[libav.git] / doc / faq.texi
1 \input texinfo @c -*- texinfo -*-
3 @settitle Libav FAQ
4 @titlepage
5 @center @titlefont{Libav FAQ}
6 @end titlepage
8 @top
10 @contents
12 @chapter General Questions
14 @section Why doesn't Libav support feature [xyz]?
16 Because no one has taken on that task yet. Libav development is
17 driven by the tasks that are important to the individual developers.
18 If there is a feature that is important to you, the best way to get
19 it implemented is to undertake the task yourself or sponsor a developer.
21 @section Libav does not support codec XXX. Can you include a Windows DLL loader to support it?
23 No. Windows DLLs are not portable, bloated and often slow.
24 Moreover Libav strives to support all codecs natively.
25 A DLL loader is not conducive to that goal.
27 @section I cannot read this file although this format seems to be supported by avconv.
29 Even if avconv can read the container format, it may not support all its
30 codecs. Please consult the supported codec list in the avconv
31 documentation.
33 @section Which codecs are supported by Windows?
35 Windows does not support standard formats like MPEG very well, unless you
36 install some additional codecs.
38 The following list of video codecs should work on most Windows systems:
39 @table @option
40 @item msmpeg4v2
41 .avi/.asf
42 @item msmpeg4
43 .asf only
44 @item wmv1
45 .asf only
46 @item wmv2
47 .asf only
48 @item mpeg4
49 Only if you have some MPEG-4 codec like ffdshow or Xvid installed.
50 @item mpeg1video
51 .mpg only
52 @end table
53 Note, ASF files often have .wmv or .wma extensions in Windows. It should also
54 be mentioned that Microsoft claims a patent on the ASF format, and may sue
55 or threaten users who create ASF files with non-Microsoft software. It is
56 strongly advised to avoid ASF where possible.
58 The following list of audio codecs should work on most Windows systems:
59 @table @option
60 @item adpcm_ima_wav
61 @item adpcm_ms
62 @item pcm_s16le
63 always
64 @item libmp3lame
65 If some MP3 codec like LAME is installed.
66 @end table
69 @chapter Compilation
71 @section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}
73 This is a bug in gcc. Do not report it to us. Instead, please report it to
74 the gcc developers. Note that we will not add workarounds for gcc bugs.
76 Also note that (some of) the gcc developers believe this is not a bug or
77 not a bug they should fix:
78 @url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.
79 Then again, some of them do not know the difference between an undecidable
80 problem and an NP-hard problem...
82 @chapter Usage
84 @section How do I encode single pictures into movies?
86 First, rename your pictures to follow a numerical sequence.
87 For example, img1.jpg, img2.jpg, img3.jpg,...
88 Then you may run:
90 @example
91 avconv -f image2 -i img%d.jpg /tmp/a.mpg
92 @end example
94 Notice that @samp{%d} is replaced by the image number.
96 @file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc...
98 If you have large number of pictures to rename, you can use the
99 following command to ease the burden. The command, using the bourne
100 shell syntax, symbolically links all files in the current directory
101 that match @code{*jpg} to the @file{/tmp} directory in the sequence of
102 @file{img001.jpg}, @file{img002.jpg} and so on.
104 @example
105 x=1; for i in *jpg; do counter=$(printf %03d $x); ln -s "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
106 @end example
108 If you want to sequence them by oldest modified first, substitute
109 @code{$(ls -r -t *jpg)} in place of @code{*jpg}.
111 Then run:
113 @example
114 avconv -f image2 -i /tmp/img%03d.jpg /tmp/a.mpg
115 @end example
117 The same logic is used for any image format that avconv reads.
119 @section How do I encode movie to single pictures?
121 Use:
123 @example
124 avconv -i movie.mpg movie%d.jpg
125 @end example
127 The @file{movie.mpg} used as input will be converted to
128 @file{movie1.jpg}, @file{movie2.jpg}, etc...
130 Instead of relying on file format self-recognition, you may also use
131 @table @option
132 @item -c:v ppm
133 @item -c:v png
134 @item -c:v mjpeg
135 @end table
136 to force the encoding.
138 Applying that to the previous example:
139 @example
140 avconv -i movie.mpg -f image2 -c:v mjpeg menu%d.jpg
141 @end example
143 Beware that there is no "jpeg" codec. Use "mjpeg" instead.
145 @section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
147 For multithreaded MPEG* encoding, the encoded slices must be independent,
148 otherwise thread n would practically have to wait for n-1 to finish, so it's
149 quite logical that there is a small reduction of quality. This is not a bug.
151 @section How can I read from the standard input or write to the standard output?
153 Use @file{-} as file name.
155 @section -f jpeg doesn't work.
157 Try '-f image2 test%d.jpg'.
159 @section Why can I not change the framerate?
161 Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.
162 Choose a different codec with the -c:v command line option.
164 @section How do I encode Xvid or DivX video with avconv?
166 Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
167 standard (note that there are many other coding formats that use this
168 same standard). Thus, use '-c:v mpeg4' to encode in these formats. The
169 default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
170 a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
171 force the fourcc 'xvid' to be stored as the video fourcc rather than the
172 default.
174 @section Which are good parameters for encoding high quality MPEG-4?
176 '-mbd rd -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
177 things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.
179 @section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
181 '-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
182 but beware the '-g 100' might cause problems with some decoders.
183 Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.
185 @section Interlaced video looks very bad when encoded with avconv, what is wrong?
187 You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
188 material, and try '-top 0/1' if the result looks really messed-up.
190 @section How can I read DirectShow files?
192 If you have built Libav with @code{./configure --enable-avisynth}
193 (only possible on MinGW/Cygwin platforms),
194 then you may use any file that DirectShow can read as input.
196 Just create an "input.avs" text file with this single line ...
197 @example
198 DirectShowSource("C:\path to your file\yourfile.asf")
199 @end example
200 ... and then feed that text file to avconv:
201 @example
202 avconv -i input.avs
203 @end example
205 For ANY other help on Avisynth, please visit the
206 @uref{http://www.avisynth.org/, Avisynth homepage}.
208 @section How can I join video files?
210 A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by
211 merely concatenating them.
213 Hence you may concatenate your multimedia files by first transcoding them to
214 these privileged formats, then using the humble @code{cat} command (or the
215 equally humble @code{copy} under Windows), and finally transcoding back to your
216 format of choice.
218 @example
219 avconv -i input1.avi -same_quant intermediate1.mpg
220 avconv -i input2.avi -same_quant intermediate2.mpg
221 cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
222 avconv -i intermediate_all.mpg -same_quant output.avi
223 @end example
225 Notice that you should either use @code{-same_quant} or set a reasonably high
226 bitrate for your intermediate and output files, if you want to preserve
227 video quality.
229 Also notice that you may avoid the huge intermediate files by taking advantage
230 of named pipes, should your platform support it:
232 @example
233 mkfifo intermediate1.mpg
234 mkfifo intermediate2.mpg
235 avconv -i input1.avi -same_quant -y intermediate1.mpg < /dev/null &
236 avconv -i input2.avi -same_quant -y intermediate2.mpg < /dev/null &
237 cat intermediate1.mpg intermediate2.mpg |\
238 avconv -f mpeg -i - -same_quant -c:v mpeg4 -acodec libmp3lame output.avi
239 @end example
241 Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
242 allow concatenation, and the transcoding step is almost lossless.
243 When using multiple yuv4mpegpipe(s), the first line needs to be discarded
244 from all but the first stream. This can be accomplished by piping through
245 @code{tail} as seen below. Note that when piping through @code{tail} you
246 must use command grouping, @code{@{ ;@}}, to background properly.
248 For example, let's say we want to join two FLV files into an output.flv file:
250 @example
251 mkfifo temp1.a
252 mkfifo temp1.v
253 mkfifo temp2.a
254 mkfifo temp2.v
255 mkfifo all.a
256 mkfifo all.v
257 avconv -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
258 avconv -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
259 avconv -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
260 @{ avconv -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &
261 cat temp1.a temp2.a > all.a &
262 cat temp1.v temp2.v > all.v &
263 avconv -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
264 -f yuv4mpegpipe -i all.v \
265 -same_quant -y output.flv
266 rm temp[12].[av] all.[av]
267 @end example
269 @section -profile option fails when encoding H.264 video with AAC audio
271 @command{avconv} prints an error like
273 @example
274 Undefined constant or missing '(' in 'baseline'
275 Unable to parse option value "baseline"
276 Error setting option profile to value baseline.
277 @end example
279 Short answer: write @option{-profile:v} instead of @option{-profile}.
281 Long answer: this happens because the @option{-profile} option can apply to both
282 video and audio. Specifically the AAC encoder also defines some profiles, none
283 of which are named @var{baseline}.
285 The solution is to apply the @option{-profile} option to the video stream only
286 by using @url{http://libav.org/avconv.html#Stream-specifiers-1, Stream specifiers}.
287 Appending @code{:v} to it will do exactly that.
289 @chapter Development
291 @section Are there examples illustrating how to use the Libav libraries, particularly libavcodec and libavformat?
293 Yes. Read the Developers Guide of the Libav documentation. Alternatively,
294 examine the source code for one of the many open source projects that
295 already incorporate Libav at (@url{projects.html}).
297 @section Can you support my C compiler XXX?
299 It depends. If your compiler is C99-compliant, then patches to support
300 it are likely to be welcome if they do not pollute the source code
301 with @code{#ifdef}s related to the compiler.
303 @section Is Microsoft Visual C++ supported?
305 No. Microsoft Visual C++ is not compliant to the C99 standard and does
306 not - among other things - support the inline assembly used in Libav.
307 If you wish to use MSVC++ for your
308 project then you can link the MSVC++ code with libav* as long as
309 you compile the latter with a working C compiler. For more information, see
310 the @emph{Microsoft Visual C++ compatibility} section in the Libav
311 documentation.
313 There have been efforts to make Libav compatible with MSVC++ in the
314 past. However, they have all been rejected as too intrusive, especially
315 since MinGW does the job adequately. None of the core developers
316 work with MSVC++ and thus this item is low priority. Should you find
317 the silver bullet that solves this problem, feel free to shoot it at us.
319 We strongly recommend you to move over from MSVC++ to MinGW tools.
321 @section Can I use Libav under Windows?
323 Yes, but the Cygwin or MinGW tools @emph{must} be used to compile Libav.
324 Read the @emph{Windows} section in the Libav documentation to find more
325 information.
327 @section Can you add automake, libtool or autoconf support?
329 No. These tools are too bloated and they complicate the build.
331 @section Why not rewrite Libav in object-oriented C++?
333 Libav is already organized in a highly modular manner and does not need to
334 be rewritten in a formal object language. Further, many of the developers
335 favor straight C; it works for them. For more arguments on this matter,
336 read @uref{http://www.tux.org/lkml/#s15, "Programming Religion"}.
338 @section I do not like the LGPL, can I contribute code under the GPL instead?
340 Yes, as long as the code is optional and can easily and cleanly be placed
341 under #if CONFIG_GPL without breaking anything. So for example a new codec
342 or filter would be OK under GPL while a bug fix to LGPL code would not.
344 @section I'm using Libav from within my C++ application but the linker complains about missing symbols which seem to be available.
346 Libav is a pure C project, so to use the libraries within your C++ application
347 you need to explicitly state that you are using a C library. You can do this by
348 encompassing your Libav includes using @code{extern "C"}.
350 See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
352 @section I'm using libavutil from within my C++ application but the compiler complains about 'UINT64_C' was not declared in this scope
354 Libav is a pure C project using C99 math features, in order to enable C++
355 to use them you have to append -D__STDC_CONSTANT_MACROS to your CXXFLAGS
357 @section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
359 You have to implement a URLProtocol, see @file{libavformat/file.c} in
360 Libav and @file{libmpdemux/demux_lavf.c} in MPlayer sources.
362 @bye