When Watercolor is hit by light, some of the light is converted into heat, some of the light is reflected and some of the light passes through the color.

If you now want to take a watercolor painting (or any other painting, that is) and to put it on a different background - it would be fine to have a method to tell your scanner you want to scan not only the color, but also the transparency.

This tool is a step into this direction that both gives impressive results and will necessarily remain imperfect:
Colours can have different transparency values for red, for green and for blue - or even for only small parts of the frequency spectre of light representing these colors. But all widespread image formats don't support this feature.

Using it is rather simple:
 - Draw an image on transparent paper
 - Put a white sheet of paper behind it and scan the image
 - now put a black sheet of paper behind it instead and scan it again
 - then execute the following command:
watercolor2rgba FirstImage SecondImage OutputFile

The output file will contain the image, but not the transparent paper.

Note: Please use a lossless format like PNG for scanning the image: JPEG will create an approximation of your image that is optimized on saving disk space, not on accuracy - which normally is fine. But if it makes both scanned images differ from each other - this might be detected as an extreme transparency variation.

To make installation easier a ppa has been created: http://launchpad.net/~peterpall/+archive/watercolor2rgba