Hello, I've made an effect that converts pixels to a single color and adjusts the alpha value based on deviation from that color. Essentially it's a grayscale to alpha converter, capable of extracting black or white objects from a background of the other color, but it could be used with other colors as well. Should the image contain alpha values already, it can be blended with the calculated alpha. The resulting image has only one RGB color, with all variation in the alpha channel only.
Download the plugin
Menu: Effects > Color
Why this plugin? There are multiple plugin packs (1, 2) that advertise begin able to perform the same task or similar tasks. However, I've found that they do not function quite as I'd like, or maybe I've misunderstood how to use them. Nevertheless, this plugin is as easy to use as it gets.
To install, extract the folder and run the batch install file, or locate your paint.net effects and paste the .dll file there manually.
I would also appreciate any feedback on the plugin! Maybe you have an edge case for which it doesn't work, or an improvement that should be made.
EDIT: Updated and renamed plugin, title and description