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3.1.3 Using a USB Camera with Python

Last Version: 10/09/2025

This guide shows how to connect a USB camera, identify the correct device node, and capture and display video frames using Python.

Hardware Connection​

  1. Connect your USB camera to the development board:

  2. Connect the board to a monitor via HDMI cable to view the video output

Check Device Number​

  1. List all video devices:

    ls /dev/video*

    Example output:

  2. Unplug the camera and list again:

    ls /dev/video*

    Example output:

    • In this example, /dev/video20 and /dev/video21 are detected.
    • It is recommended to use the lower-numbered device (here /dev/video20) for capturing video frames.

Optional: Verify with v4l2-ctl tool:

  1. List all devices:

    v4l2-ctl --list-devices

    Example output:

  2. Check the details of /dev/video20:

    v4l2-ctl -d /dev/video20 --all

    If you see a Video Capture field, this node is for capturing video frames:

  3. Check the details of /dev/video21:

    v4l2-ctl -d /dev/video21 --all

    In this case, the output contains UVC Payload Header Metadata, this node captures video metadata, not video frames.

Set Up a Python Virtual Environment​

python3 -m venv ~/test1
source ~/test1/bin/activate
pip install opencv-python

Capture & Display Video​

Save the following code as opencv_test.py:

import cv2

# Open the camera, 0 is the default device index; if multiple cameras exist, change to 1, 2, etc.
cap = cv2.VideoCapture('/dev/video20')

# Check if successfully opened
if not cap.isOpened():
    print("Unable to open camera")
    exit()

while True:
    # Read a frame
    ret, frame = cap.read()
    if not ret:
        print("Unable to receive frame, exiting...")
        break

    # Display the frame
    cv2.imshow('USB Camera', frame)

    # Press 'q' key to exit
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# Release resources
cap.release()
cv2.destroyAllWindows()

Run inside the virtual environment:

python opencv_test.py

Note: Always use the full device path (e.g., /dev/video20) in VideoCapture(). Do not use the plain number 20.