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I have a video registration of a conference: fixed camera and fixed background. Is there a mean to keep the person talking and substitute the background with another image? I am wondering whether is possible to compare pixel by pixel the "empty" shot with the shots with people in the scene, in order to remove the background. I do not want to use chroma key or green screen because the background has not uniform colors.

Stefano Pellegrini
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    Have you thought of Rotoscoping the talking person? I think it's well suited for what you're after. Comparing pixel by pixel even if possible would leave a lot of mess (leftovers) in the shot I think. – Aardo Sep 16 '15 at 10:54

1 Answers1

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The compositor is able to compare pixel by pixel two images (and movie frames too). With the help of a difference node you'll be able to isolate the fixed (given) background from the subject.

Here's a possible node setup:

enter image description here

Obiuvsly the result can't be as good as it would be with the use green screen, expecially if the subject has some colors in common with the background. See for example in the center of Suzanne how the despecle node wasn't able to reduce the dust.

Furthermore the test video in the image above was made with a CGI camera, that produce less noisy images. In a real footage, each frame sligthely differ from the previous one, and this would certently not help in making a sharp mask.

Anyway you can use this technique as a starting point and integrate with some manual masking, composite filters (ex. blurs, tweaking the despecle,..) or even build your own algorithm to take in account also the previous and following frame's pixel colour.

Carlo
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    In my experience, unless the shot was originally planned for keying out the background (ie. Greenscreen or any evenly lit flat background), you are in for a lot of time/work cleaning up the edges. Rotoscoping might be hard to avoid. The results with the difference key are going to be uneven if there is video compression involved... If the camera moves, even slightly the difference key won't work. –  Sep 16 '15 at 14:25
  • And even simple exposure changes (from auto settings on camera) can ruin the subtraction process. – 3pointedit Sep 17 '15 at 13:32