I was trying to fix near plane clipping issues in the parallel projection mode. It is very easy to get into a scenario where, in the parallel projection mode, the camera is at a position where there are one or more actors BEHIND it. In this scenario, Zoom - out does not help to put those actors in FRONT so that they dont get clipped by the near plane. In perspective projection this problem does not occur because as we zoom out, the camera actually moves farther from the focal point. So the actors BEHIND the camera do come in FRONT eventually.
Looking at the Dolly method in the vtkInteractorStyleTrackballCamera, it is clear that the camera does not move in the parallel projection but does move in the perspective projection. By removing this check for projection mode and moving the camera irrespective of the projection mode, I am able to get rid of the near plane clipping issue in the parallel projection. I am still setting the parallel scale to ensure the zoom in/out makes objects appear larger/smaller in the parallel projection.
My question is why is the camera not moved in the parallel projection when zooming in/out in the VTK source code. Is there anything inherently wrong ?