Problems with EVR custom mixer

Can anybody provide accurate details on what the MS EVR Mixer is doing on MFT_MESSAGE_SET_D3D_MANAGER?

I have implemented a custom mixer and handled all APIs specified in the Custom Mixers article, but the mixer fails to build properly in DirectShow graph or MF topology.

If I create an MFVideoMixer9 instance and pass ProcessMessage(MFT_MESSAGE_SET_D3D_MANAGER, ...), SetInputType(...), SetOutputType(...) to the IMFTransform interface; and InitServicePointers(...), ReleaseServicePointers() to the IMFTopologyServiceLookupClient interface, the custom mixer works fine.

It appears that the difference happens as a result of the ProcessMessage(MFT_MESSAGE_SET_D3D_MANAGER, ...) call.

Larry