Thanks for sharing @jaswantp!
I had a go on integrating vtk+ImGui: GitHub - phcerdan/vtkImGuiAdapter: Adapter to add imgui to an existing vtkRenderWindow using SDL2.
vtkImGuiAdapter provides two classes: vtkImGuiSDL2OpenGLRenderWindow (subclass of the vtkSDL2OpenGLRendeWindow available since 9.0) and vtkImGuiSDL2RenderWindowInteractor.
vtkImGuiAdapter takes the path to override Frame and let ImGui loop control the re-rendering. The render window is owned by VTK, and ImGui writes on it. I am sure optimizations can be done by someone with more OpenGL knowledge than myself to avoid some unnecessary re-renderings.
It has worked for me pretty well to put up some demos really quick.
Here is the code example that generates that gif.
Link to the discourse post where I first introduced it