Let's assume we have an image with a solid-color background like this:
I want to remove that solid-color background -- essentially un-flatten the image, and make it as if the red background layer never existed. The magic wand tool at tolerance=0 wouldn't work because then it wouldn't remove the red from under the transparent parts of the image. I know it's possible to remove the solid background though, because I've done it before by manually selecting colors/transparencies and drawing them over the background to see if the color matched a part of the image. Is there any sort of built-in behavior or a plugin to automate this for me? What I'm looking for is something that loops through all of the pixels and makes them the color/transparency they would have been if the background wasn't there.
