I often try to figure out how shots were done that I see in films, and I enjoy learning about the different crafts and tricks in cinematography, but now and then I see something that leaves me scratching my head for days, and I know of no places where you can check online on how they do things in films.
There was this one shot I saw a while back in a film (can't remember what it was called) where they had a low angle dolly shot tracking slowly around a table where a guy was drinking coffee in a train station or something. What got me was that the shot seemed to contain both 'real time' and time lapse.
The whole station with people passing by happened in the blurred-time-lapse way you can expect any time lapse to do, but the guy drinking the coffee as the main subject in the shot was doing so in 'real time', while the rest of the world just blurred and rushed by him, all in a single dolly track.
Any thoughts? I can only think that it was a completely posted success with a combination of the time lapse and the real coffee shot, but then it offers the question of how the tracking was done. I don't know.
This was most likely done with a motion control rig. The camera is placed on a special, computer controlled, motorized dolly and track system. The move is created and stored on the computer to be able to replicate it perfectly over and over again. The background plate (i.e. people moving in the background) is shot at one frame rate/shutter angle and the foreground plate (i.e. the man sitting at the table) is shot at a normal frame rate. The foreground was most likely shot in front of a chroma key screen. Each shot begins with a sync mark, usually a red led light flashed at the lens, so the shots can then be composited later.
This is a common technique used in films and commercials. David Fincher used it to great effect in Panic Room.
If a motion control rig was used, you don't necessarily need the green screen. Though, it can help speed the roto work between layers in post, if you have a clean back plate and a foreground plate; a difference key will notice which pixels between the two don't match, and key them as if it was a chromakey, fairly automatically.
To mix normal speed action and time lapse action with motion control adds complexity since the rig has to move exactly the same path as the real-time pass, but much more slowly for the time lapse path, I should think.
Check out the Artist DVD's of Michelle Gondry and Spike Jonze for some behind the scenes of this kind of stuff.
I agree that a difference matte can be easier for production but would I think with the background moving at a different speed you would want to get make getting your matte as easy as possible. We do this kind of stuff quite a bit and it's really no big deal to throw up a chroma key backdrop once the camera etc. is all set up.
As for the speed ramped background, your motion control operator will be able to calculate the speed difference based on how fast you want your background to move and enter that info into their system. We really like Pacific Motion out of Burbank.
And I agree with the Gondry and Jonze DVD's. They are great.