transparent background video

I haven’t tried it yet, but just as you would with keying out color for a custom alpha using nodes with any image, you’d do the same with video.

This is assuming you haven’t tried using your alpha channel yet…
If you’re using Blender, you’d take your color output of your image texture (video ) node and isolate your negative empty area from your non alpha subject matter, probably using rgb curves node

then use a black and white node, then a brightness and contrast node if needed, and an invert node if needed,

then use that as the “fac” on a mix shader node where you have your original video (image texture color again) plugged into the bottom socket and a transparency node plugged into the upper shader socket which outputs to your material output node.

Hope that helps.
I’ll try it soon

Awailable with the new release. Article.

If you have a sphere or any mesh object that covers background you can’t see transperency. You need to remove all object from the background of the scene.