In response to a StackOverflow question:
This is code to build a Direct3D wrapper DLL, intercepting all calls to Direct3D interface functions so that you can draw your own objects to display over the game. Just plop the DLL into the same folder as the game's executable, and it should load it as if it were the real d3d9.dll file. It still forwards all calls to the real one in system32, just allows stuff to happen in between. original stackoverflow answer