[vtkusers] Clipping problem (?) trying to use vtkCubeSource as selection marker

[Elvis Stansvik]

Hi all,

I'm rendering a cylindrical volume and need to let the user select a
segment of the volume.

As a "selection marker" I thought I'd, for simplicity, just use a
semi-transparent cube with "no shading" which I put around the segment
of the volume to indicate the selection:

    vtkSmartPointer<vtkCubeSource> m_selectionMarker;

which I then configure with:

    auto selectionMarkerMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
    selectionMarkerMapper->SetInputConnection(m_selectionMarker->GetOutputPort());

    auto selectionMarkerActor = vtkSmartPointer<vtkActor>::New();
    selectionMarkerActor->SetMapper(selectionMarkerMapper);
    selectionMarkerActor->GetProperty()->SetColor(0.0, 0.0, 1.0);
    selectionMarkerActor->GetProperty()->SetOpacity(0.1);
    selectionMarkerActor->GetProperty()->SetAmbient(1.0);
    selectionMarkerActor->GetProperty()->SetDiffuse(0.0);
    selectionMarkerActor->GetProperty()->SetSpecular(0.0);

    renderer()->AddActor(selectionMarkerActor);

Now, my interactor style is such that the camera is locked to move on
a cylinder around my subject volume, and the camera is always uses
parallel projection.

When the camera is modified (zoomed or panned "around" my subject), I
do something like this to update the selection marker cube source:

    int *windowSize = GetRenderWindow()->GetSize();

    // Half window width in world coordinates
    auto halfWindowWidth = renderer()->GetActiveCamera()->GetParallelScale()
            * (double(windowSize[0]) / windowSize[1]);

    m_selectionMarker->SetBounds(
        -halfWindowWidth, halfWindowWidth,
        -halfWindowWidth, halfWindowWidth,
        start,
        end
    );

where start and end are the start/end of the selection.

This seems to work fine, and the selection has the look I'm after (see
looks_fine.png).

But, if I zoom out quite a bit, I get a strange look (see looks_strange.png).

Anyone know what is happening here? Is the cube clipped perhaps? Each
time the camera moves or is zoomed, I also reset the camera clipping
range with

    renderer->ResetCameraClippingRange()

So I wouldn't expect the cube to be clipped...

Thanks for any advice.

Elvis
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