faq.texi

ffmpeg源码分析

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

@settitle FFmpeg FAQ
@titlepage
@sp 7
@center @titlefont{FFmpeg FAQ}
@sp 3
@end titlepage


@chapter General Problems

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

Even if ffmpeg can read the file format, it may not support all its
codecs. Please consult the supported codec list in the ffmpeg
documentation.

@section How do I encode JPEGs to another format ?

If the JPEGs are named img1.jpg, img2.jpg, img3.jpg,..., use:

@example
  ffmpeg -i img%d.jpg /tmp/a.mpg
@end example

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

@file{img%03d.jpg} generates @file{img001.jpg}, @file{img002.jpg}, etc...

The same system is used for the other image formats.

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

No. FFmpeg only supports open source codecs. Windows DLLs are not
portable, bloated and often slow.

@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 filename.

@section Why does ffmpeg not decode audio in VOB files ?

The audio is AC3 (a.k.a. A/52). AC3 decoding is an optional component in ffmpeg
as the component that handles AC3 decoding (liba52) is currently released under
the GPL. If you have liba52 installed on your system, enable AC3 decoding
with @code{./configure --enable-a52}. Take care: by
enabling AC3, you automatically change the license of libavcodec from
LGPL to GPL.

@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 installed like ffdshow or XviD
@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
@item mp3
if some MP3 codec like LAME is installed
@end table

@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://mplayerhq.hu/~michael/bttv-420-2.4.26.patch}). 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://mplayerhq.hu/~michael/bttv-comb-2.4.26.patch})
or (@url{http://mplayerhq.hu/~michael/bttv-comb-2.6.6.patch})
and pass 'combfilter=2'.

@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 CVS. 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 CVS".

@section -f jpeg doesn't work.

Try '-f image -img jpeg 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 ffmpeg does not work; What is wrong?

Try a 'make distclean' in the ffmpeg source directory. If this does not help see
(@url{http://ffmpeg.org/bugreports.php}).

@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 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.

@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.php}).

@section Can you support my C compiler XXX ?

No. Only GCC is supported. GCC is ported to most systems available and there
is no need to pollute the source code with @code{#ifdef}s
related to the compiler.

@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.

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

No. These tools are too bloated and they complicate the build. Moreover,
since only @samp{gcc} is supported they would add little advantages in
terms of portability.

@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://lkml.org/faq/lkmlfaq-15.html}).

@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 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.

@bye