cvtColor 函数中的 openCV 错误：断言失败 (scn == 3 || scn == 4)

Question

我想简单地加载一个视频文件，将其转换为灰度并显示它。这是我的代码：

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()

视频以灰度播放直到结束。然后它冻结，窗口变为无响应，我在终端中收到以下错误：

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

我取消了语句 print frame.shape 的注释。它继续打印 720,1028,3。但是在视频播放到最后，冻结并在一段时间后关闭并返回

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

我了解此断言失败消息通常意味着我正在尝试转换一个空图像。在开始使用 if(ret): 语句处理它之前，我添加了一个空图像检查。 （还有其他方法吗？？）

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()

这次视频播放到最后，窗口仍然冻结并在几秒钟后关闭。这次我没有收到任何错误。但是为什么窗口会冻结？我该如何解决？

Answer 1

waitKey() 部分不应依赖于帧的有效性，将其移出条件：

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