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AFX / 3D Motion Tracking / Shadows

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Jeff Klein
AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 4:18:13 pm

Hi, I 3D motion tracked some green screen footage (with track points) and brought the camera data into AFX. Everything got brought into AFX successfully and the AFX camera lined up with the footage nicely. I added a 3D floor, 3D text, and a light in AFX and things are looking good. The text is casting a shadow (from the AFX light) on the floor and I would also like the actor (from the green screen footage) to cast a shadow. Ideally I would just make the footage (which at this point is just the actor) 3D and cast shadows like you normally do in AFX. However, when you make the footage/actor 3D it's no longer position/tracked correctly.

Any suggestions on a good way to accomplish this? Thanks.


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Larry S. Evans II
Re: AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 5:56:01 pm

You could duplicate the tracked layer, use scale to "squash" it down to where it appears to be a shadow on the ground, and then use that layer as an alpha matte for a solid. The solid can then be mixed in overlay or multiply mode, have it's opacity adjusted, etc. to get a believable "shadow" look.

Larry S. Evans II

Executive Producer

Digital I Productions


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Jeff Klein
Re: AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 6:17:49 pm

Hey Larry, thanks for the response. That is an option (and possibly the best one), but the only problem with doing it that way is then the person's shadow will stay static relative to the person, while other the "real" shadows (in the case, the 3D text) will move based on the camera move/lighting.

I suppose to I could keyframe the fake shadow to try and mimic what is going on with the real shadows, but I was hoping for another solution. Thanks for the help!


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Larry S. Evans II
Re: AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 6:34:42 pm

Ah, didn't quite follow that from the original post, but yes I could see where that'd be a problem.

I don't know that this would work, but have you tried putting the tracked clip inside it's own comp and then making the comp a 3D layer? That might keep the tracking from diverging and still allow the footage to cast a shadow from the light. Just a wild idea, but if that works let me know so I can add it to my list.

Larry S. Evans II

Executive Producer

Digital I Productions


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Jeff Klein
Re: AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 7:44:11 pm

That doesn't seem to work because as soon as I turn the pre-comp into a 3d layer the same problem occurs as when you turned the original layer 3D...it isn't lined up correctly.


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Tony Silva
Re: AFX / 3D Motion Tracking / Shadows
on Sep 21, 2009 at 10:25:19 pm

seems like you'd need tracking data on your subject, maybe somewhere around the waist, then parent the shadow to that and try and stabalize the Y position.

Ideally you'd have a keyed shadow in the first place. Not sure what the subject is doing, but could you just hand roto a shadow in? or roto his shape 90degrees to the lightsource?


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