Prompted by the recent work by Maarten Lankhorst on getting his video cam to work in Wine, I did some in depth investigations on how Windows actually does this stuff.
This is a summary of what I found and some thoughts about it. I would like some comments on how to progress.
Basically applications have two obvious ways to use video capture services in Windows. One is avicap32.dll which is basically the Video for Windows way of doing this and the other one is ActiveMovie/DirectShow, soon to be obsoleted by Microsoft as well, as it seems.
Avicap32 implements two functions, one is for the enumeration of the VfW devices registered in the system. For this it does enumerate all entries in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\MediaResources\msvid eo and for compatibilty reasons with 16bit Windows also in the Drivers32 section in system.ini. I have an untested version of this code already. The enumeration function does not try to detect any devices other than what it finds in above mentioned places. I assume that a webcam driver installation would have to do that. The second function in Avicap32.dll creates a (hidden) window which handles the actual interaction with the video device as well as being an optional user interface for the video data. The communication with the video device is always by so called installable drivers, like wineoss, etc. is used for audio devices. The interface to these installable video drivers is for the video device specific parts declared in a header file msviddrv.h which is really part of the DDK. (How would we add that to the Wine headers?) In modern Windows systems there is one MS provided installable driver vfwwdm32.dll, which is basically a translation layer between the installable interface and modern WDM stream based video drivers.
Devenum uses the avicap enumeration function to find all registered VfW video sources and provide them to an application. Presumably there are other possible video source objects which would provide a direct interface to WDM stream class devices somehow and could be enumerated by devenum as well.
DirectShow seems to be somwhat complicated. Video sources can be provided from VfW again, which is provided by the "VfW Capture Filter" class in qcap.dll. This is basically a DirectShow filter wrapper around the same installable driver interface as is used in avicap32.dll (and through above mentioned vfwwdm32.dll driver an application can also access WDM stream class video capture sources indirectly. How the direct interface to WDM stream class devices is provide I haven't found out yet but according to MSDN it should be possible somehow.
For Wine purposes the VfW path seems the most interesting one. Separating the Video4Linux interface into an installable multimedia driver such as winev4l would cleanly separate platform specific code from the rest of the Wine DirectShow and Avicap implementation. Trying to emulate direct access to WDM based drivers would not be possible anyhow, as that would seem to require kernel driver support, which is mostly absent in Wine.
I could probably hack an experimental winev4l driver together based on the work already done by Maarten Lankhorst, but would first want to make sure that this is considered the way to go, before investing more time into this. While the current implementation by Maarten is not as clean and nice because of the direct V4L dependency in standard Wine DLLs, it is something which already works somehow and could be integrated into Wine in quite a short time.
What are the thoughts about this?
Rolf Kalbermatter