How to layer shots to double actors (arithmetic?)

I want to do that effect they use for motion control shots and lock-offs where the same actor is shot twice, then combined in the editing so only the actor is doubled.

I have two static shots of my room from the exact same location. The first one, I walked to the left, the second to the right. I want to "add' them together so that it looks like there's two of me, and "I" walk in together.

Do you know what I mean? Used by Michel Gondry.

I've tried the arthmetic and compound arithmetic functions with no luck.

Masking is the only way... Mask "top layer you" over "bottom layer you" until they're clear of each other, then just use a split-screen from there.

The simplest way is to put the version of you walking in on the left on V1 on the timeline, then the version of you walking on on the right on V2 on the timeline.

Then select the V2 clip, double click it to load it into the viewer and go to the 'Motion' tab at the top of the viewer. Here, find the 'crop' section and click the small black triangle next to it to reveal the crop tools. Then increase the slider for 'left' so that you reveal the left hand side of the bottom clip by removing the left hand side of the upper clip.

If the shot was locked off properly and there is no overlap between the two 'yous', it should work fine.

The split screen did work for the times when I didn't overlap, but...i did overlap a little.

Isn't there a way to take a plate, (the room with no me's in it) and use that to erase everything in a shot except what is different from that plate?

I thought there was a relatively easy operation that could accomplish this.

Thanks for the help.

Not to the best of my knowledge in FCP. What I think you think you can do is use a composite mode, but this won't accomplish what you want.

The best you could do in FCP would be to rotoscope using a keyframed mask or matte. Try the free 50 point bezier matte plug-in here : [homepage.mac.com]

I remember a discussion about this a couple of years ago, might have been on Ken Stone's site. Yes, something about a "Difference" composite mode. I don't have any way to experiment with it right now, but I think that's what was talked about. Couldn't tell you how well it works, but it's theoretically possible.

Which reminds me:
"In theory, there is no difference between theory and practice. But in practice, there is."

Scott

I think you are thinking of Key > Difference Matte. You'll need a clean plate to perform this effect. Hopefully you have a locked down a shot without any actors in frame.

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Oh - I thought they meant change the composite mode to Difference, then key out the black, which wouldn't work very well. Gonna have to experiment with the difference matte now.

I've tried the difference matte with no luck yet. I drop a clean plate into the drop box of the effect, and still can't pull "me" out of the frame.

I also realize that if I COULD get it to work as I assume it's supposed to work, then I still would have a hard time in places where my black shirt blended into the shadows.

Next time I'll wear brighter clothes...

Sounds like you didn't light the scene or were not wearing something significantly different than the background, or both. Can you reshoot?

Kevin Monahan
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Layer 1 "clean plate"
Layer 2 "green screen" on set in front of plate with actor fore ground
Layer 3 "green Screen" on set in front of plate with actor fore ground