Culling Objects Using a Culling Object Today my question might be a bit complex, but I hope that Blender has an easy solution for this. So, let's say I have a scene with several objects in it and I have a background. The background is image/video which has some objects on it. Some of them are supposed to be closer to camera than my 3D models. I have a representative 3D model made for those objects, but texturing it would be real complex. Instead what I inted to do is to use it as a culler - wherever the object is, render the background(cull the other objects). Is this possible to do? Also, a solution for both - Blender Internal and Blender Cycles is needed.
Thanks! ;)
Using Renderlayers
Position your masking objects on a separate layer. (In my case layer 2.)

Go to the Properties &gt; Renderlayers Tab. Set the Mask Layer to the layer, which contains the mask objects. Note that you have to have the layers switched on in the scene for them to be included in the render.

Rendering this setup results in the following. The mask object renders as transparent.

If you do not wish the object to have a shadow, set it to a transparent material or turn off cast shadows.
Using the Material Setup
Cycles
Set the material to holdout.

Internal
Check Transparent, use Mask mode and set Alpha to 0.0. Disable Cast Shadows.