DriverProc in User-Mode Video Capture Drivers
Like all other Win32-based user-mode drivers, user-mode video capture drivers must export a DriverProc entry point that recognizes the standard driver messages. Video capture driver clients, such as msvfw32.dll and avicap32.dll, send messages to video capture drivers by calling SendDriverMessage, which is exported by winmm.dll and described in the Win32 SDK.
When a user-mode video capture driver receives a DRV_OPEN message from avicap32.dll or msvfw32.dll, it also receives a pointer to a VIDEO_OPEN_PARMS structure. For more information about the use of this structure, see Opening Video Channels.
In addition to supporting the standard messages, the DriverProc entry point for user-mode video capture drivers must support a set of user-mode video capture driver messages.