python - Cannot capture video using OpenCV in python in Jetson Xavier

Question

I am trying to read the camera image on Jetson Xavier (ubuntu 18). I am facing a problem. When I run the following code it gives a warning and gives a black (full) image.

[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (933) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1

cam=cv2.VideoCapture(0)
if cam.isOpened():
    grab,img = cam.read()
    if grab is True:
        cv2.imshow('sample image',img)
    else:
        print(f"Image not found")
else:
    print("Camera not openedd")
       

cv2.waitKey(0) # waits until a key is pressed
cv2.destroyAllWindows() # destroys the window showing image

If I use 'dev/video0' to read the image i.e.

cam=cv2.VideoCapture('dev/video0')

I get the warning and custom error message of camera not opened

[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (711) open OpenCV | GStreamer warning: Error opening bin: no element "dev" [ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (480) isPipelinePlaying OpenCV | GStreamer warning: GStreamer: pipeline have not been created Camera not opened

Then I created gstream string and passed that to video capture as shown below. the string is as follow

gstr = 'varguscamerasrc ! video/x-raw(memory:NVMM), width=(int)1920, height=(int)1080, format=(string)NV12, framerate=(fraction)60/1 ! nvvidconv flip-method=0 ! video/x-raw, width=(int)1280, height=(int)720, format=(string)BGRx ! videoconvert ! video/x-raw, format=(string)BGR ! appsink'

cap = cv2.VideoCapture(gstr, cv2.CAP_GSTREAMER)

I get the following error

Error generated. /dvs/git/dirty/git-master_linux/multimedia/nvgstreamer/gst-nvarguscamera/gstnvarguscamerasrc.cpp, execute:645 No cameras available

(python3:15402): GStreamer-CRITICAL **: 19:08:54.835: gst_mini_object_set_qdata: assertion 'object != NULL' failed
[ WARN:0] global /home/nvidia/host/build_opencv/nv_opencv/modules/videoio/src/cap_gstreamer.cpp (933) open OpenCV | GStreamer warning: Cannot query video position: status=0, value=-1, duration=-1
Traceback (most recent call last):

I am new to jetson please guide me. Thanks