OpenCV错误：在cv :: imshow（Python）中断言失败（size.width> 0 && size.height> 0）

Question

I went trough many similar topics like mine, but found no solution for my code. First of all I simply want to run my webcam and see a picture.

import numpy as np
import cv2

cap = cv2.VideoCapture(1)

while True:
  re,img=cap.read()
  cv2.imshow("video output", img)
  k = cv2.waitKey(10)&0xFF
  if k==27:
     break
cap.release()
cv2.destroyAllWindows()

I also tried it with:

if img is not None:

I still get this error:

OpenCV Error: Assertion failed (size.width>0 && size.height>0) in cv::imshow

I have to say that I am not using my laptop webcam so its id is not 0, but it should be 1. I read about solutions like in c++: frame=cvQueryFrame(capture); if (frame.empty()) break; frame=cvQueryFrame(capture); if (frame.empty()) break; But how to do it in Python? I do not think that this will fix my problem? Has anybody a solution?

Best regards

Answer 1

According to the OpenCV docs :

cap.read() returns a bool (True/False). If frame is read correctly, it will be True.

...

Sometimes, cap may not have initialized the capture. In that case, this code shows error. You can check whether it is initialized or not by the method cap.isOpened(). If it is True, OK. Otherwise open it using cap.open().

Thus your code becomes:

import numpy as np
import cv2

device = 1
cap = cv2.VideoCapture(device)
# if capture failed to open, try again
if not cap.isOpened():
    cap.open(device)

# only attempt to read if it is opened
if cap.isOpened:
    while True:
        re, img = cap.read()
        # Only display the image if it is not empty
        if re:
            cv2.imshow("video output", img)
        # if it is empty abort
        else:
            print "Error reading capture device"
            break
        k = cv2.waitKey(10) & 0xFF
        if k == 27:
            break
    cap.release()
    cv2.destroyAllWindows()
else:
    print "Failed to open capture device"

If you still have errors try changing device to -1, 0 or 2. Otherwise, it may be an issue unrelated to OpenCV such as driver issues.