One-sided layers

//found on :http://www.mographwiki.net/After_Effects_expressions_library

Apply this expression to the opacity attribute and it'll disappear 
whenever it's flipped away from the camera. You need to have an active 
camera in your composition for this one to work.
 

a = toWorldVec([0,0,1]);
b = thisComp.activeCamera.position - toWorld(anchorPoint);
c = dot(a,b)/length(b);
if (c > 0) 0 else value