Q:

How to use a mesh to reveal an object

How can you use an object to mask out objects in blender? 
As in, a cube perhaps that hides a circle, and then the cube is animated
to reveal the circle. 
I want this cube to act like an eraser almost; Any part of the mesh it encapsulates needs to disappear, however the remaining portion of the mesh that is not "subtracted" should remain visible. Of course I want to be able to specify a specific mesh that the mask effects.
Also a way to fill in the gaps in the mesh that effect would create automatically would be needed in many instances.
Is this possible? If so, is there another way to achieve this?
Here is the link to the model.
I want the cube to be invisible, and as it rises I want it to reveal the sphere, which is also invisible while it is inside of the cube.

A:

This is fairly straight-forward, there are two things you need to do:

Make the cube subtract from the sphere, and
Make the cube invisible to the camera so it doesn't render

First, select the sphere and add a Boolean Modifier to it, setting it to difference and making sure the "Object" option is set to the cube.

Now go ahead and set the maximum draw type for the cube to Wire in the object settings.

You should get a result like this:

Scroll down a little farther and under Cycles Settings un-check all the check-boxes. 

When you render you should have a result like this:

