Query fraction of rendered pixels

I’m fairly new to VTK, so I apologize if this is already answered somewhere.

I’m rendering a large point cloud, and I quickly saturate the screen as the data size increases. I’d like to algorithmically describe this saturation, so I can look at ways to control it. Is there a way to query the render window to determine the total fraction of rendered points? I’d further prefer to handle this at the level of individual actors; is that possible?