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iChat background replacement... how is it done?

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iChat background replacement... how is it done?
on Aug 7, 2006 at 7:58:49 pm

In the Leopard preview at WWDC they showed that iChat can now do real-time background replacement without a green screen:

Just curious--how is this done? The text on Apple's site implies that you may need one of the new, built-in iSight cameras for this to work, so is it using some infrared trickery to pull a key or is it using a motion-based algorithm... or something else?

I've spent a great deal of my life trying to remove backgrounds from videos of various sorts and I'm wondering if the technique used here could be applied to Final Cut or After Effects to help with difficult keys.

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Jeff Carpenter
Re: iChat background replacement... how is it done?
on Aug 7, 2006 at 9:08:17 pm

You're thinking way too hard. The solution is in the instructions: They say to step out of frame first.

Here's my guess on how it works:

The moment when you are out of frame the computer memorizes every pixel that it sees. From then on, it only records pixels that are DIFFERENT from the ones in its memory. That's it!

As you can see in the demo, however, it's not a very "clean" key. It's certainly good enough for chatting with your friends, but I don't think you'd be using that method in After Effects anytime soon. It also makes me think that if you do this on a laptop you'd have to re-set the background key every time you adjusted or moved the laptop.

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