Create openCV VideoCapture from interface name instead of camera numbers

user_5969609 0
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The normal way to create a videocapture is this:

cam = cv2.VideoCapture(n)

where n corresponds to the number of /dev/video0, dev/video1

But because I'm building a robot that uses multiple cameras for different things, I needed to make sure that it was being assigned to the correct camera, I created udev rules that created devices with symbolic links to the correct port whenever a specific camera was plugged in.

They appear to be working because when I look in the /dev directory I can see the link:

/dev/front_cam -> video1

However I'm not sure how to actually use this now.

I thought I could just open it from the filename, as if it was a video, but cam = cv2.VideoCapture('/dev/front_cam') doesn't work.

Neither does cv2.VideoCapture('/dev/video1')

It doesn't throw an error, it does return a VideoCapture object, just not one that's opened (cam.isOpened() returns False).

udev opencv interface python
5 Answers

Each of my video4linux devices creates 2 device nodes. For example, /dev/video0 and /dev/video1 are both related to my internal webcam. When I plug in a second USB webcam, /dev/video2 and /dev/video3 both appear. However, I can only use the lower-numbered device of each pair for video capture (i.e. /dev/video0 and /dev/video2).

I watched my device arrival with udevadm monitor, and then inspected each of the camera devices with udevadm info --path=$PATH_FROM_UDEVADM_MONITOR --attribute-walk. The devices which work for video capture have ATTR{index}=="0".

Maybe instead of trying to open /dev/video1, you just need to open /dev/video0:

cam = cv2.CaptureVideo("/dev/video0")
user_123049510

If you know the model of the camera, you can look it up in /dev/v4l/by-id/.... We are using an HDMI-USB video converter, and we connect to it like this:

#! /usr/bin/env python
import os
import re
import cv2

DEFAULT_CAMERA_NAME = '/dev/v4l/by-id/usb-AVerMedia_Technologies__Inc._Live_Gamer_Portable_2_Plus_5500114600612-video-index0'

device_num = 0
if os.path.exists(DEFAULT_CAMERA_NAME):
    device_path = os.path.realpath(DEFAULT_CAMERA_NAME)
    device_re = re.compile("\/dev\/video(\d+)")
    info = device_re.match(device_path)
    if info:
        device_num = int(info.group(1))
        print("Using default video capture device on /dev/video" + str(device_num))
cap = cv2.VideoCapture(device_num)

This follows the device name symlink to the /dev/video name, then parses that for the device number.

user_36619540

A possibility that was not explored in the other answers is to use the "name" file in the /sys/class/video4linux/video*/ directories.

Example :

def get_camera(camera_name):
    cam_num = None
    for file in os.listdir("/sys/class/video4linux"):
        real_file = os.path.realpath("/sys/class/video4linux/" + file + "/name")
        with open(real_file, "rt") as name_file:
            name = name_file.read().rstrip()
        if camera_name in name:
            cam_num = int(re.search("\d+$", file).group(0))
            found = "FOUND!"
        else:
            found = "      "
        print("{} {} -> {}".format(found, file, name))
    return cam_num

Which gives :

get_camera('HUE')
FOUND! video1 -> HUE HD Pro Camera: HUE HD Pro C
    video0 -> HP HD Camera: HP HD Camera
user_27988020
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