faq.texi

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

@settitle Libav FAQ
@titlepage
@center @titlefont{Libav FAQ}
@end titlepage

@top

@contents

@chapter General Questions

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

Because no one has taken on that task yet. Libav 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 Libav 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 Libav strives to support all codecs natively.
A DLL loader is not conducive to that goal.

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

Even if avconv can read the container format, it may not support all its
codecs. Please consult the supported codec list in the avconv
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 mpeg1video
.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_s16le
always
@item libmp3lame
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 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
  avconv -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...

If you have large number of pictures to rename, you can use the
following command to ease the burden. The command, using the bourne
shell syntax, symbolically links all files in the current directory
that match @code{*jpg} to the @file{/tmp} directory in the sequence of
@file{img001.jpg}, @file{img002.jpg} and so on.

@example
  x=1; for i in *jpg; do counter=$(printf %03d $x); ln -s "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
@end example

If you want to sequence them by oldest modified first, substitute
@code{$(ls -r -t *jpg)} in place of @code{*jpg}.

Then run:

@example
  avconv -f image2 -i /tmp/img%03d.jpg /tmp/a.mpg
@end example

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

@section How do I encode movie to single pictures?

Use:

@example
  avconv -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 -c:v ppm
@item -c:v png
@item -c:v mjpeg
@end table
to force the encoding.

Applying that to the previous example:
@example
  avconv -i movie.mpg -f image2 -c:v 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 -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 -c:v command line option.

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

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 '-c:v 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 Which are good parameters for encoding high quality MPEG-4?

'-mbd rd -flags +mv4+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 avconv, 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 Libav with @code{./configure --enable-avisynth}
(only possible on MinGW/Cygwin platforms),
then you may use any file that DirectShow can read as input.

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 avconv:
@example
  avconv -i input.avs
@end example

For ANY other help on AviSynth, please visit the
@uref{http://www.avisynth.org/, AviSynth homepage}.

@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
avconv -i input1.avi intermediate1.mpg
avconv -i input2.avi intermediate2.mpg
cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
avconv -i intermediate_all.mpg output.avi
@end example

Notice that you should 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
avconv -i input1.avi -y intermediate1.mpg < /dev/null &
avconv -i input2.avi -y intermediate2.mpg < /dev/null &
cat intermediate1.mpg intermediate2.mpg |\
avconv -f mpeg -i - -c:v mpeg4 -c:a 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
avconv -i input1.flv -vn -f u16le -c:a pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
avconv -i input2.flv -vn -f u16le -c:a pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
avconv -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
@{ avconv -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 &
avconv -f u16le -c:a pcm_s16le -ac 2 -ar 44100 -i all.a \
       -f yuv4mpegpipe -i all.v \
       -y output.flv
rm temp[12].[av] all.[av]
@end example

@section -profile option fails when encoding H.264 video with AAC audio

@command{avconv} prints an error like

@example
Undefined constant or missing '(' in 'baseline'
Unable to parse option value "baseline"
Error setting option profile to value baseline.
@end example

Short answer: write @option{-profile:v} instead of @option{-profile}.

Long answer: this happens because the @option{-profile} option can apply to both
video and audio.  Specifically the AAC encoder also defines some profiles, none
of which are named @var{baseline}.

The solution is to apply the @option{-profile} option to the video stream only
by using @url{http://libav.org/avconv.html#Stream-specifiers-1, Stream specifiers}.
Appending @code{:v} to it will do exactly that.

@chapter Development

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

Yes. Read the Developers Guide of the Libav documentation. Alternatively,
examine the source code for one of the many open source projects that
already incorporate Libav 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?

Yes. Please see the @uref{platform.html, Microsoft Visual C++}
section in the Libav documentation.

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

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

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

Libav 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 @uref{http://www.tux.org/lkml/#s15, "Programming Religion"}.

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

Libav 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 Libav includes using @code{extern "C"}.

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

@section I'm using libavutil from within my C++ application but the compiler complains about 'UINT64_C' was not declared in this scope

Libav is a pure C project using C99 math features, in order to enable C++
to use them you have to append -D__STDC_CONSTANT_MACROS to your CXXFLAGS

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

You have to create a custom AVIOContext using @code{avio_alloc_context},
see @file{libavformat/aviobuf.c} in Libav and @file{libmpdemux/demux_lavf.c} in MPlayer2 sources.

@section Why is @code{make fate} not running all tests?

Make sure you have the fate-suite samples and the @code{SAMPLES} Make variable
or @code{FATE_SAMPLES} environment variable or the @code{--samples}
@command{configure} option is set to the right path.

@section Why is @code{make fate} not finding the samples?

Do you happen to have a @code{~} character in the samples path to indicate a
home directory? The value is used in ways where the shell cannot expand it,
causing FATE to not find files. Just replace @code{~} by the full path.

@bye