[vtkusers] Can I mix Boost.Python and VTK Python wrapper?

[Андрей Траченко]

Thank you, David, you helped me a lot!

Using "__this__" as a search keyword I found a very useful page [1],
describing how to convert VTK objects between C++ and Python. I've
also used another great article [2] on writing converters in terms of
Boost.Python.

Now I'm going to describe what exactly I've done.

Suppose we have a class:

class Foo {
public:
    vtkSmartPointer<vtkUnstructuredGrid> grid();
};

and we wrap it using Boost.Python:

// inside BOOST_PYTHON_MODULE(MyModule)
class_<Foo>("Foo")
    .def("grid", &Foo::grid);

then we also have to add to_python_converter like this:

// inside BOOST_PYTHON_MODULE(MyModule)
to_python_converter<vtkSmartPointer<vtkUnstructuredGrid>,
    vtkSmartPointer_to_python<vtkUnstructuredGrid> >();

and define struct vtkSmartPointer_to_python somewhere before:

template<class T>
struct vtkSmartPointer_to_python
{
    static PyObject *convert(const vtkSmartPointer<T> &p)
    {
        std::ostringstream oss;
        oss << (void*) p.GetPointer();
        std::string address_str = oss.str();

        using namespace boost::python;
        object obj = import("vtk").attr("vtkObject")(address_str);
        return incref(obj.ptr());
    }
};

Thereby we can write in Python:

from MyModule import Foo
f = Foo()
print f.grid()

Аfter all, I see two major flaws in this approach:
- every vtk class should get its own to_python_converter (I hope that
power of Boost.Python will help me to simplify this)
- I haven't ensured yet that there are no vtkSmartPointer leaks (and
currently have no idea how).

[1] http://visitusers.org/index.php?title=Python_Filters_Development
[2] http://misspent.wordpress.com/2009/09/27/how-to-write-boost-python-converters/

--
Andrew

2011/2/23 David Gobbi <david.gobbi at gmail.com>:
> Hi Andrew,
>
> It is possible to generate VTK/Python objects from the memory address
> of the C++ VTK objects. In the python wrappers, you can get the memory
> address of a VTK object like this:
>
> o = vtk.vtkObject()
> address = o.__this__
>
> and to create a python-wrapped object from a memory address, do this:
>
> o = vtk.vtkObject(address)
>
> Also, if you want an object to "look" like a VTK/Python object even if
> it isn't, you can add a __vtk__() method which will be called when a
> conversion to a VTK/Python object is required by the wrappers.
>
> If you are successful using this to mix Boost.Python and VTK/Python,
> please report back to the list because there are probably other people
> who are interested in doing the same.  More information about the
> wrappers is provided here:
> http://vtk.org/gitweb?p=VTK.git;a=blob;f=Wrapping/Python/README_WRAP.txt
>
> Cheers,
>
>  - David
>
>
> 2011/2/23 Андрей Траченко <goodrone at gmail.com>:
>> I have an application in C++, which uses VTK. Now I'm exposing C++
>> classes to Python using Boost.Python library and using my C++ classes
>> within Python scripts.
>>
>> Now I want to pass VTK objects between C++ and Python code. Is there
>> any correct way to do it? As far as I understand, vtkSmartPointer is
>> used as a "smart pointer" in terms of Boost.Python to wrap VTK C++
>> classes.
>>
>> Currently I'm swapping vtkUnstructuredGrid and other stuff using
>> temporary files, and it's way too slow on large datasets, so I hope I
>> could build a "bridge" between C++ and Python for VTK.
>>
>> Thanks!
>> Andrew
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