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\input texinfo @c -*- texinfo -*-
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@settitle FFmpeg FAQ
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@titlepage
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@sp 7
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@center @titlefont{FFmpeg FAQ}
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@sp 3
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@end titlepage
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@chapter General Questions
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@section When will the next FFmpeg version be released? / Why are FFmpeg releases so few and far between?
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Like most open source projects FFmpeg suffers from a certain lack of
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manpower. For this reason the developers have to prioritize the work
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they do and putting out releases is not at the top of the list, fixing
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bugs and reviewing patches takes precedence. Please don't complain or
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request more timely and/or frequent releases unless you are willing to
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help out creating them.
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@section I have a problem with an old version of FFmpeg; where should I report it?
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Nowhere. Upgrade to the latest release or if there is no recent release upgrade
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to Subversion HEAD. You could also try to report it. Maybe you will get lucky and
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become the first person in history to get an answer different from "upgrade
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to Subversion HEAD".
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@section Why doesn't FFmpeg support feature [xyz]?
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Because no one has taken on that task yet. FFmpeg development is
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driven by the tasks that are important to the individual developers.
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If there is a feature that is important to you, the best way to get
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it implemented is to undertake the task yourself or sponsor a developer.
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@section FFmpeg does not support codec XXX. Can you include a Windows DLL loader to support it?
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No. Windows DLLs are not portable, bloated and often slow.
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Moreover FFmpeg strives to support all codecs natively.
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A DLL loader is not conducive to that goal.
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@section My bug report/mail to ffmpeg-devel/user has not received any replies.
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Likely reasons
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@itemize
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@item We are busy and haven't had time yet to read your report or
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investigate the issue.
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@item You didn't follow bugreports.html.
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@item You didn't use Subversion HEAD.
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@item You reported a segmentation fault without gdb output.
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@item You describe a problem but not how to reproduce it.
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@item It's unclear if you use ffmpeg as command line tool or use
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libav* from another application.
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@item You speak about a video having problems on playback but
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not what you use to play it.
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@item We have no faint clue what you are talking about besides
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that it is related to FFmpeg.
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@end itemize
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@section Is there a forum for FFmpeg? I do not like mailing lists.
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You may view our mailing lists with a more forum-alike look here:
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@url{http://dir.gmane.org/gmane.comp.video.ffmpeg.user},
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but, if you post, please remember that our mailing list rules still apply there.
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@section I cannot read this file although this format seems to be supported by ffmpeg.
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Even if ffmpeg can read the container format, it may not support all its
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codecs. Please consult the supported codec list in the ffmpeg
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documentation.
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@section Which codecs are supported by Windows?
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Windows does not support standard formats like MPEG very well, unless you
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install some additional codecs.
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The following list of video codecs should work on most Windows systems:
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@table @option
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@item msmpeg4v2
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.avi/.asf
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@item msmpeg4
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.asf only
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@item wmv1
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.asf only
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@item wmv2
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.asf only
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@item mpeg4
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Only if you have some MPEG-4 codec like ffdshow or Xvid installed.
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@item mpeg1
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.mpg only
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@end table
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Note, ASF files often have .wmv or .wma extensions in Windows. It should also
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be mentioned that Microsoft claims a patent on the ASF format, and may sue
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or threaten users who create ASF files with non-Microsoft software. It is
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strongly advised to avoid ASF where possible.
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The following list of audio codecs should work on most Windows systems:
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@table @option
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@item adpcm_ima_wav
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@item adpcm_ms
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@item pcm
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always
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@item mp3
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If some MP3 codec like LAME is installed.
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@end table
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@chapter Compilation
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@section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}
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This is a bug in gcc. Do not report it to us. Instead, please report it to
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the gcc developers. Note that we will not add workarounds for gcc bugs.
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Also note that (some of) the gcc developers believe this is not a bug or
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not a bug they should fix:
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@url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.
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Then again, some of them do not know the difference between an undecidable
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problem and an NP-hard problem...
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@chapter Usage
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@section ffmpeg does not work; what is wrong?
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Try a @code{make distclean} in the ffmpeg source directory before the build. If this does not help see
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(@url{http://ffmpeg.org/bugreports.html}).
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@section How do I encode single pictures into movies?
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First, rename your pictures to follow a numerical sequence.
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For example, img1.jpg, img2.jpg, img3.jpg,...
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Then you may run:
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@example
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  ffmpeg -f image2 -i img%d.jpg /tmp/a.mpg
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@end example
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Notice that @samp{%d} is replaced by the image number.
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@file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc...
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The same logic is used for any image format that ffmpeg reads.
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@section How do I encode movie to single pictures?
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Use:
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@example
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  ffmpeg -i movie.mpg movie%d.jpg
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@end example
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The @file{movie.mpg} used as input will be converted to
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@file{movie1.jpg}, @file{movie2.jpg}, etc...
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Instead of relying on file format self-recognition, you may also use
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@table @option
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@item -vcodec ppm
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@item -vcodec png
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@item -vcodec mjpeg
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@end table
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to force the encoding.
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Applying that to the previous example:
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@example
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  ffmpeg -i movie.mpg -f image2 -vcodec mjpeg menu%d.jpg
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@end example
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Beware that there is no "jpeg" codec. Use "mjpeg" instead.
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@section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
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For multithreaded MPEG* encoding, the encoded slices must be independent,
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otherwise thread n would practically have to wait for n-1 to finish, so it's
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quite logical that there is a small reduction of quality. This is not a bug.
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@section How can I read from the standard input or write to the standard output?
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Use @file{-} as file name.
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@section Why does the chrominance data seem to be sampled at a different time from the luminance data on bt8x8 captures on Linux?
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This is a well-known bug in the bt8x8 driver. For 2.4.26 there is a patch at
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(@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-420-2.4.26.patch?view=co}). This may also
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apply cleanly to other 2.4-series kernels.
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@section How do I avoid the ugly aliasing artifacts in bt8x8 captures on Linux?
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Pass 'combfilter=1 lumafilter=1' to the bttv driver. Note though that 'combfilter=1'
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will cause somewhat too strong filtering. A fix is to apply (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.4.26.patch?view=co})
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or (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.6.6.patch?view=co})
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and pass 'combfilter=2'.
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@section -f jpeg doesn't work.
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Try '-f image2 test%d.jpg'.
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@section Why can I not change the framerate?
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Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.
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Choose a different codec with the -vcodec command line option.
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@section How do I encode Xvid or DivX video with ffmpeg?
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Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
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standard (note that there are many other coding formats that use this
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same standard). Thus, use '-vcodec mpeg4' to encode in these formats. The
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default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
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a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
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force the fourcc 'xvid' to be stored as the video fourcc rather than the
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default.
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@section How do I encode videos which play on the iPod?
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@table @option
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@item needed stuff
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-acodec libfaac -vcodec mpeg4 width<=320 height<=240
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@item working stuff
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4mv, title
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@item non-working stuff
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B-frames
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@item example command line
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ffmpeg -i input -acodec libfaac -ab 128kb -vcodec mpeg4 -b 1200kb -mbd 2 -flags +4mv -trellis 2 -aic 2 -cmp 2 -subcmp 2 -s 320x180 -title X output.mp4
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@end table
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@section How do I encode videos which play on the PSP?
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@table @option
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@item needed stuff
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-acodec libfaac -vcodec mpeg4 width*height<=76800 width%16=0 height%16=0 -ar 24000 -r 30000/1001 or 15000/1001 -f psp
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@item working stuff
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4mv, title
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@item non-working stuff
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B-frames
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@item example command line
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ffmpeg -i input -acodec libfaac -ab 128kb -vcodec mpeg4 -b 1200kb -ar 24000 -mbd 2 -flags +4mv -trellis 2 -aic 2 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -title X -f psp output.mp4
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@item needed stuff for H.264
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-acodec libfaac -vcodec libx264 width*height<=76800 width%16=0? height%16=0? -ar 48000 -coder 1 -r 30000/1001 or 15000/1001 -f psp
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@item working stuff for H.264
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title, loop filter
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@item non-working stuff for H.264
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CAVLC
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@item example command line
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ffmpeg -i input -acodec libfaac -ab 128kb -vcodec libx264 -b 1200kb -ar 48000 -mbd 2 -coder 1 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -title X -f psp -flags loop -trellis 2 -partitions parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 output.mp4
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@item higher resolution for newer PSP firmwares, width<=480, height<=272
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-vcodec libx264 -level 21 -coder 1 -f psp
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@item example command line
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ffmpeg -i input -acodec libfaac -ab 128kb -ac 2 -ar 48000 -vcodec libx264 -level 21 -b 640kb -coder 1 -f psp -flags +loop -trellis 2 -partitions +parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 -g 250 -s 480x272 output.mp4
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@end table
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@section Which are good parameters for encoding high quality MPEG-4?
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'-mbd rd -flags +4mv+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
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things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.
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@section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
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'-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
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but beware the '-g 100' might cause problems with some decoders.
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Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.
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@section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?
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You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
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material, and try '-top 0/1' if the result looks really messed-up.
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@section How can I read DirectShow files?
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If you have built FFmpeg with @code{./configure --enable-avisynth}
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(only possible on MinGW/Cygwin platforms),
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then you may use any file that DirectShow can read as input.
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(Be aware that this feature has been recently added,
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so you will need to help yourself in case of problems.)
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Just create an "input.avs" text file with this single line ...
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@example
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  DirectShowSource("C:\path to your file\yourfile.asf")
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@end example
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... and then feed that text file to FFmpeg:
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@example
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  ffmpeg -i input.avs
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@end example
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For ANY other help on Avisynth, please visit @url{http://www.avisynth.org/}.
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@section How can I join video files?
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A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by
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merely concatenating them.
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Hence you may concatenate your multimedia files by first transcoding them to
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these privileged formats, then using the humble @code{cat} command (or the
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equally humble @code{copy} under Windows), and finally transcoding back to your
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format of choice.
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@example
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ffmpeg -i input1.avi -sameq intermediate1.mpg
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ffmpeg -i input2.avi -sameq intermediate2.mpg
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cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
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ffmpeg -i intermediate_all.mpg -sameq output.avi
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@end example
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Notice that you should either use @code{-sameq} or set a reasonably high
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bitrate for your intermediate and output files, if you want to preserve
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video quality.
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Also notice that you may avoid the huge intermediate files by taking advantage
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of named pipes, should your platform support it:
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@example
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mkfifo intermediate1.mpg
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mkfifo intermediate2.mpg
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ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &
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ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &
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cat intermediate1.mpg intermediate2.mpg |\
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ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi
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@end example
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Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
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allow concatenation, and the transcoding step is almost lossless.
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When using multiple yuv4mpegpipe(s), the first line needs to be discarded
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from all but the first stream. This can be accomplished by piping through
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@code{tail} as seen below. Note that when piping through @code{tail} you
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must use command grouping, @code{@{  ;@}}, to background properly.
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For example, let's say we want to join two FLV files into an output.flv file:
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@example
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mkfifo temp1.a
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mkfifo temp1.v
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mkfifo temp2.a
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mkfifo temp2.v
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mkfifo all.a
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mkfifo all.v
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ffmpeg -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
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ffmpeg -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
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ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
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@{ ffmpeg -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &
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cat temp1.a temp2.a > all.a &
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cat temp1.v temp2.v > all.v &
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ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
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       -f yuv4mpegpipe -i all.v \
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       -sameq -y output.flv
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rm temp[12].[av] all.[av]
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@end example
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@section FFmpeg does not adhere to the -maxrate setting, some frames are bigger than maxrate/fps.
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Read the MPEG spec about video buffer verifier.
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@section I want CBR, but no matter what I do frame sizes differ.
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You do not understand what CBR is, please read the MPEG spec.
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Read about video buffer verifier and constant bitrate.
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The one sentence summary is that there is a buffer and the input rate is
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constant, the output can vary as needed.
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@section How do I check if a stream is CBR?
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To quote the MPEG-2 spec:
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"There is no way to tell that a bitstream is constant bitrate without
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examining all of the vbv_delay values and making complicated computations."
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@chapter Development
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@section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
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Yes. Read the Developers Guide of the FFmpeg documentation. Alternatively,
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examine the source code for one of the many open source projects that
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already incorporate FFmpeg at (@url{projects.html}).
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@section Can you support my C compiler XXX?
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It depends. If your compiler is C99-compliant, then patches to support
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it are likely to be welcome if they do not pollute the source code
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with @code{#ifdef}s related to the compiler.
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@section Is Microsoft Visual C++ supported?
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No. Microsoft Visual C++ is not compliant to the C99 standard and does
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not - among other things - support the inline assembly used in FFmpeg.
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If you wish to use MSVC++ for your
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project then you can link the MSVC++ code with libav* as long as
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you compile the latter with a working C compiler. For more information, see
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the @emph{Microsoft Visual C++ compatibility} section in the FFmpeg
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documentation.
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There have been efforts to make FFmpeg compatible with MSVC++ in the
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past. However, they have all been rejected as too intrusive, especially
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since MinGW does the job adequately. None of the core developers
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work with MSVC++ and thus this item is low priority. Should you find
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the silver bullet that solves this problem, feel free to shoot it at us.
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We strongly recommend you to move over from MSVC++ to MinGW tools.
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@section Can I use FFmpeg or libavcodec under Windows?
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Yes, but the Cygwin or MinGW tools @emph{must} be used to compile FFmpeg.
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Read the @emph{Windows} section in the FFmpeg documentation to find more
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information.
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To get help and instructions for building FFmpeg under Windows, check out
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the FFmpeg Windows Help Forum at
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@url{http://ffmpeg.arrozcru.org/}.
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@section Can you add automake, libtool or autoconf support?
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No. These tools are too bloated and they complicate the build.
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@section Why not rewrite ffmpeg in object-oriented C++?
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FFmpeg is already organized in a highly modular manner and does not need to
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be rewritten in a formal object language. Further, many of the developers
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favor straight C; it works for them. For more arguments on this matter,
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read "Programming Religion" at (@url{http://www.tux.org/lkml/#s15}).
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@section Why are the ffmpeg programs devoid of debugging symbols?
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The build process creates ffmpeg_g, ffplay_g, etc. which contain full debug
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information. Those binaries are stripped to create ffmpeg, ffplay, etc. If
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you need the debug information, used the *_g versions.
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@section I do not like the LGPL, can I contribute code under the GPL instead?
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Yes, as long as the code is optional and can easily and cleanly be placed
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under #if CONFIG_GPL without breaking anything. So for example a new codec
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or filter would be OK under GPL while a bug fix to LGPL code would not.
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@section I want to compile xyz.c alone but my compiler produced many errors.
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Common code is in its own files in libav* and is used by the individual
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codecs. They will not work without the common parts, you have to compile
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the whole libav*. If you wish, disable some parts with configure switches.
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You can also try to hack it and remove more, but if you had problems fixing
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the compilation failure then you are probably not qualified for this.
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@section I'm using libavcodec from within my C++ application but the linker complains about missing symbols which seem to be available.
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FFmpeg is a pure C project, so to use the libraries within your C++ application
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you need to explicitly state that you are using a C library. You can do this by
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encompassing your FFmpeg includes using @code{extern "C"}.
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See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
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@section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
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You have to implement a URLProtocol, see libavformat/file.c in FFmpeg
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and libmpdemux/demux_lavf.c in MPlayer sources.
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@section I get "No compatible shell script interpreter found." in MSys.
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The standard MSys bash (2.04) is broken. You need to install 2.05 or later.
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@section I get "./configure: line <xxx>: pr: command not found" in MSys.
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The standard MSys install doesn't come with pr. You need to get it from the coreutils package.
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@section I tried to pass RTP packets into a decoder, but it doesn't work.
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RTP is a container format like any other, you must first depacketize the
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codec frames/samples stored in RTP and then feed to the decoder.
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@section Where can I find libav* headers for Pascal/Delphi?
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see @url{http://www.iversenit.dk/dev/ffmpeg-headers/}
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@section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?
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see @url{http://svn.ffmpeg.org/michael/trunk/docs/}
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@section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?
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Even if peculiar since it is network oriented, RTP is a container like any
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other. You have to @emph{demux} RTP before feeding the payload to libavcodec.
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In this specific case please look at RFC 4629 to see how it should be done.
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@section AVStream.r_frame_rate is wrong, it is much larger than the framerate.
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r_frame_rate is NOT the average framerate, it is the smallest framerate
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that can accurately represent all timestamps. So no, it is not
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wrong if it is larger than the average!
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For example, if you have mixed 25 and 30 fps content, then r_frame_rate
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will be 150.
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@bye