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@chapter Input Devices
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@c man begin INPUT DEVICES
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Input devices are configured elements in Libav which allow to access
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the data coming from a multimedia device attached to your system.
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When you configure your Libav build, all the supported input devices
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are enabled by default. You can list all available ones using the
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configure option "--list-indevs".
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You can disable all the input devices using the configure option
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"--disable-indevs", and selectively enable an input device using the
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option "--enable-indev=@var{INDEV}", or you can disable a particular
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input device using the option "--disable-indev=@var{INDEV}".
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The option "-formats" of the ff* tools will display the list of
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supported input devices (amongst the demuxers).
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A description of the currently available input devices follows.
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@section alsa
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ALSA (Advanced Linux Sound Architecture) input device.
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To enable this input device during configuration you need libasound
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installed on your system.
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This device allows capturing from an ALSA device. The name of the
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device to capture has to be an ALSA card identifier.
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An ALSA identifier has the syntax:
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@example
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hw:@var{CARD}[,@var{DEV}[,@var{SUBDEV}]]
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@end example
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where the @var{DEV} and @var{SUBDEV} components are optional.
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The three arguments (in order: @var{CARD},@var{DEV},@var{SUBDEV})
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specify card number or identifier, device number and subdevice number
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(-1 means any).
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To see the list of cards currently recognized by your system check the
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files @file{/proc/asound/cards} and @file{/proc/asound/devices}.
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For example to capture with @file{ffmpeg} from an ALSA device with
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card id 0, you may run the command:
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@example
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ffmpeg -f alsa -i hw:0 alsaout.wav
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@end example
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For more information see:
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@url{http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html}
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@section bktr
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BSD video input device.
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@section dv1394
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Linux DV 1394 input device.
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@section fbdev
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Linux framebuffer input device.
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The Linux framebuffer is a graphic hardware-independent abstraction
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layer to show graphics on a computer monitor, typically on the
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console. It is accessed through a file device node, usually
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@file{/dev/fb0}.
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For more detailed information read the file
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Documentation/fb/framebuffer.txt included in the Linux source tree.
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To record from the framebuffer device @file{/dev/fb0} with
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@file{ffmpeg}:
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@example
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ffmpeg -f fbdev -r 10 -i /dev/fb0 out.avi
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@end example
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You can take a single screenshot image with the command:
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@example
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ffmpeg -f fbdev -vframes 1 -r 1 -i /dev/fb0 screenshot.jpeg
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@end example
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See also @url{http://linux-fbdev.sourceforge.net/}, and fbset(1).
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@section jack
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JACK input device.
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To enable this input device during configuration you need libjack
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installed on your system.
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A JACK input device creates one or more JACK writable clients, one for
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each audio channel, with name @var{client_name}:input_@var{N}, where
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@var{client_name} is the name provided by the application, and @var{N}
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is a number which identifies the channel.
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Each writable client will send the acquired data to the Libav input
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device.
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Once you have created one or more JACK readable clients, you need to
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connect them to one or more JACK writable clients.
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To connect or disconnect JACK clients you can use the
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@file{jack_connect} and @file{jack_disconnect} programs, or do it
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through a graphical interface, for example with @file{qjackctl}.
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To list the JACK clients and their properties you can invoke the command
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@file{jack_lsp}.
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Follows an example which shows how to capture a JACK readable client
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with @file{ffmpeg}.
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@example
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# Create a JACK writable client with name "ffmpeg".
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$ ffmpeg -f jack -i ffmpeg -y out.wav
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# Start the sample jack_metro readable client.
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$ jack_metro -b 120 -d 0.2 -f 4000
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# List the current JACK clients.
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$ jack_lsp -c
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system:capture_1
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system:capture_2
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system:playback_1
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system:playback_2
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ffmpeg:input_1
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metro:120_bpm
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# Connect metro to the ffmpeg writable client.
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$ jack_connect metro:120_bpm ffmpeg:input_1
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@end example
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For more information read:
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@url{http://jackaudio.org/}
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@section libdc1394
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IIDC1394 input device, based on libdc1394 and libraw1394.
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@section oss
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Open Sound System input device.
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The filename to provide to the input device is the device node
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representing the OSS input device, and is usually set to
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@file{/dev/dsp}.
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For example to grab from @file{/dev/dsp} using @file{ffmpeg} use the
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command:
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@example
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ffmpeg -f oss -i /dev/dsp /tmp/oss.wav
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@end example
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For more information about OSS see:
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@url{http://manuals.opensound.com/usersguide/dsp.html}
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@section sndio
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sndio input device.
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To enable this input device during configuration you need libsndio
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installed on your system.
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The filename to provide to the input device is the device node
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representing the sndio input device, and is usually set to
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@file{/dev/audio0}.
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For example to grab from @file{/dev/audio0} using @file{ffmpeg} use the
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command:
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@example
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ffmpeg -f sndio -i /dev/audio0 /tmp/oss.wav
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@end example
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@section video4linux and video4linux2
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Video4Linux and Video4Linux2 input video devices.
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The name of the device to grab is a file device node, usually Linux
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systems tend to automatically create such nodes when the device
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(e.g. an USB webcam) is plugged into the system, and has a name of the
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kind @file{/dev/video@var{N}}, where @var{N} is a number associated to
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the device.
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Video4Linux and Video4Linux2 devices only support a limited set of
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@var{width}x@var{height} sizes and framerates. You can check which are
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supported for example with the command @file{dov4l} for Video4Linux
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devices and the command @file{v4l-info} for Video4Linux2 devices.
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If the size for the device is set to 0x0, the input device will
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try to autodetect the size to use.
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Only for the video4linux2 device, if the frame rate is set to 0/0 the
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input device will use the frame rate value already set in the driver.
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Video4Linux support is deprecated since Linux 2.6.30, and will be
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dropped in later versions.
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Follow some usage examples of the video4linux devices with the ff*
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tools.
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@example
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# Grab and show the input of a video4linux device, frame rate is set
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# to the default of 25/1.
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ffplay -s 320x240 -f video4linux /dev/video0
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# Grab and show the input of a video4linux2 device, autoadjust size.
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ffplay -f video4linux2 /dev/video0
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# Grab and record the input of a video4linux2 device, autoadjust size,
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# frame rate value defaults to 0/0 so it is read from the video4linux2
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# driver.
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ffmpeg -f video4linux2 -i /dev/video0 out.mpeg
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@end example
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@section vfwcap
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VfW (Video for Windows) capture input device.
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The filename passed as input is the capture driver number, ranging from
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0 to 9. You may use "list" as filename to print a list of drivers. Any
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other filename will be interpreted as device number 0.
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@section x11grab
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X11 video input device.
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This device allows to capture a region of an X11 display.
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The filename passed as input has the syntax:
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@example
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[@var{hostname}]:@var{display_number}.@var{screen_number}[+@var{x_offset},@var{y_offset}]
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@end example
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@var{hostname}:@var{display_number}.@var{screen_number} specifies the
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X11 display name of the screen to grab from. @var{hostname} can be
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ommitted, and defaults to "localhost". The environment variable
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@env{DISPLAY} contains the default display name.
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@var{x_offset} and @var{y_offset} specify the offsets of the grabbed
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area with respect to the top-left border of the X11 screen. They
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default to 0.
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Check the X11 documentation (e.g. man X) for more detailed information.
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Use the @file{dpyinfo} program for getting basic information about the
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properties of your X11 display (e.g. grep for "name" or "dimensions").
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For example to grab from @file{:0.0} using @file{ffmpeg}:
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@example
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ffmpeg -f x11grab -r 25 -s cif -i :0.0 out.mpg
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# Grab at position 10,20.
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ffmpeg -f x11grab -25 -s cif -i :0.0+10,20 out.mpg
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@end example
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@c man end INPUT DEVICES