I'm currently enrolled in a coursera.org course which has the presenter standing in front of the camera with a completely black background behind them. While presenting, he would drop directly on the screen as if there's a transparent piece of glass in front of him as shown in the screenshot. I'm wondering how is this possible and what kind of software/hardware could allow this as I'm sure he is not typing backwards. Is the picture inverted and he's really drawing in a glass in front of him or there's another trick?enter image description here