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ffmpeg:main-ffmpeg

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ffmpeg is a very powerful command line tool to manipulate audio and video. Here are some command to acquire, cut, split, copy, transcode, compose video and audio files. The following commands assume using Linux.

To capture the content of a part of a the screen of with the upper left corner x=50,y=150 :

ffmpeg -video_size 1024x768 -framerate 25 -f x11grab -i :0.0+50,150 output.mp4

To capture the content of a specific window, it's a bit complicated on linux as we can use the window's title (-title argurment). So, we first need to know the id of the window :

wmctrl -l
0x02200002  0 rizen5 XdndCollectionWindowImp
0x02200003  0 rizen5 unity-launcher
0x02200004  0 rizen5 unity-panel
0x02200005  0 rizen5 unity-dash
0x02200006  0 rizen5 Hud
0x01a0000a -1 rizen5 Desktop
0x058000af  0 rizen5 emacs@rizen5
0x06400015  0    N/A V-REP PRO EDU - New file - rendering: 2 ms (8.0 fps) - SIMULATION STOPPED

The first field is the id of the windows. For example, here, we will record the content of the V-REP window, so the id is 0x06400015. The location on the screen and the size of the window is given by :

xwininfo -shape -id 0x06400015
 
xwininfo: Window id: 0x6400015 "V-REP PRO EDU - New file - rendering: 2 ms (8.0 fps) - SIMULATION STOPPED"
 
  Absolute upper-left X:  241
  Absolute upper-left Y:  196
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 1545
  Height: 810
  Depth: 24
  Visual: 0x27
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x6400014 (not installed)
  Bit Gravity State: NorthWestGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +241+196  -134+196  -134-74  +241-74
  -geometry 1545x810+241-74
 
  No window shape defined
  No border shape defined

The window as a size of 1545×810 and starts at (241,196)

ffmpeg -video_size 1545x810 -framerate 25 -f x11grab -i :0.0+241,196 output.mp4

It can be wise to change to a common video display size , e.g. 1366×768, 1600×900 or 1920×1200. this can be done by either two ways, using the name (title) of the window :

# change the size and location
wmctrl -r "V-REP PRO EDU" -e 0,100,100,1600,900
# or change only size
xdotool search --name "V-REP PRO EDU" windowsize 1600 900
 
# when not working with window's name (not resizing the right window)
# use id instead of name
wmctrl -i -r 0x06800015 -e 0,100,100,1600,900

Then do not forget to re run xwininfo as the location and the size might have been changed by the display manager.

Get the audio device:

arecord -l
**** List of CAPTURE Hardware Devices ****
card 1: Device [USB Audio Device], device 0: USB Audio [USB Audio]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Generic [HD-Audio Generic], device 0: ALC892 Analog [ALC892 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Generic [HD-Audio Generic], device 2: ALC892 Alt Analog [ALC892 Alt Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Recording 30 seconds with with audio:

xcorn=100
ycorn=80
duration=00:00:30
soundcard=hw:2 # hw:1
nbchan=2 # 1
ffmpeg -y -video_size 1600x900 -f x11grab -i ${DISPLAY}.0+${xcorn},${ycorn} -f alsa -thread_queue_size 16384 -ac ${nbchan} -i ${soundcard} -acodec aac -strict experimental -t ${duration} ./output.mp4

Fullscreen recording with audio for 30 seconds:

duration=00:00:30
soundcard=hw:2 # hw:1
nbchan=2 # 1
ffmpeg -y -video_size 1920x1050 -f x11grab -i ${DISPLAY}.0+0,0 -f alsa -thread_queue_size 16384 -ac ${nbchan} -i ${soundcard} -acodec aac -strict experimental -t ${duration} ./output.mp4

It may turn out that it does not record anything in the audio channel ?!? in this case you can try pulse :

duration=00:00:30
nbchan=2 # 1
ffmpeg -y -video_size 1920x1050 -f x11grab -i ${DISPLAY}.0+0,0 -f alsa -thread_queue_size 16384 -ac ${nbchan} -i pulse  -t ${duration} ./output.mp4

Comprehensive information is available in the ffmpeg wiki.

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ffmpeg/main-ffmpeg.txt · Last modified: 2020/11/10 16:03 by admin