Error (893) can't open camera by index, python and jupyter notebook

Question

The code:

import cv2
   
cap= cv2.VideoCapture(1)

if not cap.isOpened():
    raise IOError("Cannot open webcam")

cntr =0
while True:
    ret,frame = cap.read()
    
    cntr= cntr+1;
    if ((cntr%30)==0):
            
        cv2.putText(frame,'cool' ,(50,50), cv2.FONT_HERSHEY_SIMPLEX, 0.7,(0,0,255),2)
        cv2.imshow('Text Detection Tutorial',frame)
        if cv2.waitKey(2) & 0xff == ord('q'):
             break
cap.release()
cv2.destroyAllWindows()

The error in the jupyter-notebook window:

17 raise IOError("Cannot open webcam")

OSError: Cannot open webcam

I have two devices, a built-in webcam /dev/video0 and an USB webcam /dev/video1.

They both appear listed with ls -ltrh /dev/video*

Both cameras work fine with guvcview.

I use jupyter-notebook to run the code above.

When I use cap= cv2.VideoCapture(0) it works fine. But when I use cap= cv2.VideoCapture(1) for the USB camera I get the next error in the terminal:

[ WARN:0] global /tmp/pip-req-build-ms668fyv/opencv/modules/videoio/src/cap_v4l.cpp (893) open VIDEOIO(V4L2:/dev/video1): can't open camera by index

I tried to give all permissions to /dev/video1, sudo chmod 777 /dev/video1.

I'm using linux mint.

Answer 1

Linux creates 2 files in the /dev/video for each webcam. The lower numbered one (/dev/video0 in your case ) is for capturing stream and the following one (/dev/video1 in your case) includes some data about camera.

So when you tried with the index number 0, it works fine and opens the camera with VideoCapture(0). But what about the 2nd camera ? There may be some different reasons why it is missing:

• Current driver systems doesn't support that ( not a video4linux device or old kernel version )
• It is not usb-plugged camera.

As a result, its not bacause of VideoCapture can not read; its just because of you are trying to open it with wrong index which is data file of camera.

Note: I suggest you to try VideoCapture(-1) also, this also works in some cases interestingly. Sometimes for substreams of webcams or luckiliy opening a camera's substream which has its own driver.