Well, if you are filming in darkness or semi darkness and want the muzzle flash to light up the scene for a frame or two then I can't think of a single way of faking that on a PC. Unless someone better than me has some ideas I think you're at the stage where it might have to be performed on camera i.e. for real.

Quick ideas to fool around with though are...

You might try syncing and setting off a still camera flash but that will likely set the whole frame to 'daylight'.

You might also try filming the scene in the darkness how you actually want it and also in daylight and then compositting a daylight scene still over the dark version and feathering it in to get the effect you want. But good luck trying to get the same shot exactly.

i think if you play with gamma and brightness for a frame or two, as you suggested, then you may get away with it. But any freeze frame on your movie will give the game away as there will be no dpeth to the light you've added. i.e. no shadows cast etc.

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