[jitter] Alpha Chanel on Video with black background

Hi everybody
I am trying to compose something using pictures with alpha. I am putting all together using jit.gl.videoplanes with alpha_blend. But I need to incorporate a video with an animal walking, the animal more and less is white (but it looks real not only white) and the background is totally black. Therefore, I need to add alpha channel to the background but after some hours I don’t get it. Anybody have a solution for this kind of compositions? 
I am trying to do a video with an animal without alpha and the background with only alpha channel.
The second question is what kind of video compression use alpha channel? 
Thanks a lot for the help, I am running against time (like always).