How do you place clip in background with foreground objects?

[A.D. Reis]

I'm trying to get a clip in the background of another clip while also have objects move in front of the background clip. I was wondering how is this done. I have already tried using a composite layer such as multiply and it doesn't give me the effect I'm looking for.

http://youtu.be/Fvs6YaPMQ4A?t=3m31s from this clip around 3:31-3:38 is somewhat what I"m looking for with background images going on in a foreground clip. I"m not sure if this is an image mask or what exactly.

I use Final Cut Express HD on a mac

Any help at all would be greatly appreciated. thank you.

[Eake4]

Nostromo lol, well I would use a motion mask on the top layer and have the background just under the masked layer. Its a technique which requires an intermediate amount of time but gives a stunning look to videos. Good luck

[A.D. Reis]

Nostromo lol, well I would use a motion mask on the top layer and have the background just under the masked layer. Its a technique which requires an intermediate amount of time but gives a stunning look to videos. Good luck

Ah I see. So basically mask each frame as it moves over the backgrounded section? I can see how that will take some time lol. thanks for the info.

[Eake4]

You mask the first frame, and in each frame you can just change the anchor points to different places which will create more keyframes, if your not too experienced with keyframing (which i doubt)
you should learn that first (takes a few minutes lol). And yeh, just keep changing your anchor points and different frames to match your borders of the character, building etc.

[Mister Hatt]

I don't know much about effects in FC but you can do a roto mask for the frame you want to insert it into via Mocha or any good AutoDesk effects program. You can then load that into AE or another effects specialised app, or otherwise render the mask in black/white and feed it to avisynth. Using the rotation information and any perspective stuff, you can then move the secondary clip as you normally would in a real program. If you can program at all, simple enough to write a quick Mocha parser in Perl or Ruby to generate the avisynth effects for any translations. As long as you have a good rotoscoping app, it's not that hard.