For trajectory planning purposes, it would be helpful to have a "cutout" feature in the volume rendering module to allow seeing into the volume along the outline of a sphere, cone, or trajectory to some point inside the volume. A camera-based control would be great to adjust the trajectory to the current view angle so that the cutout is continuously centered as the volume is rotated.

Static cutout is available in Mask volume effect (in SegmentEditorExtraEffects), which are already available for users.

It would be nice to make shader-based real-time cutouts available to users as well: https://github.com/pieper/VTKCustomShaders.

The custom shader implementation is now available in a self test, and works with the current builds.

https://github.com/Slicer/Slicer/blob/master/Applications/SlicerApp/Testing/Python/ShaderProperties.py

It's still an open issue to develop a useful user interface.