Let's say I have a layer with a lens flair - or something perhaps that you hate less than a lens flair. The flair was rendered on pure black and the layer's being applied using the "Screen" layer style.
So if this layer of light is applied over a scene, we don't see black -- photoshop filters out the black and applies the light nicely.
But let's say I want to have that lens flair as a separate PNG within a webpage, so that I could perhaps move it around in a DIV on top of the scene. (please overlook stylistic choices here)
So I want to save out the light layer by itself with PNG24 transparency -- but it gets difficult because those black pixels still do exist even though they're invisible. They are invisible only when on top of background layers. If I hide the other layers, the black is there, and copying and pasting will bring it into the PNG.
For years I've been creating complex alpha masks to achieve the same transparencies as I get with layer styles. It'd be great - beyond great - if I could render out the layer as it is seen. Would it ever. I finally realized I should probably ask:)
If the question isn't clear, please let me know.
It's easy: In a new document apply your flare on black backround as usual. Open the channels palette and command+click over the little layer preview of RGB, it will select the light area. Now you can cut away the black (inverting the selection) or make a mask. Now you have a flare on transparent background.