So I've got a uni project coming up and we've been asked to go out in the field and estimate the canopy cover of trees by looking up. But I figured that if I took a photo, I might be able to get my computer to do the work. Since blue/white would be the sky (no cover), and the rest could be classed as cover.

So I'm wondering if there is such software? I can make transparent background with GIMP but selecting what I want doesn't give me a percentage of the total image. I've also used ImageMagick several times but don't know much about it's potential.

Here's an image: