video 4 linux 2 notes

V4L2 -- Video For Linux Version Two

UVC

For a list of cameras that support UVC see the official Linux UVC site. Full UVC support in Linux:

I have been using a Logitech HD Pro Webcam C910, USB Device ID: 046d:0821, for my tests.

Gstreamer

Gstreamer is my favorite command-line tool for handling video.

This will display the camera view in a window:

gst-launch v4l2src ! ffmpegcolorspace ! xvimagesink

This will display the camera view in a window with a specified size (320x240):

gst-launch v4l2src ! video/x-raw-yuv,width=320,height=240 ! ffmpegcolorspace ! xvimagesink

Record video stream to a file using Motion JPEG encoding (MJPEG):

gst-launch --eos-on-shutdown v4l2src ! ffmpegcolorspace ! jpegenc ! avimux ! filesink location=video.avi

1920x1080: This will have a low frame-rate since it will essentially max-out the USB bandwidth. It's useful for capturing individual frames without compression.

gst-launch v4l2src device=/dev/video0 ! 'video/x-raw-yuv,width=1920,height=1080' ! xvimagesink

This will capture at a higher framerate, but it will undersample the pixels so you will get an effective 320x240 resolution.

gst-launch v4l2src device=/dev/video0 ! 'video/x-raw-yuv,width=640,height=480,framerate=60/1' ! xvimagesink

Record video and display the stream at the same time requires tee to split the stream. Notice how the name parameter of tee refers to a label defined at the end of the command-line. Also notice how queue is necessary so that xvimagesink will run in parallel with the filesink. Without this you would record video but you would see a window with a single frozen frame of video.

gst-launch --eos-on-shutdown v4l2src ! ffmpegcolorspace ! tee name=my_videosink ! jpegenc ! avimux ! filesink location=video.avi my_videosink. ! queue ! xvimagesink

Video test pattern:

gst-launch -v videotestsrc ! ffmpegcolorspace ! xvimagesink

gstreamer fbdevsink "ERROR: Pipeline doesn't want to pause."

If you are trying to use the framebuffer device for video playback then you may get an error like the one below. This is a permissions problem. Try adding sudo in front of the pipeline, or run the command as root.

$ gst-launch videotestsrc ! ffmpegcolorspace ! fbdevsink
Setting pipeline to PAUSED ...
ERROR: Pipeline doesn't want to pause.
Setting pipeline to NULL ...
Freeing pipeline ...

playback video on framebuffer (/dev/fbdev0)

sudo gst-launch uridecodebin uri=file:///home/noah/Videos/ct_scan_sample.flv ! ffmpegcolorspace ! fbdevsink

You can also use mplayer to play video on a framebuffer device. Be sure to specify fbdev2 if you want color.

sudo mplayer -vo fbdev2 ct_scan_sample.flv

Capture video and modify settings at the same time

Start `guvcview` with the --control_only option to display a GUI dialog to edit camera settings. This will work while another video display or capture application is already running.

guvcview --control_only --device=/dev/video0

fswebcam

`fswebcam` is a small and simple tool for grabbing still images from a camera. The world needs more apps like this. It can grab a single image or grab sequences of images in a loop. It can save images to a file or pipe them to stdout.

fswebcam --png --save fswebcam-test.png

Common UVC patterns with `uvcdynctrl`

apt-get install uvcdynctrl
uvcdynctrl --device=/dev/video1 --clist
uvcdynctrl --device=/dev/video1 --get='Focus, Auto'
uvcdynctrl --device=/dev/video1 --set='Focus, Auto' 0
uvcdynctrl --device=/dev/video1 --set='Focus (absolute)' 20

See also

• Mplayer_notes#Webcam_preview_--_play_raw_video_stream_from_video_device
• Mplayer_notes#Record_video_directly_from_video_device