How to do an overlay?

[slicebogey]

I want to say it's an overlay but I'm not certain. I'm trying to create an effect that you see on certain networks. While the main movie is playing, in the lower corner (mainly right) an advertisement for the next movie might appear of a main character walking onto the screen of just the characterwith no background but the movie that is now playing is being the background. How is this effect acheived?

[edDV]

How do they do it or how would you put a simple graphic overlay over a backgound? For the latter you use the integrated graphic overlay function in an editing application (Adobe, Sony, ULead, Pinnacle, etc.).

If you want to do it from scratch, you create your graphic in an animation program with an alpha channel. Then you shoot the live action over green screen and create a masked foreground with alpha (Google "chroma key" or "Ultimatte"). Then you combine those foregrounds with your movie background in a compositing application or compositing hardware.

http://en.wikipedia.org/wiki/Compositing
http://en.wikipedia.org/wiki/Alpha_channel
http://en.wikipedia.org/wiki/Chromakey