I don't think this is directly possible (but the export options of Poser might allow this - possibly).  For one thing, the scene animation is 3D and Alpha channels are 2D.  This means that the only appropriate export involves the camera (and a 2D rendering of the animation) as you end up with a 2D representation of the 3D scene this way - comprende?

One possible solution that is very easy is to do this:

1. Hide the Ground plane (you can do this in Poser)

2. Set all of the materials of the animated objects to a flat color that differs from the background color complimentarily (by flat, I mean no specular, no highlights and by 'compliment' I mean the opposite color (black/white, red/green, blue/yellow) - something that is easily separable - think blue or green screen here).

3. Render as either animated image sequence or animation.  Although I have Cinema 4D, I'm not certain of how it handles animation in materials.  It may allow image sequences or it may only allow animation formats (e.g.: avi, mov).

What you should end up with is an animation that is basically two colors (background and animated objects).  You can then use that as an Alpha channel.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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