I would personally opt for doing the ghost effect "in post"... i.e. build your environment and ghost models, render them in separate passes, and use after effects or smoke or something to comp them together. Then you have fast, simple controls for the ghost's transparency (and parts of it that shouldn't be?), extra glow filters, and the flexibility to make changes to the ghost effect without spending a ton of render time.
That's if you're currently working on a production that needs this shot. If you're doing it just to learn, I'm really freaking new to the vfx world, so I have no clue
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Peter Srinivasan
Producer