An occlusion model enables you to mask the surfaces of a physical object so that virtual objects are not rendered over them when they are behind the object in your scene. This effect relies on depth masking and provides a much more realistic rendering for scenes that combine physical and virtual objects.
Depth masking via an occlusion model is accomplished by developing a 3D model or mesh of the regions of the object that you want to mask and then apply a depth mask shader to that model. The depth mask will prevent any objects behind the masked region from being rendered.
A volumetric occlusion model
An occlusion model can match the exact geometry of the physical object that it corresponds to, or it can be a simplified representation of the object, such as its bounding box. The latter technique is an easy way to occlude physical objects with simple geometries, like boxes, balls and cylinders and ideal for Multi Targets and Cylinder Targets.