[gdal-dev] RFC 39

[Ari Jolma]

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.