Frame Differencing Tricks with Processing

I haven’t been posting anything lately but I have a lot helpful bits of code backlogged on my machine and I am going to try to share and explain some. This first section has to do with frame differencing. The purpose of frame differencing is to constantly check the way each frame in a movie or video changes and respond accordingly. For instance, you could use frame differencing to track something that is moving on screen. That is basically what this piece of software does. It has a “palette” frame in which only moving pixels are drawn to the screen. This creates an interesting effect. Here is the source code : frame_diff_movie.pde. I also have a ‘live’ version that does this algorithm in real-time on webcam input available here : frame_diff.pde.
Here is a brief rundown on how the code itself operates. First you chose a reference frame by hitting the space bar. You usually want to do this when the movie is only showing the background and the subject has not yet moved or entered the frame. This frame is stored in [ int[] previousFrame ]. Now every-time a new frame is read from the movie file, our algorithm compares each pixel in the new frame to each corresponding pixel in the old frame. If the movementSum (amount of movement in the frame) is greater than the globally defined TOLERANCE integer, it draws that pixel to the screen. You can hit the ‘r’ to start recording into a video output and hit ’s’ to close the outgoing movie stream. Here is a video of my friend Christian trying to write his name on his bike:

http://www.vimeo.com/1282940