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Difference keying and matte pulling

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lorraine collardDifference keying and matte pulling
by on May 7, 2010 at 10:39:49 pm

Hi All,

Can anyone point me in the direction of a good tutorial, preferably video, on Difference keying. I am familiar with chroma keying but have been told to learn this technique to help my studies. I understand it is taking the two pieces of footage, and producing the difference between them. Can anyone help, in fact can anyone point me to any tutorials on pulling mattes too, any dvds etc.

Thanks you for any help offered.

thanks Lorraine

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Dave LaRondeRe: Difference keying and matte pulling
by on May 8, 2010 at 12:19:49 am

[lorraine collard] "I am familiar with chroma keying but have been told to learn this technique to help my studies."

Oh, dear, that's too bad. You see, difference keying is not something you really want to do in AE. Now, it's not the fault of the software, it's due to the footage.

You've got the basic principle: AE compares a clean plate (typically video with no subject in it) to a piece of footage (the same shot, but now with a subject in it. The effect then finds where the pixels match, and creates transparency.

Sounds great, right? Sure does! Well, then how come nobody outside of AE neophytes uses it? A Difference Key almost NEVER looks good.

To achieve complete transparency the pixels have to correspond PRECISELY from shot to shot: same position, same color, same intensity, same everything. But most cameras can't do that. Okay, a Panasonic cine camera with a $79,000 price tag (minus lens) can, but not the kinds of cameras mere mortals tend to use. Our cameras tend to have lousy color resolution and noise. We immediately have two strikes against us. Now, I'm talkin' Sony XDCams, and Panasonic P2 cameras here... Lord only knows what the result would be on one of those cute lil' Kodak HD cameras the size of a deck of cards.

Furthermore, we tend to shoot ourselves in the foot with this effect: we tend not to lock down the shots as we should, we tend to think we can build an absolutely perfect clean plate, we tend to overlook the fact that a cloud just passed over, thereby ruining our clean plate... a host of things. And there's Strike Three.

AE's online help does a good job of describing the Difference Key: just look in the effects section under Keying, and you'll find it. Heck, it's dead simple to use: just a couple of parameters to tweak. So you know what that means? The success of the key is up to the quality of the footage, that's what... and we've been over that already.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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lorraine collardRe: Difference keying and matte pulling
by on May 12, 2010 at 5:53:56 pm

Thanks for your help Dave, I have since tried this in both programs and have to say that fusion did a slightly better job in this case, however, like you said, this technique is rarely going to satisfy and I can now understand why.

many thanks, Lorraine

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