Udacity - Hand under notes video effect In a typical Udacity course, the hand written course videos are displayed with the writers hand beneath a semi-transparant view of the notes (1:02 in video linked below).

I want to replicate this to record some tutorial videos, but am stuck on how Udacity managed to achieve this effect.

Here is a video of their set up:
http://blog.udacity.com/2013/06/coffee-break-episode-6-inside-recording.html

And here is some more information from one of the teachers:
https://medium.com/teaching-learning/1d7c77b857de

From what I gather, they have a Tablet that they draw on, which they then use some form of screen capture software to record the video of the notes. 

At the same time, they are recording a video of the teacher writing on the tablet.

With these two videos they seem to be chroma keying the screen capture out of the video of the teacher's hand.

How are they achieving this effect? If they are chroma keying the screen capture out of the teacher's hand video, how can this be achieved?

Thanks
Most likely, they are simply using an output of the notes with transparency.  If you output the digital version of the notes with a transparent background you could just layer it over the video once properly aligned and it would work fine.  

Alternately, a quicker process if they do not support alpha is to just apply a blend between the layers.  Since the background matches the video except for where the hand is, it would blend very smoothly.

Finally, what is known as a luma-key could also be used on the screen capture file to generate transparency anywhere that the luminosity (brightness) differs from the empty background.

Any of these options could be used and they are all extremely easy to use, though the first requires the most advanced technique to generate the alpha content, but would also produce the best results.