PeerStreamer

\input texinfo @c -*- texinfo -*-

@settitle FFmpeg FAQ

@titlepage

@sp 7

@center @titlefont{FFmpeg FAQ}

@sp 3

@end titlepage

@chapter General Questions

@section When will the next FFmpeg version be released? / Why are FFmpeg releases so few and far between?

Like most open source projects FFmpeg suffers from a certain lack of

manpower. For this reason the developers have to prioritize the work

they do and putting out releases is not at the top of the list, fixing

bugs and reviewing patches takes precedence. Please don't complain or

request more timely and/or frequent releases unless you are willing to

help out creating them.

@section I have a problem with an old version of FFmpeg; where should I report it?

Nowhere. Upgrade to the latest release or if there is no recent release upgrade

to Subversion HEAD. You could also try to report it. Maybe you will get lucky and

become the first person in history to get an answer different from "upgrade

to Subversion HEAD".

@section Why doesn't FFmpeg support feature [xyz]?

Because no one has taken on that task yet. FFmpeg development is

driven by the tasks that are important to the individual developers.

If there is a feature that is important to you, the best way to get

it implemented is to undertake the task yourself or sponsor a developer.

@section FFmpeg does not support codec XXX. Can you include a Windows DLL loader to support it?

No. Windows DLLs are not portable, bloated and often slow.

Moreover FFmpeg strives to support all codecs natively.

A DLL loader is not conducive to that goal.

@section My bug report/mail to ffmpeg-devel/user has not received any replies.

Likely reasons

@itemize

@item We are busy and haven't had time yet to read your report or

investigate the issue.

@item You didn't follow bugreports.html.

@item You didn't use Subversion HEAD.

@item You reported a segmentation fault without gdb output.

@item You describe a problem but not how to reproduce it.

@item It's unclear if you use ffmpeg as command line tool or use

libav* from another application.

@item You speak about a video having problems on playback but

not what you use to play it.

@item We have no faint clue what you are talking about besides

that it is related to FFmpeg.

@end itemize

@section Is there a forum for FFmpeg? I do not like mailing lists.

You may view our mailing lists with a more forum-alike look here:

@url{http://dir.gmane.org/gmane.comp.video.ffmpeg.user},

but, if you post, please remember that our mailing list rules still apply there.

@section I cannot read this file although this format seems to be supported by ffmpeg.

Even if ffmpeg can read the container format, it may not support all its

codecs. Please consult the supported codec list in the ffmpeg

documentation.

@section Which codecs are supported by Windows?

Windows does not support standard formats like MPEG very well, unless you

install some additional codecs.

The following list of video codecs should work on most Windows systems:

@table @option

@item msmpeg4v2

.avi/.asf

@item msmpeg4

.asf only

@item wmv1

.asf only

@item wmv2

.asf only

@item mpeg4

Only if you have some MPEG-4 codec like ffdshow or Xvid installed.

@item mpeg1

.mpg only

@end table

Note, ASF files often have .wmv or .wma extensions in Windows. It should also

be mentioned that Microsoft claims a patent on the ASF format, and may sue

or threaten users who create ASF files with non-Microsoft software. It is

strongly advised to avoid ASF where possible.

The following list of audio codecs should work on most Windows systems:

@table @option

@item adpcm_ima_wav

@item adpcm_ms

@item pcm

always

@item mp3

If some MP3 codec like LAME is installed.

@end table

@chapter Compilation

@section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}

This is a bug in gcc. Do not report it to us. Instead, please report it to

the gcc developers. Note that we will not add workarounds for gcc bugs.

Also note that (some of) the gcc developers believe this is not a bug or

not a bug they should fix:

@url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.

Then again, some of them do not know the difference between an undecidable

problem and an NP-hard problem...

@chapter Usage

@section ffmpeg does not work; what is wrong?

Try a @code{make distclean} in the ffmpeg source directory before the build. If this does not help see

(@url{http://ffmpeg.org/bugreports.html}).

@section How do I encode single pictures into movies?

First, rename your pictures to follow a numerical sequence.

For example, img1.jpg, img2.jpg, img3.jpg,...

Then you may run:

@example

  ffmpeg -f image2 -i img%d.jpg /tmp/a.mpg

@end example

Notice that @samp{%d} is replaced by the image number.

@file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc...

The same logic is used for any image format that ffmpeg reads.

@section How do I encode movie to single pictures?

Use:

@example

  ffmpeg -i movie.mpg movie%d.jpg

@end example

The @file{movie.mpg} used as input will be converted to

@file{movie1.jpg}, @file{movie2.jpg}, etc...

Instead of relying on file format self-recognition, you may also use

@table @option

@item -vcodec ppm

@item -vcodec png

@item -vcodec mjpeg

@end table

to force the encoding.

Applying that to the previous example:

@example

  ffmpeg -i movie.mpg -f image2 -vcodec mjpeg menu%d.jpg

@end example

Beware that there is no "jpeg" codec. Use "mjpeg" instead.

@section Why do I see a slight quality degradation with multithreaded MPEG* encoding?

For multithreaded MPEG* encoding, the encoded slices must be independent,

otherwise thread n would practically have to wait for n-1 to finish, so it's

quite logical that there is a small reduction of quality. This is not a bug.

@section How can I read from the standard input or write to the standard output?

Use @file{-} as file name.

@section Why does the chrominance data seem to be sampled at a different time from the luminance data on bt8x8 captures on Linux?

This is a well-known bug in the bt8x8 driver. For 2.4.26 there is a patch at

(@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-420-2.4.26.patch?view=co}). This may also

apply cleanly to other 2.4-series kernels.

@section How do I avoid the ugly aliasing artifacts in bt8x8 captures on Linux?

Pass 'combfilter=1 lumafilter=1' to the bttv driver. Note though that 'combfilter=1'

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})

or (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.6.6.patch?view=co})

and pass 'combfilter=2'.

@section -f jpeg doesn't work.

Try '-f image2 test%d.jpg'.

@section Why can I not change the framerate?

Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.

Choose a different codec with the -vcodec command line option.

@section How do I encode Xvid or DivX video with ffmpeg?

Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4

standard (note that there are many other coding formats that use this

same standard). Thus, use '-vcodec mpeg4' to encode in these formats. The

default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want

a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will

force the fourcc 'xvid' to be stored as the video fourcc rather than the

default.

@section How do I encode videos which play on the iPod?

@table @option

@item needed stuff

-acodec libfaac -vcodec mpeg4 width<=320 height<=240

@item working stuff

4mv, title

@item non-working stuff

B-frames

@item example command line

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

@end table

@section How do I encode videos which play on the PSP?

@table @option

@item needed stuff

-acodec libfaac -vcodec mpeg4 width*height<=76800 width%16=0 height%16=0 -ar 24000 -r 30000/1001 or 15000/1001 -f psp

@item working stuff

4mv, title

@item non-working stuff

B-frames

@item example command line

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

@item needed stuff for H.264

-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

@item working stuff for H.264

title, loop filter

@item non-working stuff for H.264

CAVLC

@item example command line

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

@item higher resolution for newer PSP firmwares, width<=480, height<=272

-vcodec libx264 -level 21 -coder 1 -f psp

@item example command line

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

@end table

@section Which are good parameters for encoding high quality MPEG-4?

'-mbd rd -flags +4mv+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',

things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.

@section Which are good parameters for encoding high quality MPEG-1/MPEG-2?

'-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'

but beware the '-g 100' might cause problems with some decoders.

Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.

@section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?

You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced

material, and try '-top 0/1' if the result looks really messed-up.

@section How can I read DirectShow files?

If you have built FFmpeg with @code{./configure --enable-avisynth}

(only possible on MinGW/Cygwin platforms),

then you may use any file that DirectShow can read as input.

(Be aware that this feature has been recently added,

so you will need to help yourself in case of problems.)

Just create an "input.avs" text file with this single line ...

@example

  DirectShowSource("C:\path to your file\yourfile.asf")

@end example

... and then feed that text file to FFmpeg:

@example

  ffmpeg -i input.avs

@end example

For ANY other help on Avisynth, please visit @url{http://www.avisynth.org/}.

@section How can I join video files?

A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by

merely concatenating them.

Hence you may concatenate your multimedia files by first transcoding them to

these privileged formats, then using the humble @code{cat} command (or the

equally humble @code{copy} under Windows), and finally transcoding back to your

format of choice.

@example

ffmpeg -i input1.avi -sameq intermediate1.mpg

ffmpeg -i input2.avi -sameq intermediate2.mpg

cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg

ffmpeg -i intermediate_all.mpg -sameq output.avi

@end example

Notice that you should either use @code{-sameq} or set a reasonably high

bitrate for your intermediate and output files, if you want to preserve

video quality.

Also notice that you may avoid the huge intermediate files by taking advantage

of named pipes, should your platform support it:

@example

mkfifo intermediate1.mpg

mkfifo intermediate2.mpg

ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &

ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &

cat intermediate1.mpg intermediate2.mpg |\

ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi

@end example

Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also

allow concatenation, and the transcoding step is almost lossless.

When using multiple yuv4mpegpipe(s), the first line needs to be discarded

from all but the first stream. This can be accomplished by piping through

@code{tail} as seen below. Note that when piping through @code{tail} you

must use command grouping, @code{@{  ;@}}, to background properly.

For example, let's say we want to join two FLV files into an output.flv file:

@example

mkfifo temp1.a

mkfifo temp1.v

mkfifo temp2.a

mkfifo temp2.v

mkfifo all.a

mkfifo all.v

ffmpeg -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &

ffmpeg -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &

ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &

@{ ffmpeg -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &

cat temp1.a temp2.a > all.a &

cat temp1.v temp2.v > all.v &

ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \

       -f yuv4mpegpipe -i all.v \

       -sameq -y output.flv

rm temp[12].[av] all.[av]

@end example

@section FFmpeg does not adhere to the -maxrate setting, some frames are bigger than maxrate/fps.

Read the MPEG spec about video buffer verifier.

@section I want CBR, but no matter what I do frame sizes differ.

You do not understand what CBR is, please read the MPEG spec.

Read about video buffer verifier and constant bitrate.

The one sentence summary is that there is a buffer and the input rate is

constant, the output can vary as needed.

@section How do I check if a stream is CBR?

To quote the MPEG-2 spec:

"There is no way to tell that a bitstream is constant bitrate without

examining all of the vbv_delay values and making complicated computations."

@chapter Development

@section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?

Yes. Read the Developers Guide of the FFmpeg documentation. Alternatively,

examine the source code for one of the many open source projects that

already incorporate FFmpeg at (@url{projects.html}).

@section Can you support my C compiler XXX?

It depends. If your compiler is C99-compliant, then patches to support

it are likely to be welcome if they do not pollute the source code

with @code{#ifdef}s related to the compiler.

@section Is Microsoft Visual C++ supported?

No. Microsoft Visual C++ is not compliant to the C99 standard and does

not - among other things - support the inline assembly used in FFmpeg.

If you wish to use MSVC++ for your

project then you can link the MSVC++ code with libav* as long as

you compile the latter with a working C compiler. For more information, see

the @emph{Microsoft Visual C++ compatibility} section in the FFmpeg

documentation.

There have been efforts to make FFmpeg compatible with MSVC++ in the

past. However, they have all been rejected as too intrusive, especially

since MinGW does the job adequately. None of the core developers

work with MSVC++ and thus this item is low priority. Should you find

the silver bullet that solves this problem, feel free to shoot it at us.

We strongly recommend you to move over from MSVC++ to MinGW tools.

@section Can I use FFmpeg or libavcodec under Windows?

Yes, but the Cygwin or MinGW tools @emph{must} be used to compile FFmpeg.

Read the @emph{Windows} section in the FFmpeg documentation to find more

information.

To get help and instructions for building FFmpeg under Windows, check out

the FFmpeg Windows Help Forum at

@url{http://ffmpeg.arrozcru.org/}.

@section Can you add automake, libtool or autoconf support?

No. These tools are too bloated and they complicate the build.

@section Why not rewrite ffmpeg in object-oriented C++?

FFmpeg is already organized in a highly modular manner and does not need to

be rewritten in a formal object language. Further, many of the developers

favor straight C; it works for them. For more arguments on this matter,

read "Programming Religion" at (@url{http://www.tux.org/lkml/#s15}).

@section Why are the ffmpeg programs devoid of debugging symbols?

The build process creates ffmpeg_g, ffplay_g, etc. which contain full debug

information. Those binaries are stripped to create ffmpeg, ffplay, etc. If

you need the debug information, used the *_g versions.

@section I do not like the LGPL, can I contribute code under the GPL instead?

Yes, as long as the code is optional and can easily and cleanly be placed

under #if CONFIG_GPL without breaking anything. So for example a new codec

or filter would be OK under GPL while a bug fix to LGPL code would not.

@section I want to compile xyz.c alone but my compiler produced many errors.

Common code is in its own files in libav* and is used by the individual

codecs. They will not work without the common parts, you have to compile

the whole libav*. If you wish, disable some parts with configure switches.

You can also try to hack it and remove more, but if you had problems fixing

the compilation failure then you are probably not qualified for this.

@section I'm using libavcodec from within my C++ application but the linker complains about missing symbols which seem to be available.

FFmpeg is a pure C project, so to use the libraries within your C++ application

you need to explicitly state that you are using a C library. You can do this by

encompassing your FFmpeg includes using @code{extern "C"}.

See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}

@section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?

You have to implement a URLProtocol, see libavformat/file.c in FFmpeg

and libmpdemux/demux_lavf.c in MPlayer sources.

@section I get "No compatible shell script interpreter found." in MSys.

The standard MSys bash (2.04) is broken. You need to install 2.05 or later.

@section I get "./configure: line <xxx>: pr: command not found" in MSys.

The standard MSys install doesn't come with pr. You need to get it from the coreutils package.

@section I tried to pass RTP packets into a decoder, but it doesn't work.

RTP is a container format like any other, you must first depacketize the

codec frames/samples stored in RTP and then feed to the decoder.

@section Where can I find libav* headers for Pascal/Delphi?

see @url{http://www.iversenit.dk/dev/ffmpeg-headers/}

@section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?

see @url{http://svn.ffmpeg.org/michael/trunk/docs/}

@section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?

Even if peculiar since it is network oriented, RTP is a container like any

other. You have to @emph{demux} RTP before feeding the payload to libavcodec.

In this specific case please look at RFC 4629 to see how it should be done.

@section AVStream.r_frame_rate is wrong, it is much larger than the framerate.

r_frame_rate is NOT the average framerate, it is the smallest framerate

that can accurately represent all timestamps. So no, it is not

wrong if it is larger than the average!

For example, if you have mixed 25 and 30 fps content, then r_frame_rate

will be 150.

@bye