Reflective glass looks reflective because it's reflecting something. You need to mask the area somehow. Depends on the shot, but perhaps a combination of tracking and masking. Then I would probably matte in a darkened shot. If it's a space helmet then I guess some stars or a nebula or a big lens flare. Depends. You probably want to warp it somewhat to appear that it's curved glass. Bezier warp would do that nicely.
I appreciate the feedback. That's a good run down. While I was waiting I began using a semi transparent black solid, masked out, with another layer for specular that didn't look too bad. So, sounds like I am on track with about what you are saying. The shot is a straight on with just a slight amount of left and right movement (so does require tracking). Agree on the warp. I assume Bezier is already in AE (believe I've heard of it before) so will check that out with a back up of creating a slightly oversized shield in Lightwave 3D and masking it out since it would have the curve.
On an associated note, with reflection, is there a way to cast a reflection from a layer on to the helmet lense? I know the most obvious is mask it on there too but if the helmet layer could somehow be set to be the only layer that accepts a reflection it would spare the keyframing of the mask since the reflection would remain static against the moving helmet.
You can't have layers actually cause reflections on other layers in AE. AE is not an actual 3d world and thus there is no ray tracing. There is a plugin from Zaxwerks that you can use for reflections, but I don't think it's quite what you want. Motion tracking a layer that you've masked (and somewhat distorted) onto his helmet is the bets way. Don't forget to keyframe movement in the reflection when the head moves.
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