getting alpha by subtracting two images with different backgrounds

Question

I am using a program what can render color images but only without alpha information. I would like to get alpha information from those images by using two and subtracting them. I can set the background to different colors.

My idea is that if I render an image with black background and an other one with white, then I can subtract those images from each other and get an alpha channel. But it is just a theory, I don't know how to do it in practise and that if there are any standard methods / algorithms for extracting alpha information out of two images by subtracting them from each other.

I would like to use a command line program (or a very easy to use library in C++) to do this processing. I have used convert.exe from ImageMagick before, but I have never used the other utilities in ImageMagick.

Is there anyone who can recommend me a way how to do it in practise? What I am looking for is some kind of a command line solution or a C++ library with easy to understand example files what can do this.

Update: My backgrounds are computer generated, solid colors. So I can set it to 0,0,0 black. Here is an example.

Answer 1

this might be good enough if your object differs enough from the background but this looks like it might be exactly what you asked for

EDIT: the second one ends up with this command line (replace the stuff in <> with your images)

  convert <image1> <image2> -alpha off \
          \( -clone 0,1 -compose difference -composite \
             -separate -evaluate-sequence max -auto-level -negate \) \
          \( -clone 0,2 -fx "v==0?0:u/v-u.p{0,0}/v+u.p{0,0}" \) \
          -delete 0,1 +swap -compose Copy_Opacity -composite \
          <output>

You'll need to use a format that supports alpha on the output, but that's probably what you want anyway. (NOTE: I have not actually tried this for myself so it might not work with the latest imagemagick versions since I don't think that documentation is always up to date)

Or, if you actually just use pure black and white you can do it like this:

  convert <image1> <image2> -alpha off \
          \( -clone 0,1 -compose difference -composite -negate \) \
          \( -clone 0,2 +swap -compose divide -composite \) \
          -delete 0,1 +swap -compose Copy_Opacity -composite \
          <output>