https://www.youtube.com/watch?v=KKyQsnwJMrM
go to 18 minutes and 13 seconds-how do  you do it with GIMP?
if you don't want to watch to 18 minutes and 13 seconds  here is the set up:
He took multiple photo of the exact same scene holding a flash over various
sections of his scene to shine light on the area. (he was in the shot) Then
he put the photos on different layers.... I think I know.... he then
removed the pixels from the layer on top which were darker.Is that correct?
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:-)~MIKE~(-: