Luma -> Alpha with v1.18

[amindfv at mailbox.org]

I'm looking for a way to use the luma plane of one video as the alpha plane of another.

It appears the "alphacombine" element (https://gstreamer.freedesktop.org/documentation/codecalpha/alphacombine.html?gi-language=c) can do this, but it's too new even for the unstable release of GStreamer, and for a variety of reasons I'm stuck on the latest stable (1.18).

What I'm trying to do is also possible in ffmpeg with "extractplanes=y" + "alphamerge".

I'd welcome any way to do this, no matter how much of a hack. Big pipeline? Fine. Somehow using GPU/shaders? No prob.

Thanks!
Tom