Is there any plugin or ways to change the opacity of pixels based on brightness?

[slimCODE]

Is there a way or an existing plugin that would allow me to change the opacity of pixels in an image based on the brightness? Let's take a grayscale image for example. I'd like to apply a tool to either the whole image or the selected area, where the closer to white pixels are, the less their alpha value is. In this case, opacity would decrease with brightness.

Thank you.

[Sarkut]

slimCODE,

Alpha Mask Import plugin, using the image as its

own mask, with Invert Mask checked.

Or Curves+,

Set to Advanced

Key/Black - In

Alpha - Out

Edited April 8, 2010 by Sarkut