# Libav FAQ

# Table of Contents

# 1. General Questions

## 1.1 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.

## 1.2 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.

## 1.3 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.

## 1.4 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:

‘`msmpeg4v2`’

  .avi/.asf

‘`msmpeg4`’

  .asf only

‘`wmv1`’

  .asf only

‘`wmv2`’

  .asf only

‘`mpeg4`’

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

‘`mpeg1video`’

  .mpg only

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:

‘`adpcm_ima_wav`’
‘`adpcm_ms`’
‘`pcm_s16le`’

  always

‘`libmp3lame`’

  If some MP3 codec like LAME is installed.

# 2. Compilation

## 2.1 `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: 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...

# 3. Usage

## 3.1 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:

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

Notice that ‘`%d`’ is replaced by the image number.

‘`img%03d.jpg`’ means the sequence ‘`img001.jpg`’, ‘`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 `*jpg` to the ‘`/tmp`’ directory in the sequence of ‘`img001.jpg`’, ‘`img002.jpg`’ and so on.

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

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

Then run:

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

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

## 3.2 How do I encode movie to single pictures?

Use:

      avconv -i movie.mpg movie%d.jpg

The ‘`movie.mpg`’ used as input will be converted to ‘`movie1.jpg`’, ‘`movie2.jpg`’, etc...

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

‘`-c:v ppm`’
‘`-c:v png`’
‘`-c:v mjpeg`’

to force the encoding.

Applying that to the previous example:

      avconv -i movie.mpg -f image2 -c:v mjpeg menu%d.jpg

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

## 3.3 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.

## 3.4 How can I read from the standard input or write to the standard output?

Use ‘`-`’ as file name.

## 3.5 -f jpeg doesn’t work.

Try ’-f image2 test%d.jpg’.

## 3.6 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.

## 3.7 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.

## 3.8 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’.

## 3.9 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.

## 3.10 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.

## 3.11 How can I read DirectShow files?

If you have built Libav with `./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 ...

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

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

      avconv -i input.avs

For ANY other help on Avisynth, please visit the Avisynth homepage.

## 3.12 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 `cat` command (or the equally humble `copy` under Windows), and finally transcoding back to your format of choice.

    avconv -i input1.avi -same_quant intermediate1.mpg
    avconv -i input2.avi -same_quant intermediate2.mpg
    cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
    avconv -i intermediate_all.mpg -same_quant output.avi

Notice that you should either use `-same_quant` 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:

    mkfifo intermediate1.mpg
    mkfifo intermediate2.mpg
    avconv -i input1.avi -same_quant -y intermediate1.mpg < /dev/null &
    avconv -i input2.avi -same_quant -y intermediate2.mpg < /dev/null &
    cat intermediate1.mpg intermediate2.mpg |\
    avconv -f mpeg -i - -same_quant -c:v mpeg4 -acodec libmp3lame output.avi

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 `tail` as seen below. Note that when piping through `tail` you must use command grouping, `{ ;}`, to background properly.

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

    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 -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
    avconv -i input2.flv -vn -f u16le -acodec 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 -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
           -f yuv4mpegpipe -i all.v \
           -same_quant -y output.flv
    rm temp[12].[av] all.[av]

## 3.13 -profile option fails when encoding H.264 video with AAC audio

`avconv` prints an error like

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

Short answer: write ‘`-profile:v`’ instead of ‘`-profile`’.

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

The solution is to apply the ‘`-profile`’ option to the video stream only by using Stream specifiers. Appending `:v` to it will do exactly that.

# 4. Development

## 4.1 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 (projects.html).

## 4.2 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 `#ifdef`s related to the compiler.

## 4.3 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 Libav. 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 Microsoft Visual C++ compatibility section in the Libav documentation.

There have been efforts to make Libav 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.

## 4.4 Can I use Libav under Windows?

Yes, but the Cygwin or MinGW tools must be used to compile Libav. Read the Windows section in the Libav documentation to find more information.

## 4.5 Can you add automake, libtool or autoconf support?

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

## 4.6 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 "Programming Religion".

## 4.7 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.

## 4.8 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 `extern "C"`.

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

## 4.9 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

## 4.10 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 Libav and ‘`libmpdemux/demux_lavf.c`’ in MPlayer sources.

## 4.11 Why is `make fate` not running all tests?

Make sure you have the fate-suite samples and the `SAMPLES` Make variable or `FATE_SAMPLES` environment variable or the `--samples` `configure` option is set to the right path.

## 4.12 Why is `make fate` not finding the samples?

Do you happen to have a `~` 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 `~` by the full path.