Adding a shadow under a 2d object
A little background: I was involved in shooting some full body green screen footage to be composited onto a cartoon background. The cartoon background has a top light and the director would like a shadow under the keyed actor (2d object). The camera is completely locked, thank goodness.
I tried the shadow feature of Red Giant's Warp and that does great for shadows at an angle. It doesn't get director approval in this instance.
Is there any way I can gently extrude (needs to be fairly rounded, not a hard edge extrude) the 2d layer and then shine a light down to power a simple shadow on the ground plane?
Or am I better off manually tracking the feet and parenting it to a circle? Difficult because their arms come out to complicate the shape and I'd have to track it in 3d with depth for it to work on a ground plane.
I cannot track the actor. They're dancing and spinning so I don't have a clean 2d track off them.
Again, this doesn't have to be hollywood level. If I could get a feathered circle under the feet, perfect.
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Do you have have CS6? If you do, I would key the subject and turn the person into a 3D layer. Create a solid to use as the floor(enable 3D) and press AA and change "Accepts Shawdows" to only. Add a light to cast your shadow.
Lastly composite the Shadow Catcher and your person with the cartoon footage and you should have a shadow on your cartoon floor.
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In the case of casting a shadow using a light I'd actually recommend using Classic 3D rather than the ray-traced engine. You have more control.
However, I think you'd be best off in this scenario scaling your actor layer down a lot (maybe to 2%) on the y-axis. To start duplicate the actor footage, move the anchor point on the duplicate to where the actor's feet meet the floor, set the y-scale on this duplicate to 2%-ish (change as you see fit), then add a Generate > Fill effect set to black (or near black, or another color if you want to simulate some ambient light, a dark desaturated blue often works well for outdoor shots for example), add a Fast Blur and crank it up until you're happy with the result (you could add 2 Fast Blurs, one for the Horizontal Blur and one for the Vertical Blur), finally set the shadow layer to the Multiply blend mode.
You'll likely need to tweak some things to get this looking right, but it should get you most of the way there. If the talent moves toward or away from the camera then you might need to animate the transforms on the shadow layer. Also if the talent jumps very high you might want to keyframe an increase to the Fast Blur radius when they are in the air.
Also see what it looks like with the shadow layer set to a small negative scale (-2% maybe?). In the end you may want to use 2 shadow layers, one with a small positive and one with a small negative scale.