faq.texi

ffmpeg的完整源代码和作者自己写的文档。不但有在Linux的工程哦

@section My bugreport/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 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.

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 - > temp2.v < /dev/null &
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 Is there a forum for FFmpeg, I do not like mailing lists?

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

@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 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 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.


@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 Visual C++ produces many errors.

Visual C++ is not compliant to the C standard and does not support
the inline assembly used in FFmpeg.
If you wish - for whatever weird reason - to use Visual C++ for your
project then you can link the Visual C++ code with libav* as long as
you compile the latter with a working C compiler. For more information, see
the @emph{Visual C++ compatibility} section in the FFmpeg documentation.

There have been efforts to make FFmpeg compatible with Visual C++ in the
past. However, they have all been rejected as too intrusive, especially
since MinGW does the job perfectly adequately. None of the core developers
work with Visual C++ and thus this item is low priority. Should you find
the silver bullet that solves this problem, feel free to shoot it at us.

@section Can I use FFmpeg or libavcodec under Windows?

Yes, but the MinGW tools @emph{must} be used to compile FFmpeg. You
can link the resulting DLLs with any other Windows program. Read the
@emph{Native Windows Compilation} and @emph{Visual C++ compatibility}
sections in the FFmpeg documentation to find more information.

To get help and instructions for using FFmpeg under Windows, check out
the FFmpeg Windows Help Forum at
@url{http://arrozcru.no-ip.org/ffmpeg/}.

@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 strip'd 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 #ifdef CONFIG_GPL without breaking anything. So for example a new codec
or filter would be OK under GPL while a bugfix 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 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.mplayerhq.hu/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