[vtk-developers] Added support for std::vector to the wrappers

[Andras Lasso]

I find this feature very useful! In application code, we commonly need to deal with variable-length arrays. We have been using std::vector accessors in C++ and adding vtkStringArray based or GetNumberOf…/GetNth… methods or for Python, which has made the API complex and has been very inconvenient to use in Python.

These arrays typically contain up to a few ten elements, therefore, for us, convenience is much more important than efficiency.
Most often we return strings and vtkObject* in std::vector, so it would be great if vtkObject pointers would be supported, too.

Andras


From: vtk-developers <vtk-developers-bounces at public.kitware.com> On Behalf Of Todd via vtk-developers
Sent: Wednesday, September 5, 2018 8:16 PM
To: David Gobbi <david.gobbi at gmail.com>
Cc: VTK Developers <vtk-developers at vtk.org>
Subject: Re: [vtk-developers] Added support for std::vector to the wrappers

Hi David

Since std::vector types are unlikely to be small, like coordinate and bound arrays, that sounds horribly inefficient; especially when large quantities of strings and objects are being marshalled.

Why not just map the std::vector/std::array to a Python iterator object?

Todd

On 6 Sep 2018 4:20 a.m., David Gobbi <david.gobbi at gmail.com<mailto:david.gobbi at gmail.com>> wrote:
Hi Folks,

Yesterday I merged a patch that allows std::vector<T> to be used from the Python wrappers, where T can be 'std::string' or any numeric type from 'signed char' to 'float' (including typedefs to those types).

This means that if you add a C++ method to VTK that takes std::vector<std::string>, then in Python you can pass a list or any other Python sequence and the wrappers will convert it to a vector for you.

Method examples that can accept any Python sequence:
    void SetStrings(std::vector<std::string> strings);
    void SetStrings(const std::vector<std::string>& strings);

Method example that takes a Python mutable sequence (the C++ method can write back to the container, since this is pass-by-reference):
    void GetStrings(std::vector<std::string>& strings);

Method examples that return a vector (which is converted to a Python tuple):
    const std::vector<std::string>& GetStrings();
    std::vector<std::string>& GetStrings();
    std::vector<std::string> GetStrings();

Note that std::vector is not 'wrapped' per se.  Instead, what is going on here is that the wrappers are converting to/from Python container types. My primary goal was convenience rather than efficiency, we can always add a wrapped std::vector type at a later date without sacrificing the convenience of automatic conversion.

Chances are good that I'll be able to add std::vectors of VTK objects (smart or dumb pointers) and vtkVariant before VTK 9, but only if someone adds methods to VTK that take parameters of those types (as far as I know, there aren't any yet).

See the following VTK gitlab issue for additional information:
https://gitlab.kitware.com/vtk/vtk/issues/17362<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.kitware.com%2Fvtk%2Fvtk%2Fissues%2F17362&data=02%7C01%7Classo%40queensu.ca%7C0e1f3f218b5543a5524608d6138e03b5%7Cd61ecb3b38b142d582c4efb2838b925c%7C1%7C0%7C636717898021517951&sdata=9vohuaiduQLCm6pfGlMxQbXL4hYutf1fKB6SafvQE4c%3D&reserved=0>

The following is a list of VTK methods that take std::vector that have been wrapped due to this change (it is a short list, since people have historically avoided using std::vector as part of the VTK public API due to the obvious reason that they didn't want to exclude their methods from wrapping):

vtkActor::ProcessSelectorPixelBuffers
vtkAMRInformation::GetNumBlocks
vtkCompositePolyDataMapper2::ProcessSelectorPixelBuffers
vtkMapper::ProcessSelectorPixelBuffers
vtkMultiProcessStream::GetRawData
vtkMultiProcessStream::SetRawData
vtkOpenGLGlyph3DHelper::GlyphRender
vtkOpenGLIndexBufferObject::AppendTriangleIndexBuffer
vtkOpenGLIndexBufferObject::AppendLineIndexBuffer
vtkOpenGLIndexBufferObject::AppendTriangleLineIndexBuffer
vtkOpenGLIndexBufferObject::AppendPointIndexBuffer
vtkOpenGLIndexBufferObject::AppendStripIndexBuffer
vtkOpenGLIndexBufferObject::AppendEdgeFlagIndexBuffer
vtkOpenGLMoleculeMapper::ProcessSelectorPixelBuffers
vtkOpenGLPointGaussianMapper::ProcessSelectorPixelBuffers
vtkOpenGLPolyDataMapper::HandleAppleBug
vtkOpenGLPolyDataMapper::ProcessSelectorPixelBuffers
vtkOpenGLVertexBufferObject::SetShift
vtkOpenGLVertexBufferObject::SetScale
vtkOpenGLVertexBufferObject::GetShift
vtkOpenGLVertexBufferObject::GetScale
vtkOpenGLVertexBufferObject::GetPackedVBO
vtkParallelAMRUtilities::DistributeProcessInformation
vtkProp::ProcessSelectorPixelBuffers
vtkResourceFileLocator::Locate
vtkShadowMapPass::ShadowMapTransforms
vtkShadowMapPass::GetShadowMapTextureUnits
vtkSparseArray::GetUniqueCoordinates
vtkUnicodeString::utf16_str

Cheers,
 - David

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