Many of us have had attempts of filming stuff on our webcams often with disasterous outcomes - but I was wondering how you can change backgrounds?
Remember in the film "Forgetting Sarah Marshall" where the lead role would talk to his mate via his laptop webcam where he could select ANY background he chose - beaches, deserts, snow without having anything behind him? What software allows that and more importantly how does it work?
The wallpaper in my place really isn't so inspiring 
It's called chromakey. Watch the "Dr Who Confidential" programmes, as they use a lot of this for doing CGI monsters etc.
You film/record the foreground objects with a background that is plain green or plain blue.
Then in the editing suite later you mask out all that colour and substitute material from a different recording.
Not something an amateur can easily do.
Another method is where the background behind the actors is actually a fine mesh white reflective screen using the cats-eyes principle. (The same technique as used to make reflective signs on motorways.) Alongside the camera is then a movie projector which projects the background, and which is then reflected by the screen back into the camera lens. It is also projected onto the actors and their clothes, but the refection back from them is minimal and not noticable.
You can make a green or blue screen at home without super professional equipment:
For example: http://uk.youtube.com/watch?v=EGGJ2HgDiG
But you will need software that lets you "key out" the coloured background.
After Effects is the industry standard. Not sure if there's any freeware out there that does it but there are applications like this:
http://www.greenscreenwizard.com/
If you do try it, one thing that's absolutely essential is getting a strong, even light over the entire green/blue screen - you'll need at least two anglepoise lamps with very bright bulbs. The level of light on the background needs to be at least equal to the light on your subject.
Also, keying out the area around a person's hair is often difficult. It helps to slick down your hair or be bald. Picture what the person you're shooting will look like as a silhouette - if there's hair frizzing out in all directions you're going to get a lot of "noise" around their head when you try to separate them from the background.
I'm sure David Bussell has greenscreened some stuff himself with good results.
Quote: Badge @ May 27 2008, 11:36 PM BSTI'm sure David Bussell has greenscreened some stuff himself with good results.
Actually my director did - he takes care of all the technical business. Here's how our first attempt turned out...
http://www.youtube.com/watch?v=4HWepvJpG3o
That was done using a professional greenscreen and Final Cut Pro, as was this...
http://www.comedybox.tv/comedy-sketch-picks-11327
This one was a little more ambitious in scope yet not quite so polished. You can see the trouble we had with chroma key around the hair, just as Grey Bloke describes.
I'd recommend laying your hands on a copy of After Effects/Final Cut. Once you've done that there's plenty of information available on technical forums etc telling you step by step what you need to do. Just Google around.
Good luck and don't forget to post the end results!
So I was half right! That's 50% better than usual