Question

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Fast Java library for circle recognition

I’m looking for a good Java algorithm/library to recognize a circle in a bitmap, and returns the coördinates of the center of the circle. The circle has one color. (eg. green).

Thanks a lot!

Answer 1

First, I should say that I kind of agree that your question should be closed because it is a duplicate. However, I don’t quite like the answer provided since it’s overkill for such a simple problem.

I would:

• 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.