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\input texinfo @c -*- texinfo -*-
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@settitle FFmpeg Documentation
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@titlepage
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@sp 7
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@center @titlefont{FFmpeg Documentation}
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@sp 3
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@end titlepage
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@chapter Introduction
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FFmpeg is a very fast video and audio converter. It can also grab from
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a live audio/video source.
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The command line interface is designed to be intuitive, in the sense
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that FFmpeg tries to figure out all parameters that can possibly be
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derived automatically. You usually only have to specify the target
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bitrate you want.
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FFmpeg can also convert from any sample rate to any other, and resize
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video on the fly with a high quality polyphase filter.
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@chapter Quick Start
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@c man begin EXAMPLES
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@section Video and Audio grabbing
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FFmpeg can grab video and audio from devices given that you specify the input
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format and device.
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@example
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ffmpeg -f oss -i /dev/dsp -f video4linux2 -i /dev/video0 /tmp/out.mpg
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@end example
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Note that you must activate the right video source and channel before
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launching FFmpeg with any TV viewer such as xawtv
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(@url{http://bytesex.org/xawtv/}) by Gerd Knorr. You also
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have to set the audio recording levels correctly with a
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standard mixer.
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@section X11 grabbing
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FFmpeg can grab the X11 display.
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@example
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ffmpeg -f x11grab -s cif -i :0.0 /tmp/out.mpg
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@end example
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0.0 is display.screen number of your X11 server, same as
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the DISPLAY environment variable.
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@example
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ffmpeg -f x11grab -s cif -i :0.0+10,20 /tmp/out.mpg
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@end example
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0.0 is display.screen number of your X11 server, same as the DISPLAY environment
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variable. 10 is the x-offset and 20 the y-offset for the grabbing.
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@section Video and Audio file format conversion
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* FFmpeg can use any supported file format and protocol as input:
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Examples:
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* You can use YUV files as input:
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@example
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ffmpeg -i /tmp/test%d.Y /tmp/out.mpg
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@end example
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It will use the files:
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@example
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/tmp/test0.Y, /tmp/test0.U, /tmp/test0.V,
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/tmp/test1.Y, /tmp/test1.U, /tmp/test1.V, etc...
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@end example
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The Y files use twice the resolution of the U and V files. They are
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raw files, without header. They can be generated by all decent video
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decoders. You must specify the size of the image with the @option{-s} option
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if FFmpeg cannot guess it.
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* You can input from a raw YUV420P file:
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@example
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ffmpeg -i /tmp/test.yuv /tmp/out.avi
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@end example
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test.yuv is a file containing raw YUV planar data. Each frame is composed
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of the Y plane followed by the U and V planes at half vertical and
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horizontal resolution.
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* You can output to a raw YUV420P file:
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@example
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ffmpeg -i mydivx.avi hugefile.yuv
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@end example
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* You can set several input files and output files:
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@example
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ffmpeg -i /tmp/a.wav -s 640x480 -i /tmp/a.yuv /tmp/a.mpg
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@end example
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Converts the audio file a.wav and the raw YUV video file a.yuv
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to MPEG file a.mpg.
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* You can also do audio and video conversions at the same time:
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@example
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ffmpeg -i /tmp/a.wav -ar 22050 /tmp/a.mp2
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@end example
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Converts a.wav to MPEG audio at 22050Hz sample rate.
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* You can encode to several formats at the same time and define a
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mapping from input stream to output streams:
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@example
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ffmpeg -i /tmp/a.wav -ab 64k /tmp/a.mp2 -ab 128k /tmp/b.mp2 -map 0:0 -map 0:0
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@end example
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Converts a.wav to a.mp2 at 64 kbits and to b.mp2 at 128 kbits. '-map
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file:index' specifies which input stream is used for each output
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stream, in the order of the definition of output streams.
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* You can transcode decrypted VOBs:
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@example
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ffmpeg -i snatch_1.vob -f avi -vcodec mpeg4 -b 800k -g 300 -bf 2 -acodec libmp3lame -ab 128k snatch.avi
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@end example
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This is a typical DVD ripping example; the input is a VOB file, the
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output an AVI file with MPEG-4 video and MP3 audio. Note that in this
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command we use B-frames so the MPEG-4 stream is DivX5 compatible, and
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GOP size is 300 which means one intra frame every 10 seconds for 29.97fps
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input video. Furthermore, the audio stream is MP3-encoded so you need
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to enable LAME support by passing @code{--enable-libmp3lame} to configure.
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The mapping is particularly useful for DVD transcoding
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to get the desired audio language.
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NOTE: To see the supported input formats, use @code{ffmpeg -formats}.
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* You can extract images from a video:
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@example
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ffmpeg -i foo.avi -r 1 -s WxH -f image2 foo-%03d.jpeg
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@end example
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This will extract one video frame per second from the video and will
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output them in files named @file{foo-001.jpeg}, @file{foo-002.jpeg},
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etc. Images will be rescaled to fit the new WxH values.
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The syntax @code{foo-%03d.jpeg} specifies to use a decimal number
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composed of three digits padded with zeroes to express the sequence
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number. It is the same syntax supported by the C printf function, but
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only formats accepting a normal integer are suitable.
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If you want to extract just a limited number of frames, you can use the
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above command in combination with the -vframes or -t option, or in
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combination with -ss to start extracting from a certain point in time.
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@c man end
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@chapter Invocation
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@section Syntax
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The generic syntax is:
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@example
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@c man begin SYNOPSIS
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ffmpeg [[infile options][@option{-i} @var{infile}]]... @{[outfile options] @var{outfile}@}...
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@c man end
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@end example
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@c man begin DESCRIPTION
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As a general rule, options are applied to the next specified
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file. Therefore, order is important, and you can have the same
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option on the command line multiple times. Each occurrence is
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then applied to the next input or output file.
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* To set the video bitrate of the output file to 64kbit/s:
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@example
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ffmpeg -i input.avi -b 64k output.avi
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@end example
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* To force the frame rate of the output file to 24 fps:
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@example
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ffmpeg -i input.avi -r 24 output.avi
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@end example
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* To force the frame rate of the input file (valid for raw formats only)
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to 1 fps and the frame rate of the output file to 24 fps:
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@example
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ffmpeg -r 1 -i input.m2v -r 24 output.avi
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@end example
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The format option may be needed for raw input files.
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By default, FFmpeg tries to convert as losslessly as possible: It
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uses the same audio and video parameters for the outputs as the one
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specified for the inputs.
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@c man end
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@c man begin OPTIONS
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@section Main options
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@table @option
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@item -L
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Show license.
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@item -h
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Show help.
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@item -version
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Show version.
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@item -formats
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Show available formats, codecs, protocols, ...
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@item -f @var{fmt}
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Force format.
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@item -i @var{filename}
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input file name
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@item -y
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Overwrite output files.
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@item -t @var{duration}
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Restrict the transcoded/captured video sequence
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to the duration specified in seconds.
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@code{hh:mm:ss[.xxx]} syntax is also supported.
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@item -fs @var{limit_size}
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Set the file size limit.
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@item -ss @var{position}
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Seek to given time position in seconds.
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@code{hh:mm:ss[.xxx]} syntax is also supported.
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@item -itsoffset @var{offset}
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Set the input time offset in seconds.
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@code{[-]hh:mm:ss[.xxx]} syntax is also supported.
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This option affects all the input files that follow it.
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The offset is added to the timestamps of the input files.
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Specifying a positive offset means that the corresponding
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streams are delayed by 'offset' seconds.
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@item -title @var{string}
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Set the title.
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@item -timestamp @var{time}
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Set the timestamp.
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@item -author @var{string}
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Set the author.
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@item -copyright @var{string}
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Set the copyright.
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@item -comment @var{string}
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Set the comment.
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@item -album @var{string}
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Set the album.
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@item -track @var{number}
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Set the track.
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@item -year @var{number}
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Set the year.
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@item -v @var{number}
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Set the logging verbosity level.
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@item -target @var{type}
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Specify target file type ("vcd", "svcd", "dvd", "dv", "dv50", "pal-vcd",
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"ntsc-svcd", ... ). All the format options (bitrate, codecs,
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buffer sizes) are then set automatically. You can just type:
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@example
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ffmpeg -i myfile.avi -target vcd /tmp/vcd.mpg
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@end example
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Nevertheless you can specify additional options as long as you know
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they do not conflict with the standard, as in:
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@example
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ffmpeg -i myfile.avi -target vcd -bf 2 /tmp/vcd.mpg
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@end example
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@item -dframes @var{number}
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Set the number of data frames to record.
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@item -scodec @var{codec}
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Force subtitle codec ('copy' to copy stream).
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@item -newsubtitle
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Add a new subtitle stream to the current output stream.
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@item -slang @var{code}
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Set the ISO 639 language code (3 letters) of the current subtitle stream.
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@end table
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@section Video Options
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@table @option
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@item -b @var{bitrate}
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Set the video bitrate in bit/s (default = 200 kb/s).
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@item -vframes @var{number}
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Set the number of video frames to record.
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@item -r @var{fps}
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Set frame rate (Hz value, fraction or abbreviation), (default = 25).
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@item -s @var{size}
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Set frame size. The format is @samp{wxh} (ffserver default = 160x128, ffmpeg default = same as source).
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The following abbreviations are recognized:
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@table @samp
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@item sqcif
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128x96
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@item qcif
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176x144
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@item cif
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352x288
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@item 4cif
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704x576
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@item qqvga
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160x120
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@item qvga
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320x240
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@item vga
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640x480
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@item svga
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800x600
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@item xga
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1024x768
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@item uxga
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1600x1200
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@item qxga
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2048x1536
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@item sxga
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1280x1024
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@item qsxga
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2560x2048
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@item hsxga
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5120x4096
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@item wvga
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852x480
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@item wxga
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1366x768
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@item wsxga
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1600x1024
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@item wuxga
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1920x1200
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@item woxga
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2560x1600
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@item wqsxga
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3200x2048
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@item wquxga
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3840x2400
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@item whsxga
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6400x4096
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@item whuxga
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7680x4800
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@item cga
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320x200
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@item ega
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640x350
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@item hd480
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852x480
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@item hd720
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1280x720
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@item hd1080
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1920x1080
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@end table
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@item -aspect @var{aspect}
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Set aspect ratio (4:3, 16:9 or 1.3333, 1.7777).
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@item -croptop @var{size}
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Set top crop band size (in pixels).
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@item -cropbottom @var{size}
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Set bottom crop band size (in pixels).
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@item -cropleft @var{size}
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Set left crop band size (in pixels).
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@item -cropright @var{size}
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Set right crop band size (in pixels).
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@item -padtop @var{size}
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Set top pad band size (in pixels).
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@item -padbottom @var{size}
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Set bottom pad band size (in pixels).
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@item -padleft @var{size}
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Set left pad band size (in pixels).
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@item -padright @var{size}
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Set right pad band size (in pixels).
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@item -padcolor @var{hex_color}
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Set color of padded bands. The value for padcolor is expressed
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as a six digit hexadecimal number where the first two digits
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represent red, the middle two digits green and last two digits
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blue (default = 000000 (black)).
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@item -vn
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Disable video recording.
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@item -bt @var{tolerance}
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Set video bitrate tolerance (in bits, default 4000k).
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Has a minimum value of: (target_bitrate/target_framerate).
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In 1-pass mode, bitrate tolerance specifies how far ratecontrol is
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willing to deviate from the target average bitrate value. This is
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not related to min/max bitrate. Lowering tolerance too much has
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an adverse effect on quality.
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@item -maxrate @var{bitrate}
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Set max video bitrate (in bit/s).
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Requires -bufsize to be set.
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@item -minrate @var{bitrate}
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Set min video bitrate (in bit/s).
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Most useful in setting up a CBR encode:
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@example
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ffmpeg -i myfile.avi -b 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v
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@end example
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It is of little use elsewise.
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@item -bufsize @var{size}
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Set video buffer verifier buffer size (in bits).
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@item -vcodec @var{codec}
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Force video codec to @var{codec}. Use the @code{copy} special value to
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tell that the raw codec data must be copied as is.
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@item -sameq
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Use same video quality as source (implies VBR).
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@item -pass @var{n}
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Select the pass number (1 or 2). It is useful to do two pass
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encoding. The statistics of the video are recorded in the first
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pass and the video is generated at the exact requested bitrate
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in the second pass.
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On pass 1, you may just deactivate audio and set output to null,
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examples for Windows and Unix:
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@example
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ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y NUL
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ffmpeg -i foo.mov -vcodec libxvid -pass 1 -an -f rawvideo -y /dev/null
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@end example
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@item -passlogfile @var{file}
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Set two pass logfile name to @var{file}.
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@item -newvideo
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Add a new video stream to the current output stream.
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@end table
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@section Advanced Video Options
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@table @option
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@item -pix_fmt @var{format}
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Set pixel format. Use 'list' as parameter to show all the supported
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pixel formats.
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@item -sws_flags @var{flags}
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Set SwScaler flags (only available when compiled with swscale support).
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@item -g @var{gop_size}
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Set the group of pictures size.
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@item -intra
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Use only intra frames.
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@item -vdt @var{n}
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Discard threshold.
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@item -qscale @var{q}
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Use fixed video quantizer scale (VBR).
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@item -qmin @var{q}
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minimum video quantizer scale (VBR)
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@item -qmax @var{q}
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maximum video quantizer scale (VBR)
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@item -qdiff @var{q}
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maximum difference between the quantizer scales (VBR)
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@item -qblur @var{blur}
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video quantizer scale blur (VBR) (range 0.0 - 1.0)
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@item -qcomp @var{compression}
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video quantizer scale compression (VBR) (default 0.5).
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Constant of ratecontrol equation. Recommended range for default rc_eq: 0.0-1.0
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@item -lmin @var{lambda}
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minimum video lagrange factor (VBR)
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@item -lmax @var{lambda}
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max video lagrange factor (VBR)
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@item -mblmin @var{lambda}
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minimum macroblock quantizer scale (VBR)
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@item -mblmax @var{lambda}
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maximum macroblock quantizer scale (VBR)
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These four options (lmin, lmax, mblmin, mblmax) use 'lambda' units,
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but you may use the QP2LAMBDA constant to easily convert from 'q' units:
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@example
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ffmpeg -i src.ext -lmax 21*QP2LAMBDA dst.ext
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@end example
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@item -rc_init_cplx @var{complexity}
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initial complexity for single pass encoding
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@item -b_qfactor @var{factor}
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qp factor between P- and B-frames
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@item -i_qfactor @var{factor}
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qp factor between P- and I-frames
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@item -b_qoffset @var{offset}
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qp offset between P- and B-frames
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@item -i_qoffset @var{offset}
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qp offset between P- and I-frames
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@item -rc_eq @var{equation}
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Set rate control equation (@pxref{FFmpeg formula
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evaluator}) (default = @code{tex^qComp}).
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@item -rc_override @var{override}
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rate control override for specific intervals
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@item -me_method @var{method}
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Set motion estimation method to @var{method}.
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Available methods are (from lowest to best quality):
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@table @samp
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@item zero
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Try just the (0, 0) vector.
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@item phods
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@item log
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@item x1
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@item hex
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@item umh
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@item epzs
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(default method)
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@item full
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exhaustive search (slow and marginally better than epzs)
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@end table
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@item -dct_algo @var{algo}
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Set DCT algorithm to @var{algo}. Available values are:
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@table @samp
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@item 0
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FF_DCT_AUTO (default)
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@item 1
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FF_DCT_FASTINT
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@item 2
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FF_DCT_INT
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@item 3
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FF_DCT_MMX
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@item 4
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FF_DCT_MLIB
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@item 5
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FF_DCT_ALTIVEC
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@end table
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@item -idct_algo @var{algo}
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Set IDCT algorithm to @var{algo}. Available values are:
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@table @samp
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@item 0
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FF_IDCT_AUTO (default)
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@item 1
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FF_IDCT_INT
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@item 2
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FF_IDCT_SIMPLE
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@item 3
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FF_IDCT_SIMPLEMMX
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@item 4
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FF_IDCT_LIBMPEG2MMX
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@item 5
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FF_IDCT_PS2
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@item 6
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FF_IDCT_MLIB
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@item 7
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FF_IDCT_ARM
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@item 8
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FF_IDCT_ALTIVEC
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@item 9
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FF_IDCT_SH4
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@item 10
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FF_IDCT_SIMPLEARM
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@end table
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@item -er @var{n}
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Set error resilience to @var{n}.
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@table @samp
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@item 1
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FF_ER_CAREFUL (default)
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@item 2
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FF_ER_COMPLIANT
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@item 3
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FF_ER_AGGRESSIVE
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@item 4
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FF_ER_VERY_AGGRESSIVE
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@end table
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@item -ec @var{bit_mask}
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Set error concealment to @var{bit_mask}. @var{bit_mask} is a bit mask of
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the following values:
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@table @samp
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@item 1
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FF_EC_GUESS_MVS (default = enabled)
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@item 2
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FF_EC_DEBLOCK (default = enabled)
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@end table
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@item -bf @var{frames}
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Use 'frames' B-frames (supported for MPEG-1, MPEG-2 and MPEG-4).
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@item -mbd @var{mode}
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macroblock decision
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@table @samp
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@item 0
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FF_MB_DECISION_SIMPLE: Use mb_cmp (cannot change it yet in FFmpeg).
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@item 1
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FF_MB_DECISION_BITS: Choose the one which needs the fewest bits.
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@item 2
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FF_MB_DECISION_RD: rate distortion
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@end table
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@item -4mv
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Use four motion vector by macroblock (MPEG-4 only).
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@item -part
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Use data partitioning (MPEG-4 only).
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@item -bug @var{param}
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Work around encoder bugs that are not auto-detected.
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@item -strict @var{strictness}
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How strictly to follow the standards.
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@item -aic
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Enable Advanced intra coding (h263+).
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@item -umv
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Enable Unlimited Motion Vector (h263+)
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@item -deinterlace
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Deinterlace pictures.
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@item -ilme
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Force interlacing support in encoder (MPEG-2 and MPEG-4 only).
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Use this option if your input file is interlaced and you want
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to keep the interlaced format for minimum losses.
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The alternative is to deinterlace the input stream with
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@option{-deinterlace}, but deinterlacing introduces losses.
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@item -psnr
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Calculate PSNR of compressed frames.
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@item -vstats
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Dump video coding statistics to @file{vstats_HHMMSS.log}.
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@item -vstats_file @var{file}
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Dump video coding statistics to @var{file}.
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@item -vhook @var{module}
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Insert video processing @var{module}. @var{module} contains the module
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name and its parameters separated by spaces.
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@item -top @var{n}
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top=1/bottom=0/auto=-1 field first
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@item -dc @var{precision}
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Intra_dc_precision.
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@item -vtag @var{fourcc/tag}
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Force video tag/fourcc.
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@item -qphist
639
Show QP histogram.
640
@item -vbsf @var{bitstream_filter}
641
Bitstream filters available are "dump_extra", "remove_extra", "noise", "h264_mp4toannexb", "imxdump", "mjpegadump".
642
@example
643
ffmpeg -i h264.mp4 -vcodec copy -vbsf h264_mp4toannexb -an out.h264
644
@end example
645
@end table
646

    
647
@section Audio Options
648

    
649
@table @option
650
@item -aframes @var{number}
651
Set the number of audio frames to record.
652
@item -ar @var{freq}
653
Set the audio sampling frequency (default = 44100 Hz).
654
@item -ab @var{bitrate}
655
Set the audio bitrate in bit/s (default = 64k).
656
@item -ac @var{channels}
657
Set the number of audio channels (default = 1).
658
@item -an
659
Disable audio recording.
660
@item -acodec @var{codec}
661
Force audio codec to @var{codec}. Use the @code{copy} special value to
662
specify that the raw codec data must be copied as is.
663
@item -newaudio
664
Add a new audio track to the output file. If you want to specify parameters,
665
do so before @code{-newaudio} (@code{-acodec}, @code{-ab}, etc..).
666

    
667
Mapping will be done automatically, if the number of output streams is equal to
668
the number of input streams, else it will pick the first one that matches. You
669
can override the mapping using @code{-map} as usual.
670

    
671
Example:
672
@example
673
ffmpeg -i file.mpg -vcodec copy -acodec ac3 -ab 384k test.mpg -acodec mp2 -ab 192k -newaudio
674
@end example
675
@item -alang @var{code}
676
Set the ISO 639 language code (3 letters) of the current audio stream.
677
@end table
678

    
679
@section Advanced Audio options:
680

    
681
@table @option
682
@item -atag @var{fourcc/tag}
683
Force audio tag/fourcc.
684
@item -absf @var{bitstream_filter}
685
Bitstream filters available are "dump_extra", "remove_extra", "noise", "mp3comp", "mp3decomp".
686
@end table
687

    
688
@section Subtitle options:
689

    
690
@table @option
691
@item -scodec @var{codec}
692
Force subtitle codec ('copy' to copy stream).
693
@item -newsubtitle
694
Add a new subtitle stream to the current output stream.
695
@item -slang @var{code}
696
Set the ISO 639 language code (3 letters) of the current subtitle stream.
697
@item -sbsf @var{bitstream_filter}
698
Bitstream filters available are "mov2textsub", "text2movsub".
699
@example
700
ffmpeg -i file.mov -an -vn -sbsf mov2textsub -scodec copy -f rawvideo sub.txt
701
@end example
702
@end table
703

    
704
@section Audio/Video grab options
705

    
706
@table @option
707
@item -vc @var{channel}
708
Set video grab channel (DV1394 only).
709
@item -tvstd @var{standard}
710
Set television standard (NTSC, PAL (SECAM)).
711
@item -isync
712
Synchronize read on input.
713
@end table
714

    
715
@section Advanced options
716

    
717
@table @option
718
@item -map @var{input_stream_id}[:@var{sync_stream_id}]
719
Set stream mapping from input streams to output streams.
720
Just enumerate the input streams in the order you want them in the output.
721
@var{sync_stream_id} if specified sets the input stream to sync
722
against.
723
@item -map_meta_data @var{outfile}:@var{infile}
724
Set meta data information of @var{outfile} from @var{infile}.
725
@item -debug
726
Print specific debug info.
727
@item -benchmark
728
Add timings for benchmarking.
729
@item -dump
730
Dump each input packet.
731
@item -hex
732
When dumping packets, also dump the payload.
733
@item -bitexact
734
Only use bit exact algorithms (for codec testing).
735
@item -ps @var{size}
736
Set packet size in bits.
737
@item -re
738
Read input at native frame rate. Mainly used to simulate a grab device.
739
@item -loop_input
740
Loop over the input stream. Currently it works only for image
741
streams. This option is used for automatic FFserver testing.
742
@item -loop_output @var{number_of_times}
743
Repeatedly loop output for formats that support looping such as animated GIF
744
(0 will loop the output infinitely).
745
@item -threads @var{count}
746
Thread count.
747
@item -vsync @var{parameter}
748
Video sync method. Video will be stretched/squeezed to match the timestamps,
749
it is done by duplicating and dropping frames. With -map you can select from
750
which stream the timestamps should be taken. You can leave either video or
751
audio unchanged and sync the remaining stream(s) to the unchanged one.
752
@item -async @var{samples_per_second}
753
Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps,
754
the parameter is the maximum samples per second by which the audio is changed.
755
-async 1 is a special case where only the start of the audio stream is corrected
756
without any later correction.
757
@item -copyts
758
Copy timestamps from input to output.
759
@item -shortest
760
Finish encoding when the shortest input stream ends.
761
@item -dts_delta_threshold
762
Timestamp discontinuity delta threshold.
763
@item -muxdelay @var{seconds}
764
Set the maximum demux-decode delay.
765
@item -muxpreload @var{seconds}
766
Set the initial demux-decode delay.
767
@end table
768

    
769
@node FFmpeg formula evaluator
770
@section FFmpeg formula evaluator
771

    
772
When evaluating a rate control string, FFmpeg uses an internal formula
773
evaluator.
774

    
775
The following binary operators are available: @code{+}, @code{-},
776
@code{*}, @code{/}, @code{^}.
777

    
778
The following unary operators are available: @code{+}, @code{-},
779
@code{(...)}.
780

    
781
The following functions are available:
782
@table @var
783
@item sinh(x)
784
@item cosh(x)
785
@item tanh(x)
786
@item sin(x)
787
@item cos(x)
788
@item tan(x)
789
@item exp(x)
790
@item log(x)
791
@item squish(x)
792
@item gauss(x)
793
@item abs(x)
794
@item max(x, y)
795
@item min(x, y)
796
@item gt(x, y)
797
@item lt(x, y)
798
@item eq(x, y)
799
@item bits2qp(bits)
800
@item qp2bits(qp)
801
@end table
802

    
803
The following constants are available:
804
@table @var
805
@item PI
806
@item E
807
@item iTex
808
@item pTex
809
@item tex
810
@item mv
811
@item fCode
812
@item iCount
813
@item mcVar
814
@item var
815
@item isI
816
@item isP
817
@item isB
818
@item avgQP
819
@item qComp
820
@item avgIITex
821
@item avgPITex
822
@item avgPPTex
823
@item avgBPTex
824
@item avgTex
825
@end table
826

    
827
@c man end
828

    
829
@ignore
830

    
831
@setfilename ffmpeg
832
@settitle FFmpeg video converter
833

    
834
@c man begin SEEALSO
835
ffserver(1), ffplay(1) and the HTML documentation of @file{ffmpeg}.
836
@c man end
837

    
838
@c man begin AUTHOR
839
Fabrice Bellard
840
@c man end
841

    
842
@end ignore
843

    
844
@section Protocols
845

    
846
The file name can be @file{-} to read from standard input or to write
847
to standard output.
848

    
849
FFmpeg also handles many protocols specified with an URL syntax.
850

    
851
Use 'ffmpeg -formats' to see a list of the supported protocols.
852

    
853
The protocol @code{http:} is currently used only to communicate with
854
FFserver (see the FFserver documentation). When FFmpeg will be a
855
video player it will also be used for streaming :-)
856

    
857
@chapter Tips
858

    
859
@itemize
860
@item For streaming at very low bitrate application, use a low frame rate
861
and a small GOP size. This is especially true for RealVideo where
862
the Linux player does not seem to be very fast, so it can miss
863
frames. An example is:
864

    
865
@example
866
ffmpeg -g 3 -r 3 -t 10 -b 50k -s qcif -f rv10 /tmp/b.rm
867
@end example
868

    
869
@item  The parameter 'q' which is displayed while encoding is the current
870
quantizer. The value 1 indicates that a very good quality could
871
be achieved. The value 31 indicates the worst quality. If q=31 appears
872
too often, it means that the encoder cannot compress enough to meet
873
your bitrate. You must either increase the bitrate, decrease the
874
frame rate or decrease the frame size.
875

    
876
@item If your computer is not fast enough, you can speed up the
877
compression at the expense of the compression ratio. You can use
878
'-me zero' to speed up motion estimation, and '-intra' to disable
879
motion estimation completely (you have only I-frames, which means it
880
is about as good as JPEG compression).
881

    
882
@item To have very low audio bitrates, reduce the sampling frequency
883
(down to 22050 kHz for MPEG audio, 22050 or 11025 for AC3).
884

    
885
@item To have a constant quality (but a variable bitrate), use the option
886
'-qscale n' when 'n' is between 1 (excellent quality) and 31 (worst
887
quality).
888

    
889
@item When converting video files, you can use the '-sameq' option which
890
uses the same quality factor in the encoder as in the decoder.
891
It allows almost lossless encoding.
892

    
893
@end itemize
894

    
895
@bye