When integrating live action footage and cg, how to make proper shadows for actors?

Question

What is the best way to make believable shadows for moving actors that are standing in front of a green screen?

Do you need to have a perfect modeled double that follows his movements so it can cast shadows? I'm guessing it becomes very tricky when the actor's clothes and hair are also moving. The other thing is that you are probably limited to your modeling, rigging and animation skills.

Is there another way of doing so? Are there some considerations when shooting in front on a green screen regarding the shadows for the integration (with cg)?

P.S: If someone knows how they did it on Tears of Steel... that's probably a good place to look. Unfortunately I'm unable to buy the dvds for now.

Answer 1

The green screen does not remove the shadow as long as your threshold is low enough. Because it's a darker shade of green, the green will be removed, but the gray will stay.

If that method doesn't work, you can mask around the shadow and add it back in later with the compositor.

You don't need to make a full scale model and match the lighting as that's more work than it's worth.

Answer 2

Make and rig objects that resemble the actors in sufficient detail as to be able to cast convincing shadows, then animate their movement in the scene such that they coincide with the movements in 3d space of the real-life actor. Then make sure only the shadows they cast get rendered. Then finally you might have to mask out areas where the cg shadows overlap the real-life actors.

With a static camera position and green screen setup it can be possible to capture the difference in colour values between the green screen scene with and without actors, creating a difference mask.