[vtkusers] Display different ImageData Scalar to different Viewport

[Ruben Di Battista]

Hello,
I'm playing a bit with VTK and its Python bindings. I wrote down a simple
case where I generate the distance function of a sphere, I extract the
surface from it using the FlyingEdge3D algorithm, then I calculate the
curvatures using the vtkCurvatures filter, and then I'd like to show the
Mean_Curvature and the Gauss_Curvature in two different viewports.

What I tried to do is to use the `SetActiveScalars` method of the
`PointData` associated to my `ImageData` dataset, but at the end I get two
Viewports showing the same scalar (the last one activated). I also tried to
stop the program execution using a debugger and triggering the render
before the second scalar is made "active", and it correctly shows the first
scalar field. So It's like that the two mappers rendered are "linked".

Do I need to do a ShallowCopy somewhere?

Here below the `Sphere.show` method. The full code is attached.
`self.surface` is an `ImageData` dataset.

    def show(self):
        # Render Window
        rw = vtk.vtkRenderWindow()
        rw.SetSize(800, 800)

        # Defining Bounds of Viewports
        xmins = [0, 0.5]
        xmaxs = [0.5, 1]
        ymins = [0, 0]
        ymaxs = [1, 1]

        # Curvature Types
        curv_types = ['Mean_Curvature', 'Gauss_Curvature']

        for i in six.moves.range(2):
            render = vtk.vtkRenderer()
            rw.AddRenderer(render)

            # Mean Curvature View
            render.SetViewport(xmins[i], ymins[i], xmaxs[i], ymaxs[i])

            # Create mapper
            mapper = vtk.vtkPolyDataMapper()
            self.surface.GetPointData().SetActiveScalars(curv_types[i])
            mapper.SetInputData(self.surface)
            mapper.SetScalarRange(-1, 1)
            mapper.Update()

            # ColorBar
            color_bar = vtk.vtkScalarBarActor()
            color_bar.SetLookupTable(mapper.GetLookupTable())

            # Surface Actor
            actor = vtk.vtkActor()
            actor.SetMapper(mapper)
            actor.GetProperty().EdgeVisibilityOn()

            render.AddActor(actor)
            render.AddActor2D(color_bar)

        # Render Window Interactor
        rwi = vtk.vtkRenderWindowInteractor()
        rwi.SetRenderWindow(rw)

        rwi.Initialize()
        rwi.Start()


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