Using world position pass to obscure other footage.

Hello
I just created a world position pass and imported my camera data from Blender.
I was was able to successfully add volume fog to my test render pass. I want too now add footage and have it appear to intersect with the test pass. An example of what im trying to describe can be seen here.
http://www.videocopilot.net/docs/elemen ... ion_matte/
Any help or a link to point me in the right direction would be great!
I just created a world position pass and imported my camera data from Blender.
I was was able to successfully add volume fog to my test render pass. I want too now add footage and have it appear to intersect with the test pass. An example of what im trying to describe can be seen here.
http://www.videocopilot.net/docs/elemen ... ion_matte/
Any help or a link to point me in the right direction would be great!