什么时候使用ret和frame？这些变量都有什么值？我刚刚开始使用图像处理，所以如果有更多的变化一定要让我知道。
谢谢

import numpy as np
import cv2
cap = cv2.VideoCapture('Sample Lap HUL_OB_1.56.641_Graphic.mpg')

# Define the codec and create VideoWriter object
# fourcc = cv2.cv.CV_FOURCC(*'MJPG')
out = cv2.VideoWriter('output.mpg',0, 60.0, (640,480))
while(cap.isOpened()):
    ret, frame = cap.read()
    if ret==True:
         # frame = cv2.flip(frame,0)
         # write the flipped frame
        out.write(frame)
        cv2.imshow('frame',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
             break
     else:
        break
# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()