Tip on mixing 2D and 3D
by Arjo Rozendaal
In the manual there's this tutorial about the boat crossing the 2D lake. You
might have seen it. To mask the trees in the front the tutorial uses polygon objects to mask the front objects like the tree. But this is a very rough way of masking of course.
I noticed that there's no problem using two backdrop images for the camera. Just put one for the background on a great distance and put the other one close to the camera. Both objects with their shaders can be put in separate levels without problem of loosing line up with the camera. With fade mask for the foreground you can make much more accurate masking.
It might be old news but maybe useful for someone.