[vtk-developers] VTK python wrapping - returning binary buffer to python?

[David Gobbi]

On Fri, Aug 4, 2017 at 9:08 AM, Aron Helser <aron.helser at kitware.com> wrote:

>
> Sure thing. And as I trace the code more carefully, I found at least part
> of the issue.
>
> I miss-spoke - the method in question is in ParaView, an override of a
> method in VTK:
>
> ParaView/Web/Core/vtkPVWebApplication.cxx:
> vtkUnsignedCharArray* vtkPVWebApplication::StillRender(vtkSMViewProxy*
> view, int quality)
>
> However, that method is wrapped in const char* vtkPVWebApplication::StillRenderToString(),
> which does this:
>
>
> return reinterpret_cast<char*>(array->GetPointer(0));
>
> So that's where the 'char *' to python 'str' conversion happens.
>

Yes, python will interpret a returned "char *" as a c-style string.  One
way to avoid this is to return an std::string, but std::string isn't useful
when you're trying to do a zero-copy operation.


If I avoid that wrapper and return vtkUnsignedCharArray* directly, can I
> get a python 'bytes' object out of it in the Python code without copying?
>

Yes.  In Python, the vtkUnsignedCharArray (and all other array types)
support the Python buffer interface.  That's a feature that Berk added to
the wrappers a few years back.

So if you were (hypothetically) using numpy, you could use
numpy.frombuffer() to interpret the VTK array as a numpy array.  This is a
perfect zerocopy operation.

The Python builtins, however, are more cautious about zerocopy than VTK or
numpy.  So if you were to simply do this, you'd get a copy:

  b = bytes(a)  # where "a" is a VTK array object

The Python "bytes" object is supposed to own its own memory, it wasn't
designed to share.  Even the numpy .tostring() and .tobytes() methods
produce copies.

However, I suspect that you don't have to convert the array to bytes
anyway.  Just pass the VTK array object to whatever method you were
planning to pass the 'bytes' object to, and it will probably work fine.
Many methods that take 'bytes' will actually take any object that provides
a buffer interface, and that includes numpy arrays and VTK arrays.

 - David
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