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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?


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2 Answers

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.

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Thanks for the answer AJ. The part that I'm stuck on is removing the background from the raw video of the hand. In the tutorials, the background is pure white (and not the grey tablet with a screen that is being filmed). I'll keep looking. –  Jack Jan 12 at 20:20
@Jack - removing the background from the video with the hand is the wrong way to do it. You work from the digital version of the notes that knows exactly what is stuff they drew and what is background. Then you layer those notes over the video from the actual camera, just the way it is, no alterations necessary. –  AJ Henderson Jan 12 at 20:31
thanks. How would you get a pure white background though? By layering the notes over the hand video, you would see the grey finish of the tablet. –  Jack Jan 12 at 20:50
@Jack - the easiest way is just to lower the white point so that grey becomes white. If you notice the edge of the hand (at least in the video clip you linked) is a bit washed out on the brighter end. –  AJ Henderson Jan 12 at 20:54
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What with the felt-tip pen and the transparency of the writing surface, this actually looks more like they're writing on a transparent whiteboard. Probably perspex, or plexiglass.

This type: Whiteboard

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Thanks for the answer Callum, from all of the information in the links above it seems that they are definitely creating the final video using a composition of two videos and some processing in post. –  Jack Jan 12 at 20:18
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