import sensor, time, image, math
sensor.reset()

sensor.set_contrast(3)
sensor.set_gainceiling(16)
sensor.set_framesize(sensor.VGA)
sensor.set_windowing((320, 240))
sensor.set_pixformat(sensor.GRAYSCALE)

sensor.skip_frames(time = 2000)
sensor.set_auto_gain(False, value=100)

#画出特征点
def draw_keypoints(img, kpts):
    if kpts:
        print(kpts)
        img.draw_keypoints(kpts)
        img = sensor.snapshot()
        time.sleep_ms(1000)

kpts1 = None

clock = time.clock()

while (True):
    clock.tick()
    img = sensor.snapshot()
    if (kpts1 == None):
        #提取最开始的图像作为目标物体特征，kpts1保存目标物体的特征
        kpts1 = img.find_keypoints(max_keypoints=150, threshold=10, scale_factor=1.2)
        draw_keypoints(img, kpts1)
        #画出此时的目标特征
    else:
        kpts2 = img.find_keypoints(max_keypoints=150, threshold=10, normalized=True)
        if (kpts2):
            match = image.match_descriptor(kpts1, kpts2, threshold=85)
    
            if (match.count()>3):
                img.draw_cross(match.cx(), match.cy(), size=10)
                img.mask_circle([match.cx(), match.cy(), 50])
                for c in img.find_circles(threshold = 3500, x_margin = 20, y_margin = 15, r_margin = 10,r_min = 30, r_max = 40, r_step = 2):
                         img.draw_circle(c.x(), c.y(), c.r(), color = (255, 0, 0))
                         d = math.sqrt((c.x()-160)**2 + (c.y()-120)**2)
                         print(c)
                         print("distance %d" % d)
            print(kpts2, "matched:%d dt:%d"%(match.count(), match.theta()))
                      
    #打印帧率。
    print("FPS %f" % clock.fps())

已知坐标点cx(),cy()。如何求它到中心点（160，120）的距离？