Hello,
Ok, I have a clip of dark scene in which bright flashes of laser light impinge on an object.  No problem there.  To isolate the flashes of light, I've gone to a point in the clip where there is no flash, created a still image, and then used this as a child of the former track.  After setting the composite mode to difference, I get bright flashes of light on essentially a black background.  Now, without moving the camera, I light the scene normally, and create a still.  I put this on a track above the former parent/child track, and then adjust the opacity until the overal scene looks 'right'.  My question is:  Is there another (better) way of doing this?  I'm thinking along the lines of all the black in the former parent/child track becoming alpha, and then I put the parent/child track over the well lit still.