Difference Matte Effect

[PCORBET]

I have been able to use Track Matte Key with video. I have been able to use Difference Matte Effect with JPEG & PSD. I am unable to use Difference Matte Effect with video. Does anyone have any ideas as how to use this. Thanks I use PSE 7 and PRE

[RJ Johnston]

Here's a description of the effect in Adobe After Effects help on Adobe's web site:

http://livedocs.adobe.com/en_US/AfterEf ... -7b4b.html

[Bob]

Difference Matte is a keying effect. It's similar in concept to a green screen key where you film an actor in front of a green screen and the pixels that match the green screen are removed leaving just the actor. However, Difference Matte uses an image of the background rather than a green screen. You need two shots: one of the background without the actor, and another with the actor in front of that background. The Difference Matte effect then compares the two shots pixel by pixel and removes the ones that match leaving just the actor since he wasn't in the background shot.

It sounds good, but it's really only effective if your background is static which really limits it's use. If there are any differences in the background at all -- changes in lighting, motion of either the background or the target clip, wind blowing something, etc. -- the keying will be poor. You must shoot using a tripod or the equivalent to hold the image rock solid. If shooting video, you can film a few frames of just the background and then follow up with the action. In editing, you can take a snapshot of the background and use that as the difference matte. It really doesn't make sense to use video for both the matte and the target clip, although the effect won't prevent it.