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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?

Thanks

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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.

  • 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 '14 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 '14 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 '14 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 '14 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

  • 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 '14 at 20:18
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Udacity has answered your question here http://blog.udacity.com/2014/09/udacity-videos-transparent-hand.html

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    It's recommended to paraphrase the solution given at the link, so should the link go dead, future readers aren't left in the dark. – Gyan Apr 20 '16 at 12:10
  • Sad but true: the ethos of StackExchange is that answers should be sufficiently complete as to be practical despite the death of external links. A definitive external link could be a "see also" or "see for more details" but the actual answer needs to be present in the Answer. – Michael Tiemann Apr 20 '16 at 12:11
  • Yeah, it would certainly be preferable to have more details from the link, though in this case, with such a direct answer, I'm not going to mark it as needing more detail. It would be a super improvement to include some of the details, but you can't get a much better response than having the details straight from the source. – AJ Henderson Apr 21 '16 at 14:49
  • What is not clear is what they meant by overhead camera, is it the camera of the tablet, if so how does it record the back of the hand, ie if it is a camera outside ie somewhere in the room or say on top of the teachers head then how do they synchronise the relative position of it with respect to the tablet. – ishandutta2007 Aug 23 '18 at 10:13

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