Hitlinemoss

This site seems to be doing roughly the same thing as the aforementioned Feather plugin (i.e, just erasing the pixels at the edge of the image). Which is still good for a quick fix, but with this thread I'm mostly just wondering if there's a more specialized plugin for this kind of thing that preserves the non-background pixels as accurately as possible.

I actually did install this plugin pack earlier today (good work on it, by the way, I appreciate the sheer diversity of tools here) but this specific tool isn't quite what I'm looking for. It directly erases pixels from the outside of the image, which is good for quick-fixes but runs into the problem of erasing thinner lines entirely or missing some sharp concave corners. The Switch Grey to Alpha tool from the same plugin pack is *almost* what I'm after here, but the two problems there are that it it only fully works on pure white/black backgrounds (rather than other colors like blue or whatever) and that it affects the entire selected area indiscriminately (so, if it's a blue outline on a white background, the blue becomes partially transparent even at its bluest). I guess the best way to phrase what I'm looking for is, like, a plugin that lets me designate one color as the "background" and a second color as "not the background", and then have the background color erased, the non-background color untouched, and colors between the two colors gradually get more or less transparent depending on which of the two they're closer to.

To clarify what I mean: let's say I have an image like this, with a gray background: I want to replace the gray background with a transparent one. However, because the original image was drawn with an antialiased brush, the edge of the image and the background blend together, so the magic wand tool isn't quite up to the task: Now, with enough manual effort it's possible to hide these kinds of artifacts. (My current process is to create a silhouette of the image in the same color as the original edge, then use a plugin to antialias that silhouette, delete the areas overlapping with the original image, antialias it again, and delete the areas overlapping with the background.) But this is slow, and it tends to look ugly and blocky on close inspection. Ideally, I'd like some kind of plugin that I can run to easily replace the background color with transparency; I figure that, if the background is a single static color, there's some kind of algorithm you could run to just convert pixels of that color to transparency (and pixels somewhat-that-color to somewhat-transparent), right? Does a plugin along these lines exist?