cVtColor函数中的openCV错误：声明失败（scn == 3 || scn == 4）

Question

我想简单地加载视频文件，将其转换为灰度并显示它。这里是我的代码：

import numpy as np 
import cv2 

cap = cv2.VideoCapture('cars.mp4') 

while(cap.isOpened()): 
    # Capture frame-by-frame 
    ret, frame = cap.read() 
    #print frame.shape 


    # Our operations on the frame come here 
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) 

    # Display the resulting frame 
    cv2.imshow('frame',gray) 
    if cv2.waitKey(1) & 0xFF == ord('q'): 
     break 


# When everything done, release the capture 
cap.release() 
cv2.destroyAllWindows() 

视频播放灰度直到结束。然后它冻结和窗口变得不响应，我得到以下错误在终端：

OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /home/clive/Downloads/OpenCV/opencv-2.4.9/modules/imgproc/src/color.cpp, line 3737 
Traceback (most recent call last): 
    File "cap.py", line 13, in <module> 
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) 
cv2.error: /home/clive/Downloads/OpenCV/opencv-2.4.9/modules/imgproc/src/color.cpp:3737: error: (-215) scn == 3 || scn == 4 in function cvtColor 

我注释掉的语句打印frame.shape。它不断打印720,1028,3。但视频播放，直到结束后，冻结，一段时间后，关闭并返回

print frame.shape 
AttributeError: 'NoneType' object has no attribute 'shape' 

据我所知，这个断言失败按摩通常意味着我想转换一个空的图像。在使用if（ret）：语句开始处理之前，我添加了对空图像的检查。 （任何其他方式？）

import numpy as np 
import cv2 

cap = cv2.VideoCapture('cars.mp4') 

while(cap.isOpened()): 
    # Capture frame-by-frame 
    ret, frame = cap.read() 
    #print frame.shape 

    if(ret): #if cam read is successfull 
     # Our operations on the frame come here 
     gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) 

     # Display the resulting frame 
     cv2.imshow('frame',gray) 
     if cv2.waitKey(1) & 0xFF == ord('q'): 
      break 
    else: 
     break 

# When everything done, release the capture 
cap.release() 
cv2.destroyAllWindows() 

这次视频播放直到结束，窗口仍然冻结并在几秒钟后关闭。这一次我没有得到任何错误。但为什么窗口会冻结？我该如何解决这个问题？

Answer 1

的waitKey（）部分不应该依赖于框架的有效性，将其移出的条件：

import numpy as np 
import cv2 

cap = cv2.VideoCapture('cars.mp4') 

while(cap.isOpened()): 
    # Capture frame-by-frame 
    ret, frame = cap.read() 
    #print frame.shape 

    if(ret): #if cam read is successfull 
     # Our operations on the frame come here 
     gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY) 

     # Display the resulting frame 
     cv2.imshow('frame',gray) 
    else: 
     break 

    # this should be called always, frame or not. 
    if cv2.waitKey(1) & 0xFF == ord('q'): 
     break