问如何冻结picamera预览

EN

from picamera import PiCamera
from time import sleep
import RPi.GPIO as GPIO
import matplotlib.pyplot as plt
import matplotlib.image as mpimg

def btn_press(channel):
    camera.start_preview()
    camera.annotate_text="3..."
    sleep(1)
    camera.annotate_text="2..."
    sleep(1)
    camera.annotate_text="1..."
    sleep(1)
    camera.capture("image.jpg")
    camera.annotate_text="Pic taken"
    sleep(5)
    camera.stop_preview()
    pic=mpimg.imread("image.jpg")
    plt.imshow(pic)
    plt.show()
    #print("pic taken")

camera = PiCamera()
camera.led=False
camera.annotate_text_size=100

btn=20

GPIO.setmode(GPIO.BCM)
GPIO.setup(20, GPIO.IN)

GPIO.add_event_detect(btn, GPIO.BOTH, callback=btn_press, bouncetime=8000)