First, create a scene containing volumes, hits, and trajectories.
/vis/scene/create /vis/scene/add/volume /vis/scene/add/hits /vis/scene/add/trajectories
Open the OpenGL stored viewer to use the backing store, which will save the image even when another window is moved on top.
/vis/open OGLSX
The immediate viewer does not use the backing store.
/vis/open OGLIX
If the immediate viewer is covered up, the scene needs to be manually refreshed to redraw the image.
/vis/scene/refresh