Remove identical pixels in two layers/images with free software

Question

I've inherited a large set of button icons at work, which I'd like to use in another project, but with a different button background. The trouble is that the original files have been lost in history, so all I have is the flat PNGs.

What I've managed to do is extract a blank button background. Looking at this from a developer point of view, what I'd like to do is "diff" two images (or two layers on a same image), and turn the pixels which are identical in both images transparent.

For example, out of these two pictures I would like a transparent image/layer with just the spade on it:

I realize that the result wouldn't be perfect, especially around blurred edges on the icon, but it would get me a long way. I have about 50 of these, in three button states = 150 images, so bonus for any method which would allow me to play with tolerance.

I'm mainly using Paint.NET, but any free piece software would do. I don't have access to photoshop.

EDIT

I suddenly remembered that I'm actually a software developer and not a graphic designer, so I wrote a program to do this in batch. The C# source code can be found at: http://pastebin.com/mKmtdHT4

Problem solved for me, but I'll leave this open for some time in case somebody has a more generally applicable solution to pass on to other people who have the same problem.

Answer 1

This is relatively easy with ImageMagick's compare:

$ compare -compose src Background.png Icon.png diff.png

With your files, this would result:

---

Although, convert gives you a broader control on the output, e.g. the threshold. Here a B&W version is created, which is more usable for masking, and then the spade is extracted using the created mask:

$ convert Background.png Icon.png -compose difference -composite \ 
          -separate -background black -compose plus -flatten \
          -threshold 0 diff.png
$ convert Icon.png diff.png -compose CopyOpacity -composite spade.png

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