Make an object only visible from a certain angle? I am modeling a cartoon character with a mouth that is designed to only be displayed on a single cheek, not in the middle. Its parent is a head mesh. I made two copies of the mouth: one on the left cheek of the head, the other on the right. I am wondering if there is any way to make the mouth on the side of the head that the camera is not primarily facing disappear? (i.e: if the camera is directed to the left side of the head, then the left mouth is visible, but the right one is not, and vice-versa.)

Because the camera is primarily facing the left side of the model, I need the right mouth (higlighted) to disappear.



I am using Blender Internal.

Blend file: Blend
The answer is using a Light Path node, just like Todd just said, but I'm using a different approach and a different node factor, since according to attached images, this has to happen while both mouths would be visible from a 3/4 angle:
1- Create a plane to go directly in the center of your character, The cube in the image here represents your character's head, the two spheres represent the mouth.
2- Assign a simple transparent material to the plane.
3- Mix a transparent shader to the material of the mouths (the spheres in this example), add a Light Path node, Add node>Input>Light Path, use the Transparent depth as a factor. This will make the object that this material assigned to, disappears when it falls behind an object with a transparent material, which is the plane here. Check the image below.
That's it check the animated GIF below to see how this will work, no need to say that from a front view, both of the mouths will show, so you can extrude the far edge of the transparent plane to the left or right to solve that issue.