the BCC Make Alpha Key filter creates a new alpha channel from one of the
existing channels in the image and then applies levels and gamma correction to
the new alpha channel.
The built in PixelChooser, with integrated mocha planar tracking allows for
greater control over which pixels are used to create the alpha channel. You
can create many useful effects by making a Full On (completely opaque) alpha
channel and using the PixelChooser to control which parts of the image key