Along with Extract, you might want to try the Luma Key effect or the free Unmult Plugin.
Unmult’s was created for creating alpha channels for Knoll lens flares, it just drops out black and creates an alpha. Generally when I used it, I would add a levels effect after it, switch to the alpha channel and crush the blacks and increase the whites a little more to strengthen the alpha.
Johnny Cuevas, Editor
Thinkck.com
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.