How to get feedback from MediaCapture API in Windows 8

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Metro-design

WinRT

I'm trying to do face tracking apps in Windows 8 app. And I aware that in order to access the camera stream, I have to use MediaCapture.AddEffectAsync() which has to be done incorporate with C++ as shown in http://code.msdn.microsoft.com/windowsapps/Media-Capture-Sample-adf87622.

Now I'm able to access the stream and track the face, but I'm not able to pass/return the tracking face coordinate to C# for further processing.

One way I've tried is to invoke C# method in C++ to pass back the tracking coordinate. But no avail, because Visual Studio doesn't allow circular dependencies between projects.

Another way is to constantly pull the data from C++, but this is not what I want.

Maybe there is another more straight forward way to access the camera buffer, I would like to know.

So my question is, how to get back the tracking coordinate? Or What is the right way to do it? Thanks!