A virtual webcam
There are a lot of fun things you can do with Linux. What I propose you today is to stream an arbitrary video as if it were your webcam and microphone output.
Some sort of disclaimer
Okay, so the draft of this blog post has been sitting on my hard drive for months now, and if I don’t post it yet, I’ll never post it. The reason I didn’t is because it is incomplete, as there’s a tiny something I don’t understand. I lost interest in this so I won’t look for the solution myself, but if you find it you’re welcome to share it with me! Anyway, let’s see this.
Setup a virtual webcam
First, install v4l2loopback. It’s a kernel module for Linux we will use to create a virtual webcam.
Then we will check for already existing webcams. Like almost every devices in UNIX, they’re located in
/dev/ and their names are
video followed by a number.
$ ls /dev/ | grep video video0
We see here that I have one webcam:
Let’s load v4l2loopback:
Now we’ll see if our virtual webcam is there:
$ ls /dev/ | grep video videvide
Yes, it is!
Setup a virtual microphone
Actually, we won’t create a microphone, we’ll create a soundcard. Microphones are handled by soundcards, which deal with the collected data.
To create a virtual soundcard we will proceed similarly as we did with the webcam, first we list the connected soundcards:
$ cat /proc/asound/cards 0 [Intel ]: HDA-Intel - HDA Intel HDA Intel at 0xfdff4000 irq 27 1 [NVidia ]: HDA-Intel - HDA NVidia HDA NVidia at 0xfcffc000 irq 17 2 [H2300 ]: USB-Audio - HP Webcam HD 2300 Hewlett Packard HP Webcam HD 2300 at usb-0000:00:1a.7-5, high speed
As you see, I have three soundcards.
Next we load the kernel module:
And then, I got a new soundcard!
$ cat /proc/asound/cards 0 [Intel ]: HDA-Intel - HDA Intel HDA Intel at 0xfdff4000 irq 27 1 [NVidia ]: HDA-Intel - HDA NVidia HDA NVidia at 0xfcffc000 irq 17 2 [H2300 ]: USB-Audio - HP Webcam HD 2300 Hewlett Packard HP Webcam HD 2300 at usb-0000:00:1a.7-5, high speed 3 [Loopback ]: Loopback - Loopback Loopback 1
It’s the one called Loopback, remember its number: 3.
Stream video to the virtual webcam
ffmpeg to extract a stream from a file and input it to the virtual webcam, in this case
/dev/video1. You don’t have to, but you should read at least its synopsis.
ffmpeg -re -i 'your/file.avi' -f v4l2 /dev/video1
If it doesn’t work, you’ll have to explicitly set options for the input file, read
ffmpeg‘s manpage to know those.
Ok, this is the part I’m unsure about. I don’t get why I have to specify
hw:3,1 . If this setting doesn’t work for you, well, try trial and error. And I you know why it’s this and not anything else, I’ll be glad to hear why!
$ ffmpeg -i some/test/file.mp3 -f alsa hw:3,1
Both sound and video
First, list your streams with
ffprobe, you’ll get something like this:
$ ffprobe your/file.avi Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 512x384 [SAR 1:1 DAR 4:3], 1005 kb/s, 29.97 fps, 29.97 tbr, 29.97 tbn, 29.98 tbc Stream #0:1: Audio: mp3 (U / 0x0055), 48000 Hz, stereo, s16p, 133 kb/s
It means that the first stream of the first file (
#0:0) is a video whereas the second (
#0:1) is the audio stream.
And then you can use this with ffmpeg’s option
-map to specify where to output respectively the video stream and the audio stream:
$ ffmpeg -i "your/file.avi" -map 0:0 -f v4l2 /dev/video1 -map 0:1 -f alsa hw:3,1
And that’s it! now you have a virtual webcam and a virtual microphone you can use, for example, to stream videos on videochats.