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Any tricks for faking shadows for 2d composites?

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Carey PaganAny tricks for faking shadows for 2d composites?
by on Nov 10, 2015 at 3:41:15 pm

Hey there folks,

I've been attempting to composite some green screen footage into a 3d scene and its looking pretty nice, but the shadow situation is a dead giveaway.

As the composited footage is a texture on a plane with alpha channel, the shadow only really works if the light is coming from front or back. But my subject is lit from the side, and if I try match the light direction, the shadow all but disappears as the plane has no depth in 3D space.

Ofcourse there is no real depth information, just a 2D paper cutout as far as C4D is concerned.
But I figure if it were more of a gingerbread man than a paper cutout, it would be convincing enough for a soft shadow.

So need some way to 'extrude' this 2D movie on the z-axis for the light source only.
As it's a Prores 4444 movie with alpha, splines and tracing are out.
The only thing I can think of is using the alpha channel as a displacement map on high-poly plane that's hidden from the camera with a composite tag, but I can't get it to work. I just end up with weird squiggly lines for shadows. I have a hunch that the calculations for casting shadow through the alpha are getting screwed up by the addition of the displacement via same alpha map.

Anyone got any tips on how this might be done?
I'm aware of doing keys for real shadows etc but my footage is too noisy in the shadows around the feet to get anything useable. I need to fake it.

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Adam TrachtenbergRe: Any tricks for faking shadows for 2d composites?
by on Nov 10, 2015 at 4:08:37 pm

It's not free, but you could use this plugin to generate a spline for extrusion:

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Carey PaganRe: Any tricks for faking shadows for 2d composites?
by on Nov 15, 2015 at 1:19:57 am

Thanks but that looks a bit processor intensive for the quick and dirty effect I'm after.
I ended up just making some roughly human sized squashed cylinders, dropping them in a null with the plane and turning off 'seen by camera' as well as re-keying the footage to include a little bit of darkness around the feet from the green screen.
Looks convincing enough.

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