This is a much faster way to grab frames from a video. Seeking aftering the -i flag causes the video to load all frames right up to $frameTime. In a large movie file this is extremely slow. Instead, you can seek quickly before the input file is loaded with -i, but this isn't quite as accurate as having the -ss after the -i. But, there's a fast & accurate way to seek ahead, which is to quick-seek to a point close to $frameTime, and then accurately seek from there to $frameTime using a second -ss option. See http://ffmpeg.org/trac/ffmpeg/wiki/Seeking%20with%20FFmpeg for a thorough explanation. This makes grabbing frames more than a minute or two into a large movie 100x (or more) faster as it gets progressively slower the further into the video the frame is with -i before -ss.