I'm trying to create an alpha channel on a completely black screen using the gstreamer alpha good plugin.

I created a jpeg with a black square and varying greys to try and test how I could get just the pure black to be converted to the alpha. When using this jpeg I could not get the black square to be transparent, although I could get some of the greys to be transparent.

I thought 
alpha method=custom target-r=0 target-g=0 target-b=0
would work but it doesn't.

The following almost works but only works on greys:
alpha method=custom target-r=0 target-b=0 target-g=0 black-sensitivity=128 white-sensitivity=0

Let me know if I can provide any further information.

Andy