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I am writing an app (running on ARM Ubuntu) that needs to be able to do image processing on the video stream from a webcam. The easiest way that I can get the data is through the output stream on a process, so I am planning to host a child process, run a command, and consume the output. My current plan is to use FFMPEG, but I can use any command-line tool that works on ARM. There are really two parts to my question:

  1. What FFMPEG (or other utility, if there is a better one) command do I need to use from the command line to get a stream of pixel data from a webcam?
  2. What format will the output be in (preferably just RGB data, no encodings or headers)? I am going to need to parse it in my app and need to know the encoding format.

Any information would be appreciated!

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With FFmpeg and video4linux2, you can get almost any webcam input. Make sure to use a recent version of ffmpeg from the download page or by compiling yourself. Your camera frame sizes and framerates can be checked with:

v4l2-ctl --list-formats-ext

For example, if you found out what framerate and picture size your webcam delivers:

ffmpeg -f video4linux2 -r 25 -s 640x480 -i /dev/video0 \
-c:v rawvideo -f rawvideo -pix_fmt yuv420p -

This would capture 25fps video at 640×480 pixels from the camera at /dev/video0 and output raw YUV frames encoded with the YUV 4:2:0 planar pixel format to stdout. The rawvideo encoder and format takes care of that. There will be no headers.

What you can change here:

  • Set a different framerate or -size.

  • Here, the - at the end sends the data to stdout, but you can choose a file as well if you want to.

  • To see what other pixel formats are available, run ffmpeg -pix_fmts. Their byte order is explained on fourcc.org. If you really want RGB data (although I doubt this is the default your camera outputs), you can use -pix_fmt rgb24, for example.

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  • Thanks! this is exactly what I was looking for. But shouldn't I see something in the terminal when I run it? I haven't tried it in my app yet, but shouldn't I see something? – Wasabi Fan Oct 1 '13 at 4:18
  • Yes, you should see something. If you don't get any output maybe it couldn't find the webcam. Depends on your setup – if you can paste the full uncut console output somewhere we could check. – slhck Oct 2 '13 at 17:41
  • Here's the output... not very exciting: pastebin.com/Z2hWV1uL – Wasabi Fan Oct 2 '13 at 23:07
  • I was able to capture a single image with a different utility, so I know the webcam works as /dev/video0 – Wasabi Fan Oct 2 '13 at 23:09
  • You're using a terribly outdated version of ffmpeg. As I mentioned in my post, please download a recent one. The one you have is actually not even from FFmpeg but a fork that released it under the wrong name. – slhck Oct 3 '13 at 5:55
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Definitely check out VLC's command line options. I'm not really familiar with it's output streams but I seem to have read that it could do pretty much what you're trying to do.

VLC homepage

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