QVTKWidget render window is outside main Qt app window

Hi all,

I put together a VTK viewer in Python. I also extended QVTKWidget to host the viewer in a Qt app. It all looks and works fine with Qt 5.9.7 and VTK 8.1.2 on windows (in a conda environment).

If I set up the “same” (Qt, Python, VTK versions are the same) environment on Linux, the QVTKWidget renders on its own window outside the Qt application so the interaction which is passed from Qt to VTK happens on the Qt window, while the display happens on the OpenGL2 window opened by VTK.

I have seen this behaviour before and I believe the fix I found was to change the VTK version. Is that something that you recognise?


Here just a screenshot of what happens, you can see 2 OpenGL windows separated from the main window (DVC_Configurator)


This old thread seems to be related


I tracked down the problem to these lines:

I have created a QGLWidget and inside it a vtkRenderWindow. To instruct the latter to use a pre-existing window one uses

self._RenderWindow = vtk.vtkRenderWindow()
WId = self.winId()

Now, it seems in Windows the third line works as expected, i.e. if I comment it out the vtkRenderWindow opens its own window. In Linux the third line doesn’t seem to have effect.

Any ideas?


I’m a bit confused by your question… you are asking about QVTKWidget (which is a C++ class), but that code looks like it came from QVTKRenderWindowInteractor (which is a Python class, totally unrelated to the C++ QVTKWidget).

Are you using your own, custom class to connect VTK and Qt? If so, then you can try adapting the following code from QVTKRenderWindowInteractor.py:

    WId = self.winId()

    # Python2
    if type(WId).__name__ == 'PyCObject':
        from ctypes import pythonapi, c_void_p, py_object

        pythonapi.PyCObject_AsVoidPtr.restype  = c_void_p
        pythonapi.PyCObject_AsVoidPtr.argtypes = [py_object]

        WId = pythonapi.PyCObject_AsVoidPtr(WId)

    # Python3
    elif type(WId).__name__ == 'PyCapsule':
        from ctypes import pythonapi, c_void_p, py_object, c_char_p

        pythonapi.PyCapsule_GetName.restype = c_char_p
        pythonapi.PyCapsule_GetName.argtypes = [py_object]

        name = pythonapi.PyCapsule_GetName(WId)

        pythonapi.PyCapsule_GetPointer.restype  = c_void_p
        pythonapi.PyCapsule_GetPointer.argtypes = [py_object, c_char_p]

        WId = pythonapi.PyCapsule_GetPointer(WId, name)


Hi David,

that’s why I need help!

The confusion is because (apparently) I accidentally created a Python class with the same name of an unrelated C++ class. At any rate the code is here and it is a class I modified from some code I found on internet (I don’t exactly remember where) to pass the interaction from Qt to VTK.

It did its job nicely on Windows, but as I tried to use it on Linux the VTK render window would pop up in a different window, i.e. making it unusable.

The code you posted is exactly in my class. But the SetWindowInfo does not seem to work on Linux.

However I think this is an old “hack” and I should instead use the QVTKRenderWindowInteractor to pass the interaction from Qt to VTK. That’s what I did and my new code works on Linux and Windows.



Hi David,

I now see that my class QVTKWidget in facts is a modification of the QVTKRenderWindowInteractor.py where I added a method to get also the alt key modifier and used it in stead of the one to get only shift and ctrl.

Now, the plot thickens. The same self._RenderWindow.SetWindowInfo(str(int(WId))) does not work in my case.

I believe I’ll change my class to inherit from VTK’s and then add the alt event modifier to it.


Your class uses the native vtkRenderWindowInteractor:

self._Iren = vtk.vtkRenderWindowInteractor()

but the QVTKRenderWindowInteractor uses the ‘generic’ interactor:

self._Iren = vtkGenericRenderWindowInteractor()

The difference is that the ‘native’ interactor gets events directly from the windowing system, but the ‘generic’ interactor gets all of its events from the Qt widget. The ‘generic’ interactor is the one that plays nice with Qt.



Thanks David.

I realised that the best solution was to extend the QVTKRenderWindowInteractor to get the alt modifier as well. BTW why don’t you get it as well by default?

In this way I will be keeping my code up to date with VTK code base rather than using a (random) version I found online.

Here my solution.



I don’t use this class, myself. But as you can see from the code comments, lots of people have contributed to it over the years. Info on submitting code to VTK is here.