Sorry for posting this answer many years after the fact, but that question was asked again in 2012 and then closed for being a duplicate. As I said in my answer to that duplicate, I think the answer given here is overkill for such a simple problem, so is my simpler solution:
- Find all connected green blobs. This is not completely trivial, but not that hard either.
- For each blob, find the point with the minimum x-coordinate and the one with the maximum x-coordinate and do the same in y. Then you easily get the dimension of the bounding box for the blob. If the x and y dimensions are different, you don't have a circle. However, you should allow for some small differences since drawing a circle with pixels will result in an imperfect circle.
- Finally, if you have a circle, the center of the bounding box is (approximately) the circle's center and the radius is half the bounding box width. You can draw such a circle and see how many of the green pixels fit in/out of your circle. Again, you must allow for some imperfections.
