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Motion detection with still background, moving subject

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Hyung Jin Lee
Motion detection with still background, moving subject
on Sep 11, 2009 at 1:10:40 am


I have a footage with a completely static background and a subject that moves about. I need to mask the subject out from the background so that I can add effects to the background.

Because of poor lighting, the contrast between the edge of the subject and the background isn't too good at times, and motion tracking points around the edge of the subject fails because of that.

I wonder if there is another tool to accomplish this? I can't imagine this is very difficult, at least in terms of the algorithm involved -- say, if a pixel (or an area of the video) remains constant from one frame to the next, then consider it as part of the background; otherwise, if the pixel changes, consider it as part of the subject. Something along those lines.

So far the only option I had was to manually mask the subject; however, with 12 seconds of footage at 30 fps, you could imagine that the progress isn't very satisfactory.

Any suggestions would be very much appreciated.

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Stuart Elith
Re: Motion detection with still background, moving subject
on Sep 11, 2009 at 4:39:22 am

Unfortunately, the general consensus seems to be that rotoscoping (masking) is your best option in these circumstances. There is an effect, Difference Matte, which tries to do what you are saying by comparing pixels, but it doesn't provide good results usually, for many reasons. Most people don't bother with it.

Many will suggest you go for a reshoot for best results, with a greenscreen background you can key out. It certainly makes things less painful... otherwise, I think you're stuck with roto.

But you will find that 12 seconds actually doesn't take that long... and you will learn through the process, roto does get a lot quicker once you get into it (though it is always a manual procedure).

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Dave LaRonde
Re: Motion detection with still background, moving subject
on Sep 11, 2009 at 2:55:44 pm

Stuart speaks the truth. Read his reply very carefully. It sounds like you've just run into a basic truth about producing good-looking effects:

Dave's Stock Answer #2:

When you're out on a shoot, and you say, "we'll fix this in post" without knowing PRECISELY HOW you're going to fix it in post, don't shoot it! You'll only end up shooting it over again.

Since post typically costs three times the cost of production, fixing something in post is not a way to save money, but rather a way to spend more of it.

And, before you say "well fix it in post," always consider who's doing the work, especially if you're the one doing the editing.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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Kim Segel
Re: Motion detection with still background, moving subject
on Sep 16, 2009 at 3:14:04 am

Try the "Time Difference" effect.

Paint out any leftover stuff and you might add a garbage matte to help.

Good luck...

A computer once beat me at chess, but it was no match for me at kick boxing.

--Emo Philips

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