Mummy sand effect

Question

I am working on an effect but I'm not sure if motion does this kind of effect. I am trying to turn an arm that I shot on green screen in to a smoking arm. Much like the Mummy sand effect when the face comes out of the sand and trys to eat the plane. I have keyed out the green. I then used the Stencil Luma Blend Mode to fill the arm with smoke but you can tell it's deffinitly an arm filled with smoke. What can I do to get the smoke to look like an arm but not an Arm thats filled with smoke.
15' MacBook Pro   Mac OS X (10.4.7)   2.16 GHz Intel Core Duo, 2GB DDR2 SDRAM

Answer 1

I'm not really good at Motion, but here's how the effect was done in The Mummy (paraphrased - but if you have the DVD, they show how it was done):
A 3D particle system was used to generate the sand cloud. Then, a displacement effect was added to the particle system; the displacement map being generated from the face of the actor.
In Motion, I'd think you'd use some particle emitters to make a smoke cloud that is roughly the size and shape of the arm. Then, I'd use the arm that you've keyed out as the displacement source in applying the Displace filter. You may even want to experiment with using only one channel in your arm image/movie, as certain channels may give a more effective displacement. The Channel Swap filter might be useful for this.
However, Motion is still a 2D particle system with a 2D displacement filter, so your results won't look as good as what was achieved in The Mummy.
Please also note that I haven't tried any of the above steps; I'm just giving you my idea of how I'd go about trying to achieve this effect in Motion (and like I said, I'm not very good with Motion in the first place - I'm more of an After Effects fan).