rename B as method_layer and C as result_layer to be more
explicit on the role of each argument.
I've changed the names to Input, Method and Output. For example the
layer object pointers are pLayerInput etc.
2) It would perhaps be prudent to add a char** papszOptions argument as a
provision for future needs.
I've added it.
3) I'm wondering if it would not be more appropriate to separate the creation of
fields of the output layer in a separate method that might be called, or not,
before the real operation. For example IdentifyPrepare() for Identity(). Of
course this would complicate a bit the implementation of the operation because
it will need to look for which fields exist in the output layer and how the
input and method layer fields map to those fields. But the establishment of the
mapping and the setting of the output feature from the fields of the input and
method features could be made in common methods
This way the XXXXXPrepare() would not need to check that CreateField()
succeeds because XXXX() could work with an output layer with no fields at all.
Checking that there are not fields with same name would not be necessary (in
that case the value from B would override the value from A).
I've created a new method into Feature class: SetFieldsFrom, which uses
a map. It is similar to SetFrom, which uses a map. (thus the field copy
could be optimized into one function)
The layer methods now first check the existence of fields in the result
layer. If there are fields, it is assumed that that is what the user
wants. If there are no fields, a complete set of fields are created for
fields in input and method layers (in some cases only input layer). This
behavior could later be overridden with options.
4) It would be nice to able to provide a progress callback to get progress
report (and also being able to interrupt) for such potentially long operations.
I agree. I've added these to the API, but I've not yet added the calls.
This requires for example the inclusion of gdal.h into ogr_api.h.
5) A few remarks on the implementation itself (only code review, no actual
testing). You can likely wait for comments from others before heading to fix
them...
a) CreateField() and CreateFeature() return code should be tested and acted
upon. Likely interrupt the whole process ? (not strictly necessary for
CreateField() is you adopt the idea developed in point 3))
I've added tests. I've used the "goto done;" style, where done is a
label just before the release of used resources and exit from the
method. I've tried to be careful to leave no memory leaks.
b) I think there's something wrong with OGRGeometry *Bfilter =
B->GetSpatialFilter(). When you later do
B->SetSpatialFilter(x_geom->Intersection(Bfilter)), I believe it will destroy
the previous installed filter, and thus Bfilter will point to a garbage object.
You likely need to clone the initial spatial filter (and delete it after
restoring it at the end of the method)
Yes. I checked all calls to GetSpatialFilter and now clone the initial
one and destroy it at the end.
c) Each time you do a new OGRFeature(), you need do delete it (after the call to
CreateFeature()).
They are now deleted.
d) GetGeometryRef() can return NULL (even for spatial layers). Currently it is
never checked, and there are a few places where it is dereferenced.
The validity of the geometry is checked. If there is no geometry, the
next feature is fetched.
e) Lines like x_geom_diff = x_geom_diff->Difference(y_geom) (or geom =
geom->Union(y_geom)) will leak the initial x_geom_diff object. You likely need
to do :
OGRGeometry* x_geom_diff_new = x_geom_diff->Difference(y_geom);
delete x_geom_diff;
x_geom_diff = x_geom_diff_new;
Fixed.
f) The object returned by GetNextFeature() should be deleted after use.
It is deleted, even if the iteration is continued.
g) You should be careful with SetGeometryDirectly(). You cannot pass directly
the geometry belonging to another feature because this will segfault when you
free the said feature.
Fixed
h) Nothing wrong, but calls to UnsetField() are unnecessary for a feature that
was just created and never set before (all the lines z->UnsetField(...) )
The unnecessary code is removed.
_______________________________________________
gdal-dev mailing list
[email protected]
http://lists.osgeo.org/mailman/listinfo/gdal-dev
