Mac OS X 10.5: Using iChat Backdrop Effects
Backdrop Effects is a Mac OS X 10.5 Leopard iChat feature that "removes" the background of your video chat and replaces it with a picture of your choosing, leaving you in the foreground. It works much like a "green screen" used by actors in a Hollywood studio.
This works by taking a snapshot of the background when you are not chatting. Since the computer knows what the background is, parts of it can be replaced with images or movies. Since you were not part of the background when the snapshot happened, you will not be ignored, hence you or any other object will be able to move "on top of" the background.
Tips for using iChat Backdrop Effects
To avoid "holes" in your foreground image (in other words, in your image), make the background as uniform in color as possible, and make that color as different from colors in the foreground as possible. Your background image should not include color shades that match your clothing, hair, or skin tone. For example, if your Backdrop image is mostly a bright blue sky, and you are wearing a similarly bright blue shirt, "holes" may appear in your shirt where it overlaps the background sky.
If any part of the background moves after the detection, the moving objects will not be part of the background anymore. "Holes" will appear. You should pay attention to shadows that are part of the background but move when the user moves back in front of the camera. Make sure not to move your camera after the background snapshot takes place.
Reflective surfaces in the background should be avoided.
A dark room makes it harder for this effect to work. Make your area as well-lit as possible.
Note: You can press Command-Option-R to reset the background at any time in iChat.