Chroma Key – kinda – for every background?

[Flying-V]

I've been using VEGAS Pro 8 for years for basic Video Editing.
As I don't edit Videos very often, please bare with me if the following is pretty basic. Thank you.

Imagine in a studio you direct a camera to a colorful and complex backdrop (like photo of a city) and start recording.
Camera keeps perspective perfectly steady.
Now a person walks into the scene.
Mathematically speaking every subsequent frame with person has in common with the initial frames the area around the person. So it should be possible to subtract the very pixels from subsequent frames that match pixels of the initial frame when the person was not present.
Is there some sort of software or plugin that is able to compare a give frame with subsequent frames in order to remove/replace common areas completely – analogous to Chromakey?
Can this be achieved with some sort of Mask defined by a frame, perhaps?

Thank you very much in advance.

[Cornucopia]

Not sure about other tools, but After Effects (and by extension, Premiere Pro) can do this, using a Difference Matte. Also, I am pretty sure AVISynth can do this as well, with some combination of overlay/layer and compare, and median to help create the proper mask.


Scott

[Flying-V]

Thank you very much!

Sometimes it already helps to search for the right keywords.
So I found a great little Tutorial to apply the idea in Vegas.

Just in case you are looking at this thread...
This is a quick and helpful video about how to use a couple of filters to get going:

https://www.youtube.com/watch?v=979wGcXoBWI

[jagabo]

Beware of issues like shadows and other variations in the lighting. I was writing software to do this with webcams 25 years ago. Every now and then the background would become visible again. I kept looking for a bug in the code. Eventually I realized it was a partly cloudy day so the amount of light entering the window of my office changed with the cloudiness.