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

I am running the following code on Raspberry Pi with pi camera, I have the broadcom drivers for it and all, but I am getting an error. Perhaps something to do with the dimensions of the video feed, but I do not know how to set it on Linux.

Code:

import cv2
import numpy as np

cap = cv2.VideoCapture()
while True:
    ret, img = cap.read()
    cv2.imshow('img', img)
    if cv2.waitKey(0) & 0xFF == ord('q):
        break

Error:

OpenCV Error: Assertion failed (size.width>0 && size.height>0) in imshow, 
file /home/pi/opencv-3.3.0/modules/highgui/src/window.cpp, line 325
Traceback (most recent call last):
  File "check_picam_with_opencv.py", line 10, in <module>
    cv2.imshow('img', img)
cv2.error: /home/pi/opencv-3.3.0/modules/highgui/src/window.cpp:325: error: 
(-215) size.width>0 && size.height>0 in function imshow

Answer

Provide an id to VideoCapture.

cap = cv2.VideoCapture(0)

Also check the value of ret, see if it’s TRUE or FALSE

print (ret)

Edit:

To capture a video, you need to create a VideoCapture object. Its argument can be either the device index or the name of a video file. Device index is just the number to specify which camera.

cap = cv2.VideoCapture(0)

To check whether the cap has been initialized or not, you can use cap.isOpened() function, which returns True for successful initialization and False for failure.

if cap.isOpened() == False:
    print ("VideoCapture failed")

cap.read() returns a bool (True/False). If frame is read correctly, it will be True. So you can check end of the video by checking this return value.

ret, frame = cap.read()
if ret == False:
    print("Frame is empty")

Further reading here.