BASHandSlash.com - The Call of Duty 4 FAQ
"Infinity Ward has created its
own proprietary graphics engine-based on the Quake graphics engine. The new software is heavily modified though. This "new" engine introduces cinematic quality graphics and sound . You can now get realistic depth of field, rim-lighting, character self-shadowing, texture streaming and physics-enabled effects like death scenes which combine rag doll with death animations."
I realize I didn't say it was based off the "Quake engine" an error on my part.
But you can see that based off the similar Quake commands and the same config files used for the both games.
Yes, its proprietary, but based of Quake's engine.