The question here is how to capture a JPEG still image from a V4L2 (Video for Linux 2) webcam, using Linux tools. The webcam in question is a Creative Live!Cam Optia; because it is a UVC (USB video class) compatible camera, the solution should at least work with all other UVC webcams, too.

Working alternative

Instructions

The only working alternative is, for now, to use gstreamer (to show the stream) and a screenshot tool. You cannot use all video sinks, as some are not accessible to the screenshot tools. What can be captured by the screenshot tools is the “ximagesink” video sink. How to do it:

1. Execute this command in a terminal while your webcam is connected:

   gst-launch v4l2src ! jpegdec ! ffmpegcolorspace ! ximagesink

2. Now in another terminal, create a screenshot by executing the commandscrot.
3. You may now write a script that gets a screenshot like this and handles / processes it, like uploading to a server.

Background Infos

To test things about gstreamer and screenshots, you can also use the test video source for testing. The following worked for creating screenshots like above:

gst-launch videotestsrc ! queue ! ximagesink

In my tests, the following video sinks did not work to create screenshots like above:

• autovideosink (shows green are in screenhosts)
• dfbvideosink (shows green are in screenhosts)
• glimagesink (shows green are in screenhosts)
• xvimagesink (shows green are in screenhosts)
• sdlvideosink (does not work at all)
• gconfvideosink (does not work at all)

I found many very good snippets for working with gstreamer. And a lot of ways to create screenshots in Linux.

Perhaps working alternatives

fswebcam

It is possible to use fswebcam (http://www.firestorm.cx/fswebcam).

Take screenshots with gstreamer

You may use gstreamer to take the screenshots. A webcam that serves JPEG images just serves JPEG images one after the other. The following worked to view the first captured image from a file that contains JPEG after JPEG:

gst-launch v4l2src ! filesink location=file.jpg

kuickshow file.jpg

This is even a very promising path, as you can combine some JPGs to reduce the noise.

Use gstreamer to pipe to vlc

You may use gstreamer with a filesink and then pipe that to vlc. The following created basic screen output here:

gst-launch v4l2src ! jpegdec ! autovideosink

The following recorded and played an avi video:

gst-launch v4l2src ! avimux ! filesink location=video.avi

xine video.avi

(Perhaps you need instead something like this or similar, but VLC cannot play this, saying: “avi demuxer error: avi module discarded (invalid file)”:)

gst-launch v4l2src device=/dev/video0 ! avimux ! filesink location=video.avi

The following recorded and played an avi video, using forwarding to stdout:

gst-launch v4l2src ! avimux ! fdsink | cat > video.avi

xine video.avi

(This however makes xine not recognize the format.)

And now, piping to vlc media player is possible by:

gst-launch v4l2src ! avimux ! fdsink | vlc -

(But this is not possible yet.) (You may need the “ffmpegcolorspace” element, it’s important.) (See here for the documentation: http://wiki.videolan.org/Uncommon_uses.)

Something comparable should be possible with xine:

gst-launch v4l2src ! avimux ! fdsink | xine stdin://

(However, xine does not recognize the file format and there’s yet a way to find
how to tell it explicitly about that. See here for that:
http://dvd.sourceforge.net/xine-howto/en_GB/html/howto.html#toc10.17 )

Non-working alternatives

ffmpeg to capture and convert

It is also possible to use ffmpeg to capture and transcode the stream from a V4L2 device. The command would be something like:

ffmpeg -v 100 -f video4linux2 -s 320x240 -i /dev/video0 -f audio_device -i /dev/dsp2 -f m4v test.m4v

This however leads to an error:

[video4linux2 @ 0xb7f64610]Cannot find a proper format.

Which might mean that the camera uses a compressed format. See this mailinglist contribution.

See for that and a discussion of alternatives: http://lists.berlios.de/pipermail/linux-uvc-devel/2007-September/002164.html

You may then pipe the ouput to vlc.

transcode to capture and convert

It is possible to use transcode and and pipe it’s output to VLC (as seen here:
http://wiki.videolan.org/Uncommon_uses ) and stream it from there.

On the transcode side something like:

transcode -x v4l2,null -g 640x480 -i /dev/video0 -w 4000 -y ffmpeg -F mjpeg -o test.avi

This leads however to this error:

[import_v4l2.so]: no usable pixel format supported by card

As also documented here: http://lists.berlios.de/pipermail/linux-uvc-devel/2007-September/002164.html

gstreamer

gstreamer is no alternative as it was impossible to create a network stream from it.

uvc-streamer

It is possible to use UVC-Streamer ( http://naaa.de/uvc_streamer.htm ), which works for V4L2 devices; however there were compilation errors.

mjpeg-streamer

It is possible to use MJPEG-Streamer, which is the successor to UVC-Streamer; however, compile errors happened in both the 35 and 51 revisions.

uvccapture

It is possible to use uvccapture; however, this is rather a hack and has errors when compiling.

MPEG4IP

MPEG4IP might be a possibility, but their tarball is corrupt and their development stopped.

ucview

You may try ucview (an application based on the unicap framework). There are .debs available; however, I wasn’t able to get an image (though it supports MJPEG); it would be the most comfortable software for capturing selected frames.

luvcview

You may try luvcview.