Plugin searched - Substracting Color to Transparency

[BlackRedDead]

I simply search a Plugin that can make this:

into this:

(were White is supposed to be transparency 😉

So it should be able to "measure" the "brightness" and "color" of a Pixel of a given area or a set Colorhue and change it into alpha channel - while it should work inverted for my example (as there we start from black and not white!)

I wonder if something like that doesn't already exist?
(yes i searched for "color to alpha" and stuff like that but really haven't found anything similar to what i'm searching!)

[MJW]

Try my Color Clearer plugin.