I was looking at the IGI algorithm for VPL rendering. A well known issue is the spikes\singularities resulting from a high Geometry factor in the rendering equation. The issue is well laid out by Simon Brown at his blog:
"The second issue is a problem because the optimal b (one that removes just enough energy to hide the singularities) will vary with light path density in the scene, and exposing a parameter for something generally means it will be set to something very inoptimal."
A simple issue that comes to mind with a fixed limit value is that just scaling the scene will require a different limit value to get the same effect.
I was looking at literature concerning this and didn't find much help, probably I missed some important hints, not sure. Any thoughts?