To get started: Have a look in the examples: Libraries>Video>Frame Differencing. In the console output you will see a number. This is the amount of sum of the values of the pixels that have changed compared to the previous video frame. You could introduce a threshold and activate your projector, when this value is greater than the threshold, say for 5 frames in a row. There are better ways to do this. (Adaptive background subtraction, blob tracking, person tracking). But this approach is cheap and can get you quite far.