Casting real shadows in CG environment?

[mark_wilkins]

As played in front of the green (or blue) screen he cast a shadow on a green/blue screen floor. A matte was created from the shape of the shadow. This matted shape was used to darken the floor of the 3d model.

How wrong am I? Is there a better way to do this?
Many of the software color-keying applications out there will give you a semitransparent matte where you cast a shadow on the colored background. The trick is that if you're casting a shadow on a complex shape, you need to build a mock-up of that shape and then ensure that it registers correctly with the CG elements.

-- Mark

[bartrobinson]

You've hit on one of the main techniques for getting shadows from matted foregrounds to backgrounds, cg or live action. A lot of the time people don't even want to mess with trying to pull a matte for a shadow maybe because it doesn't exist or the background doesn't match with the environment at all, so they scrap it. In those cases you could do a few things. Create a cg stand-in, animated to match your character/foreground object and just use it for it's shadow casting. Also likely would be to create 3d stand-in geometry that matches the live action background for the composite or use the cg background geometry itself (depending on how the background was accomplished obviously) to catch those shadows. Also sometimes it might just be easiest to animate a shadow matte by hand with splines or something, then maybe warp it to follow the background.

[mark_wilkins]

A lot of the time people don't even want to mess with trying to pull a matte for a shadow
Note to the original poster: Bart's suggestions are all good, but of course you should know that they're all a lot of work. :)

Which is why digital artists get paid the big bucks <cough>.

-- Mark

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