Since matte objects came up in the stream, I thought a quick explanation may be in order.

A matte object is a special kind of object which occludes things behind it, but is also transparent. The purpose is to make a hole in the image.

The term "matte" comes from visual effects, such as blue/green screening, to knock out areas from a scene to aid in compositing. In a moving image, this is known as a "travelling matte".

Matte objects have no colour, only alpha. This is a good reference on the m...ehind working with matte objects.