Well, you can use CameraProject to get screen coords from 3D coords. However, if you use it with an entity's position, then it'll still be able to move partly off-screen, I guess.
Personally, I wouldn't bother with any of this. I'd just have a fixed cam and tweak how far the invaders can move left/right until it looks right. :)
Personally, I wouldn't bother with any of this. I'd just have a fixed cam and tweak how far the invaders can move left/right until it looks right. :)