def calc_precision_recall(contours_a, contours_b, threshold):
        x = contours_a
        y = contours_b

        t1 = np.tile(x, (len(y),1))
        t2 = np.repeat(y, len(x), axis=0)
        t = np.concatenate( (t1,t2), axis=1)

        dx = (t[...,0] - t[...,2]) * (t[...,0] - t[...,2])
        dy = (t[...,1] - t[...,3]) * (t[...,1] - t[...,3])
        thr = (dx + dy) < (threshold * threshold)

        blocks = np.split(thr, len(y))
        block_has_hit = [np.any(x == True) for x in blocks]
        top_count = np.sum(block_has_hit)

        precision_recall = top_count/len(y)
        return precision_recall, top_count, len(y)