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python users NEVER do any nessecary checks, so they get those errors.

your videocapture did not open, or it could not retrieve a frame.

cam=cv2.VideoCapture(1); # try 0 here instead
if not cap.isOpened():
    print("no capture!")
    return

while(True): 
    retval,img=cam.read();
    if retval == False:
        print("no capture!")
        return

again, you HAVE TO check the outcome of reading resources, opening streams, etc. (same for cv2.imread() or similar)

if you have only a single webcam attached to your box, the only valid camera id would be 0, not 1.

python users NEVER do any nessecary checks, so they get those errors.

your videocapture did not open, or it could not retrieve a frame.

cam=cv2.VideoCapture(1); # try 0 here instead
if not cap.isOpened():
    print("no capture!")
    return

while(True): 
    retval,img=cam.read();
    if retval == False:
        print("no capture!")
image!")
        return

again, you HAVE TO check the outcome of reading resources, opening streams, etc. (same for cv2.imread() or similar)

if you have only a single webcam attached to your box, the only valid camera id would be 0, not 1.