cVtColor函數中的openCV錯誤：聲明失敗（scn == 3 || scn == 4）

Question

我想簡單地加載視頻文件，將其轉換爲灰度並顯示它。這裏是我的代碼：

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() 

視頻播放灰度直到結束。然後它凍結和窗口變得不響應，我得到以下錯誤在終端：

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 

我註釋掉的語句打印frame.shape。它不斷打印720,1028,3。但視頻播放，直到結束後，凍結，一段時間後，關閉並返回

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

據我所知，這個斷言失敗按摩通常意味着我想轉換一個空的圖像。在使用if（ret）：語句開始處理之前，我添加了對空圖像的檢查。 （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() 

這次視頻播放直到結束，窗口仍然凍結並在幾秒鐘後關閉。這一次我沒有得到任何錯誤。但爲什麼窗口會凍結？我該如何解決這個問題？

Answer 1

的waitKey（）部分不應該依賴於框架的有效性，將其移出的條件：

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