Shoot the background footage first. Make sure the image is properly in focus, and the lighting is what you want ( day time, morning, night, raining, sunny,etc) You can always defocus and muck with color, etc in post.
The best thing to do is to shoot with multiple cameras from a moving vehicle to assure that the images are in "sync" when you do cuts between various windows. OR, just shoot the same path at the same speed multiple times, and lock up later if you need to.
Then Shoot the VAN on green screen. You can then time the positions of the plates in post depending on the edit.
When you shoot green screen the most import thing is to blow out the green and keep the lighting on the green flat with no hot spots in the lighting. Generally you can spot the green screen 5 or 6 feet or more away from the VAN so that the green is very blown out, a solid color of green and its far enough away so if you use a 50mm or similar lens for close ups in the van the green screen is out of focus helping the green to be blown out and a smooth color.
Once you are all set you can then play with "passing vehicles and shadows" to give the illusion of things going by... cars, light poles, building shadows etc... with the use of specials and flags, etc. ( as well as rocking the van etc )
The other reason for shooting the mattes first is to assure that you light the van in similar light and style so it the two images match. ( the van and the background footage )
Once you have this done, then its just a matter of keying out the green and spotting the background where you want it in post and you don't have to "play" the video in real time while shooting the VAN scenes. so its a bunch of easier for the crew and actors.
This will give you the option of doing this quite well... or as "cheezy" as you want in post.
...much easier than running a video projection and trying to get it "right" live.
cheers
geo
Georgia Hilton MPE MPSE CAS
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