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Hough Circle Transform is the best option to detect circle in OpenCV. If you are getting expected circle with false circle using Hough Circle Transform you can easily filter out the true circle. For this first manually find out the radius of three circle you are expecting. Then find out the ratios between the three circle and these ratios will be constant even if you re-size your image. Then using Hough Circle Transform and find out the circles in the image. Then filter out co-centred circle from detected circle. For each co-centred circle(with next) check whether it's radius ratio comes in the range of your predefined value, if so store it as true circle.

Thanks....

Hough Circle Transform is the best option to detect circle in OpenCV. If you are getting expected circle with false circle using Hough Circle Transform you can easily filter out the true circle. For this first manually find out the radius of three circle you are expecting. Then find out the ratios between the three circle and these ratios will be constant even if you re-size your image. Then using Hough Circle Transform and find out the circles in the image. Then filter out co-centred circle from detected circle. For each co-centred circle(with next) check whether it's radius ratio comes in the range of your predefined value, if it comes so store it as true circle.

Thanks....Hope this is helpful.