Hi @FrancoisW
You haven’t said if your application is to be open source or commercial.
If you go down the C++ route, using QT for commercial purposes requires a license but Paraview provides an example of how to do it. Otherwise you can use wxWidgets and find your own way. Also setting VTK_MODULE_ENABLE_VTK_GUISupportMFC=ON in cmake generates a vtkMFCWindow class in the source, which shows how to wrap vtkWin32OpenGLRenderWindow.
AFAIK Activiz licenses cost €500 to €1000 per annum and Activiz doesn’t handle string conversion to/from UTF-8 for VTK, so you’re stuck with ANSI text only.
If you’re already familiar with Java that might be your best bet. You can build VTK with VTK_WRAP_JAVA=ON and study some of the examples.
https://vtk.org/Wiki/VTK/Java_Wrapping
You have to create/compile the VTK jar manually on Windows. VTK 9 and Java- vtk.jar not automatically generated. Also location of wrapper dlls - #19 by toddy