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I guess a similar task already exists in the tutorials: http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/bounding_rects_circles/bounding_rects_circles.html#bounding-rects-circles

The function threshold can be simply replaced by canny, example here:

http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/find_contours/find_contours.html#find-contours