GStreamer transparent video background

[Stivius]

Hello!
I was trying to use GStreamer along with GTK+3 library to put the video into
created GTK window. But the problem is that I want this video to be with the
transparent background. For example, if a window has 400x400 size and video
has 1366x768 then the video will be resized to fit into 400x400 keeping
aspect ratio. It leads to a black background which I couldn't remove
programmatically. I googled it many times, trying to find the answer but no
result. I can do my GTK window transparent (or at least use another color
than black) but the video will overlap this window and background color will
stay black. I have attached the screenshot.
Could please point me how to achieve it? Maybe there are some workarounds. 

Regards, Stivius

<http://gstreamer-devel.966125.n4.nabble.com/file/t378012/photo_2017-10-11_01-02-33.jpg> 



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