How to emulate realistic lighting with Illuminate 2D Shape?

Can confirm 2 and 3.
About 1, let me check again.
I think the posterization was present in the preview but I wish to check that again.

Sorry for the delay.

  1. The posterization effect is visible in Gmic preview.
    When I magnify the preview the effect seems less intense.
  2. Yes, the effect is present when the filter has been applied.
  3. Exported file has the same effect.