This notebook contains code to detect the tip of an air-pen* in front of a camera which allows us to write in the air in a video. The concept air-pen: We choose a color which is not present in the background and find a pin pointed object with a tip of this color. We detect the trajectory of this point and mark point on the video where this point has travelled. Once done, press Q to save this video. While the video is running, we can see the writing and ourselves on the screen, so that we have an idea of what we are writing. We finally get a video which looks like we wrote in the air.