Just subtract the frames and retain only those where the sum of the difference image is more than some specified amount. See my attached demo below in blue text.
% Demo macro to extract frames and get frame means from an avi movie
% and save individual frames to separate image files.
% Then rebuilds a new movie by recalling the saved images from disk.
% Also computes the mean gray value of the color channels
% And detects the difference between a frame and the previous frame.
