1 \input texinfo @c -*- texinfo -*-
5 @center @titlefont{Libav FAQ}
12 @chapter General Questions
14 @section When will the next Libav version be released? / Why are Libav releases so few and far between?
16 Like most open source projects Libav suffers from a certain lack of
17 manpower. For this reason the developers have to prioritize the work
18 they do and putting out releases is not at the top of the list, fixing
19 bugs and reviewing patches takes precedence. Please don't complain or
20 request more timely and/or frequent releases unless you are willing to
21 help out creating them.
23 @section I have a problem with an old version of Libav; where should I report it?
24 Nowhere. We do not support old Libav versions in any way, we simply lack
25 the time, motivation and manpower to do so. If you have a problem with an
26 old version of Libav, upgrade to the latest git snapshot. If you
27 still experience the problem, then you can report it according to our
28 @uref{http://libav.org/bugreports.html, bug reporting guidelines}.
30 @section Why doesn't Libav support feature [xyz]?
32 Because no one has taken on that task yet. Libav development is
33 driven by the tasks that are important to the individual developers.
34 If there is a feature that is important to you, the best way to get
35 it implemented is to undertake the task yourself or sponsor a developer.
37 @section Libav does not support codec XXX. Can you include a Windows DLL loader to support it?
39 No. Windows DLLs are not portable, bloated and often slow.
40 Moreover Libav strives to support all codecs natively.
41 A DLL loader is not conducive to that goal.
43 @section My bug report/mail to libav-devel/user has not received any replies.
47 @item We are busy and haven't had time yet to read your report or
48 investigate the issue.
49 @item You did not follow our
50 @uref{http://libav.org/bugreports.html, bug reporting guidelines}.
51 @item You didn't use git master.
52 @item You reported a segmentation fault without gdb output.
53 @item You describe a problem but not how to reproduce it.
54 @item It's unclear if you use ffmpeg as command line tool or use
55 libav* from another application.
56 @item You speak about a video having problems on playback but
57 not what you use to play it.
58 @item We have no faint clue what you are talking about besides
59 that it is related to Libav.
62 @section Is there a forum for Libav? I do not like mailing lists.
64 You may view our mailing lists with a more forum-alike look here:
65 @url{http://dir.gmane.org/gmane.comp.video.ffmpeg.user},
66 but, if you post, please remember that our mailing list rules still apply there.
68 @section I cannot read this file although this format seems to be supported by ffmpeg.
70 Even if ffmpeg can read the container format, it may not support all its
71 codecs. Please consult the supported codec list in the ffmpeg
74 @section Which codecs are supported by Windows?
76 Windows does not support standard formats like MPEG very well, unless you
77 install some additional codecs.
79 The following list of video codecs should work on most Windows systems:
90 Only if you have some MPEG-4 codec like ffdshow or Xvid installed.
94 Note, ASF files often have .wmv or .wma extensions in Windows. It should also
95 be mentioned that Microsoft claims a patent on the ASF format, and may sue
96 or threaten users who create ASF files with non-Microsoft software. It is
97 strongly advised to avoid ASF where possible.
99 The following list of audio codecs should work on most Windows systems:
106 If some MP3 codec like LAME is installed.
112 @section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}
114 This is a bug in gcc. Do not report it to us. Instead, please report it to
115 the gcc developers. Note that we will not add workarounds for gcc bugs.
117 Also note that (some of) the gcc developers believe this is not a bug or
118 not a bug they should fix:
119 @url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.
120 Then again, some of them do not know the difference between an undecidable
121 problem and an NP-hard problem...
125 @section ffmpeg does not work; what is wrong?
127 Try a @code{make distclean} in the ffmpeg source directory before the build.
128 If this does not help see our
129 @uref{http://libav.org/bugreports.html, bug reporting guidelines}.
131 @section How do I encode single pictures into movies?
133 First, rename your pictures to follow a numerical sequence.
134 For example, img1.jpg, img2.jpg, img3.jpg,...
138 ffmpeg -f image2 -i img%d.jpg /tmp/a.mpg
141 Notice that @samp{%d} is replaced by the image number.
143 @file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc...
145 If you have large number of pictures to rename, you can use the
146 following command to ease the burden. The command, using the bourne
147 shell syntax, symbolically links all files in the current directory
148 that match @code{*jpg} to the @file{/tmp} directory in the sequence of
149 @file{img001.jpg}, @file{img002.jpg} and so on.
152 x=1; for i in *jpg; do counter=$(printf %03d $x); ln -s "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
155 If you want to sequence them by oldest modified first, substitute
156 @code{$(ls -r -t *jpg)} in place of @code{*jpg}.
161 ffmpeg -f image2 -i /tmp/img%03d.jpg /tmp/a.mpg
164 The same logic is used for any image format that ffmpeg reads.
166 @section How do I encode movie to single pictures?
171 ffmpeg -i movie.mpg movie%d.jpg
174 The @file{movie.mpg} used as input will be converted to
175 @file{movie1.jpg}, @file{movie2.jpg}, etc...
177 Instead of relying on file format self-recognition, you may also use
183 to force the encoding.
185 Applying that to the previous example:
187 ffmpeg -i movie.mpg -f image2 -vcodec mjpeg menu%d.jpg
190 Beware that there is no "jpeg" codec. Use "mjpeg" instead.
192 @section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
194 For multithreaded MPEG* encoding, the encoded slices must be independent,
195 otherwise thread n would practically have to wait for n-1 to finish, so it's
196 quite logical that there is a small reduction of quality. This is not a bug.
198 @section How can I read from the standard input or write to the standard output?
200 Use @file{-} as file name.
202 @section -f jpeg doesn't work.
204 Try '-f image2 test%d.jpg'.
206 @section Why can I not change the framerate?
208 Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.
209 Choose a different codec with the -vcodec command line option.
211 @section How do I encode Xvid or DivX video with ffmpeg?
213 Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
214 standard (note that there are many other coding formats that use this
215 same standard). Thus, use '-vcodec mpeg4' to encode in these formats. The
216 default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
217 a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
218 force the fourcc 'xvid' to be stored as the video fourcc rather than the
221 @section How do I encode videos which play on the iPod?
225 -acodec libfaac -vcodec mpeg4 width<=320 height<=240
228 @item non-working stuff
230 @item example command line
231 ffmpeg -i input -acodec libfaac -ab 128k -vcodec mpeg4 -b 1200k -mbd 2 -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -s 320x180 -metadata title=X output.mp4
234 @section How do I encode videos which play on the PSP?
238 -acodec libfaac -vcodec mpeg4 width*height<=76800 width%16=0 height%16=0 -ar 24000 -r 30000/1001 or 15000/1001 -f psp
241 @item non-working stuff
243 @item example command line
244 ffmpeg -i input -acodec libfaac -ab 128k -vcodec mpeg4 -b 1200k -ar 24000 -mbd 2 -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -metadata title=X -f psp output.mp4
245 @item needed stuff for H.264
246 -acodec libfaac -vcodec libx264 width*height<=76800 width%16=0? height%16=0? -ar 48000 -coder 1 -r 30000/1001 or 15000/1001 -f psp
247 @item working stuff for H.264
249 @item non-working stuff for H.264
251 @item example command line
252 ffmpeg -i input -acodec libfaac -ab 128k -vcodec libx264 -b 1200k -ar 48000 -mbd 2 -coder 1 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -metadata title=X -f psp -flags loop -trellis 2 -partitions parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 output.mp4
253 @item higher resolution for newer PSP firmwares, width<=480, height<=272
254 -vcodec libx264 -level 21 -coder 1 -f psp
255 @item example command line
256 ffmpeg -i input -acodec libfaac -ab 128k -ac 2 -ar 48000 -vcodec libx264 -level 21 -b 640k -coder 1 -f psp -flags +loop -trellis 2 -partitions +parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 -g 250 -s 480x272 output.mp4
259 @section Which are good parameters for encoding high quality MPEG-4?
261 '-mbd rd -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
262 things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.
264 @section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
266 '-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
267 but beware the '-g 100' might cause problems with some decoders.
268 Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.
270 @section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?
272 You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
273 material, and try '-top 0/1' if the result looks really messed-up.
275 @section How can I read DirectShow files?
277 If you have built Libav with @code{./configure --enable-avisynth}
278 (only possible on MinGW/Cygwin platforms),
279 then you may use any file that DirectShow can read as input.
281 Just create an "input.avs" text file with this single line ...
283 DirectShowSource("C:\path to your file\yourfile.asf")
285 ... and then feed that text file to ffmpeg:
290 For ANY other help on Avisynth, please visit the
291 @uref{http://www.avisynth.org/, Avisynth homepage}.
293 @section How can I join video files?
295 A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by
296 merely concatenating them.
298 Hence you may concatenate your multimedia files by first transcoding them to
299 these privileged formats, then using the humble @code{cat} command (or the
300 equally humble @code{copy} under Windows), and finally transcoding back to your
304 ffmpeg -i input1.avi -sameq intermediate1.mpg
305 ffmpeg -i input2.avi -sameq intermediate2.mpg
306 cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
307 ffmpeg -i intermediate_all.mpg -sameq output.avi
310 Notice that you should either use @code{-sameq} or set a reasonably high
311 bitrate for your intermediate and output files, if you want to preserve
314 Also notice that you may avoid the huge intermediate files by taking advantage
315 of named pipes, should your platform support it:
318 mkfifo intermediate1.mpg
319 mkfifo intermediate2.mpg
320 ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &
321 ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &
322 cat intermediate1.mpg intermediate2.mpg |\
323 ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi
326 Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
327 allow concatenation, and the transcoding step is almost lossless.
328 When using multiple yuv4mpegpipe(s), the first line needs to be discarded
329 from all but the first stream. This can be accomplished by piping through
330 @code{tail} as seen below. Note that when piping through @code{tail} you
331 must use command grouping, @code{@{ ;@}}, to background properly.
333 For example, let's say we want to join two FLV files into an output.flv file:
342 ffmpeg -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
343 ffmpeg -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
344 ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
345 @{ ffmpeg -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &
346 cat temp1.a temp2.a > all.a &
347 cat temp1.v temp2.v > all.v &
348 ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
349 -f yuv4mpegpipe -i all.v \
351 rm temp[12].[av] all.[av]
354 @section The ffmpeg program does not respect the -maxrate setting, some frames are bigger than maxrate/fps.
356 Read the MPEG spec about video buffer verifier.
358 @section I want CBR, but no matter what I do frame sizes differ.
360 You do not understand what CBR is, please read the MPEG spec.
361 Read about video buffer verifier and constant bitrate.
362 The one sentence summary is that there is a buffer and the input rate is
363 constant, the output can vary as needed.
365 @section How do I check if a stream is CBR?
367 To quote the MPEG-2 spec:
368 "There is no way to tell that a bitstream is constant bitrate without
369 examining all of the vbv_delay values and making complicated computations."
374 @section Are there examples illustrating how to use the Libav libraries, particularly libavcodec and libavformat?
376 Yes. Read the Developers Guide of the Libav documentation. Alternatively,
377 examine the source code for one of the many open source projects that
378 already incorporate Libav at (@url{projects.html}).
380 @section Can you support my C compiler XXX?
382 It depends. If your compiler is C99-compliant, then patches to support
383 it are likely to be welcome if they do not pollute the source code
384 with @code{#ifdef}s related to the compiler.
386 @section Is Microsoft Visual C++ supported?
388 No. Microsoft Visual C++ is not compliant to the C99 standard and does
389 not - among other things - support the inline assembly used in Libav.
390 If you wish to use MSVC++ for your
391 project then you can link the MSVC++ code with libav* as long as
392 you compile the latter with a working C compiler. For more information, see
393 the @emph{Microsoft Visual C++ compatibility} section in the Libav
396 There have been efforts to make Libav compatible with MSVC++ in the
397 past. However, they have all been rejected as too intrusive, especially
398 since MinGW does the job adequately. None of the core developers
399 work with MSVC++ and thus this item is low priority. Should you find
400 the silver bullet that solves this problem, feel free to shoot it at us.
402 We strongly recommend you to move over from MSVC++ to MinGW tools.
404 @section Can I use Libav or libavcodec under Windows?
406 Yes, but the Cygwin or MinGW tools @emph{must} be used to compile Libav.
407 Read the @emph{Windows} section in the Libav documentation to find more
410 @section Can you add automake, libtool or autoconf support?
412 No. These tools are too bloated and they complicate the build.
414 @section Why not rewrite ffmpeg in object-oriented C++?
416 Libav is already organized in a highly modular manner and does not need to
417 be rewritten in a formal object language. Further, many of the developers
418 favor straight C; it works for them. For more arguments on this matter,
419 read @uref{http://www.tux.org/lkml/#s15, "Programming Religion"}.
421 @section I do not like the LGPL, can I contribute code under the GPL instead?
423 Yes, as long as the code is optional and can easily and cleanly be placed
424 under #if CONFIG_GPL without breaking anything. So for example a new codec
425 or filter would be OK under GPL while a bug fix to LGPL code would not.
427 @section I want to compile xyz.c alone but my compiler produced many errors.
429 Common code is in its own files in libav* and is used by the individual
430 codecs. They will not work without the common parts, you have to compile
431 the whole libav*. If you wish, disable some parts with configure switches.
432 You can also try to hack it and remove more, but if you had problems fixing
433 the compilation failure then you are probably not qualified for this.
435 @section I'm using libavcodec from within my C++ application but the linker complains about missing symbols which seem to be available.
437 Libav is a pure C project, so to use the libraries within your C++ application
438 you need to explicitly state that you are using a C library. You can do this by
439 encompassing your Libav includes using @code{extern "C"}.
441 See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
443 @section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
445 You have to implement a URLProtocol, see @file{libavformat/file.c} in
446 Libav and @file{libmpdemux/demux_lavf.c} in MPlayer sources.
448 @section I get "No compatible shell script interpreter found." in MSys.
450 The standard MSys bash (2.04) is broken. You need to install 2.05 or later.
452 @section I get "./configure: line <xxx>: pr: command not found" in MSys.
454 The standard MSys install doesn't come with pr. You need to get it from the coreutils package.
456 @section Where can I find libav* headers for Pascal/Delphi?
458 see @url{http://www.iversenit.dk/dev/ffmpeg-headers/}
460 @section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?
462 see @url{http://www.ffmpeg.org/~michael/}
464 @section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?
466 Even if peculiar since it is network oriented, RTP is a container like any
467 other. You have to @emph{demux} RTP before feeding the payload to libavcodec.
468 In this specific case please look at RFC 4629 to see how it should be done.
470 @section AVStream.r_frame_rate is wrong, it is much larger than the framerate.
472 r_frame_rate is NOT the average framerate, it is the smallest framerate
473 that can accurately represent all timestamps. So no, it is not
474 wrong if it is larger than the average!
475 For example, if you have mixed 25 and 30 fps content, then r_frame_rate