Recording Amnesia on Mac

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Sorry, I never used a mac so I can't offer any real advice here, but - just a crazy idea Have you tried setting the screen brightness outside of the game? Like, in your graphic card settings or whatever the mac equivalent of that is?

If nothing else helps you might also be able to easlily change the brightness of the video afterwards using free video editing software like VirtualDub. Google tells me a similar program for Mac would be AviDemux, but you probably know that better than me