More Webcams, Fewer Problems

Posted in Linux, Robotics - September 14, 2017 - 0 Comment

I want to have two applications using the same webcam under Linux, in my case an app using Kivy to render the webcam view and let people interact with it, and ROS.

My first hope had been to have a Kivy app that subscribed to a ROS image topic and displayed the resulting image, but I’ve spent two days beating on it and only found a variety of ways to make a python program give me a segfault.

The new plan is to use v4l2loopback to create two virtual cameras, and have them both fed from /dev/video0 (the actual camera).

sudo modprobe v4l2loopback devices=2

That gets me the two devices. The thing is, these devices don’t actually output any video. They’re just fake video devices that a program can write to. There are a lot of instructions on feeding them with ffmpeg. For some reason, my computer says it has ffmpeg installed, but locate can’t locate it. Instead, I’ve used gstreamer to set up /dev/video1 as a video device that ROS’s usb_cam node can handle, and configured the launch file for usb_cam to read /dev/video1

gst-launch v4l2src device=/dev/video0 ! video/x-raw-yuv,width=640,height=480,framerate=15/1 ! v4l2sink device=/dev/video1

That solves half of my problem. The other half is having the second video device get fed, and getting Kivy to read /dev/video2.

gst-launch v4l2src device=/dev/video0 ! video/x-raw-yuv,width=640,height=480,framerate=15/1 ! tee name=t ! v4l2sink device=/dev/video1 t. ! queue ! v4l2sink device=/dev/video2

Adding the tee and queue gives me two video devices playing the same video, both fed by /dev/video0. The usb_cam ROS node can read either one of them. The basic Kivy camera app that I bashed together doesn’t work, though. It seems to default to trying to open /dev/video0, and then failing because the gst-launch invocation is using it.

AnchorLayout:
anchor_x:"center"
anchor_y:"center"
   Camera:
      id: camera
      resolution:(640,480)
      play: True
      index:2
   KeyboardListener:
      id:kbd_lstn

Adding index:2 to my Kivy app’s .kv file gets me a kivy app with my video in it. As long as my ROS nodes are looking at /dev/video1 and my Kivy app is looking at /dev/video2, no one steps on anyone’s toes, and they both can operate at the same time.

Gizmosmith