I have a shadow on a solid coloured background. I know that the colour of the shadow is black (#000000), and I know exactly what the background colour is. How can I remove the colored background and convert the shadow into a black image with alpha? Given that I know what the colour of the shadow is the problem should be exactly solvable for an alpha-shadow that gives me back the original image when I overlay it on the background colour.
Using paint.NET, or Gimp, or any other free software. I can find lots of approximate solutions, that "work" even when the colour of the shadow is not known (Leaving some of the background colour "lingering" in the result), but I don't want that. I want the exact solution when I do know the colour of the shadow and the background.