[libav.git] / doc / faq.texi
1 \input texinfo @c -*- texinfo -*-
3 @settitle FFmpeg FAQ
4 @titlepage
5 @sp 7
6 @center @titlefont{FFmpeg FAQ}
7 @sp 3
8 @end titlepage
11 @chapter General Questions
13 @section When will the next FFmpeg version be released? / Why are FFmpeg releases so few and far between?
15 Like most open source projects FFmpeg suffers from a certain lack of
16 manpower. For this reason the developers have to prioritize the work
17 they do and putting out releases is not at the top of the list, fixing
18 bugs and reviewing patches takes precedence. Please don't complain or
19 request more timely and/or frequent releases unless you are willing to
20 help out creating them.
22 @section I have a problem with an old version of FFmpeg; where should I report it?
23 Nowhere. Upgrade to the latest release or if there is no recent release upgrade
24 to Subversion HEAD. You could also try to report it. Maybe you will get lucky and
25 become the first person in history to get an answer different from "upgrade
26 to Subversion HEAD".
28 @section Why doesn't FFmpeg support feature [xyz]?
30 Because no one has taken on that task yet. FFmpeg development is
31 driven by the tasks that are important to the individual developers.
32 If there is a feature that is important to you, the best way to get
33 it implemented is to undertake the task yourself or sponsor a developer.
35 @section FFmpeg does not support codec XXX. Can you include a Windows DLL loader to support it?
37 No. Windows DLLs are not portable, bloated and often slow.
38 Moreover FFmpeg strives to support all codecs natively.
39 A DLL loader is not conducive to that goal.
41 @section My bug report/mail to ffmpeg-devel/user has not received any replies.
43 Likely reasons
44 @itemize
45 @item We are busy and haven't had time yet to read your report or
46 investigate the issue.
47 @item You didn't follow bugreports.html.
48 @item You didn't use Subversion HEAD.
49 @item You reported a segmentation fault without gdb output.
50 @item You describe a problem but not how to reproduce it.
51 @item It's unclear if you use ffmpeg as command line tool or use
52 libav* from another application.
53 @item You speak about a video having problems on playback but
54 not what you use to play it.
55 @item We have no faint clue what you are talking about besides
56 that it is related to FFmpeg.
57 @end itemize
59 @section Is there a forum for FFmpeg? I do not like mailing lists.
61 You may view our mailing lists with a more forum-alike look here:
62 @url{http://dir.gmane.org/gmane.comp.video.ffmpeg.user},
63 but, if you post, please remember that our mailing list rules still apply there.
65 @section I cannot read this file although this format seems to be supported by ffmpeg.
67 Even if ffmpeg can read the container format, it may not support all its
68 codecs. Please consult the supported codec list in the ffmpeg
69 documentation.
71 @section Which codecs are supported by Windows?
73 Windows does not support standard formats like MPEG very well, unless you
74 install some additional codecs.
76 The following list of video codecs should work on most Windows systems:
77 @table @option
78 @item msmpeg4v2
79 .avi/.asf
80 @item msmpeg4
81 .asf only
82 @item wmv1
83 .asf only
84 @item wmv2
85 .asf only
86 @item mpeg4
87 Only if you have some MPEG-4 codec like ffdshow or Xvid installed.
88 @item mpeg1
89 .mpg only
90 @end table
91 Note, ASF files often have .wmv or .wma extensions in Windows. It should also
92 be mentioned that Microsoft claims a patent on the ASF format, and may sue
93 or threaten users who create ASF files with non-Microsoft software. It is
94 strongly advised to avoid ASF where possible.
96 The following list of audio codecs should work on most Windows systems:
97 @table @option
98 @item adpcm_ima_wav
99 @item adpcm_ms
100 @item pcm
101 always
102 @item mp3
103 If some MP3 codec like LAME is installed.
104 @end table
107 @chapter Compilation
109 @section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}
111 This is a bug in gcc. Do not report it to us. Instead, please report it to
112 the gcc developers. Note that we will not add workarounds for gcc bugs.
114 Also note that (some of) the gcc developers believe this is not a bug or
115 not a bug they should fix:
116 @url{http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.
117 Then again, some of them do not know the difference between an undecidable
118 problem and an NP-hard problem...
120 @chapter Usage
122 @section ffmpeg does not work; what is wrong?
124 Try a @code{make distclean} in the ffmpeg source directory before the build. If this does not help see
125 (@url{http://ffmpeg.org/bugreports.html}).
127 @section How do I encode single pictures into movies?
129 First, rename your pictures to follow a numerical sequence.
130 For example, img1.jpg, img2.jpg, img3.jpg,...
131 Then you may run:
133 @example
134 ffmpeg -f image2 -i img%d.jpg /tmp/a.mpg
135 @end example
137 Notice that @samp{%d} is replaced by the image number.
139 @file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc...
141 If you have large number of pictures to rename, you can use the
142 following command to ease the burden. The command, using the bourne
143 shell syntax, symbolically links all files in the current directory
144 that match @code{*jpg} to the @file{/tmp} directory in the sequence of
145 @file{img001.jpg}, @file{img002.jpg} and so on.
147 @example
148 x=1; for i in *jpg; do counter=$(printf %03d $x); ln "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
149 @end example
151 If you want to sequence them by oldest modified first, substitute
152 @code{$(ls -r -t *jpg)} in place of @code{*jpg}.
154 Then run:
156 @example
157 ffmpeg -f image2 -i /tmp/img%03d.jpg /tmp/a.mpg
158 @end example
160 The same logic is used for any image format that ffmpeg reads.
162 @section How do I encode movie to single pictures?
164 Use:
166 @example
167 ffmpeg -i movie.mpg movie%d.jpg
168 @end example
170 The @file{movie.mpg} used as input will be converted to
171 @file{movie1.jpg}, @file{movie2.jpg}, etc...
173 Instead of relying on file format self-recognition, you may also use
174 @table @option
175 @item -vcodec ppm
176 @item -vcodec png
177 @item -vcodec mjpeg
178 @end table
179 to force the encoding.
181 Applying that to the previous example:
182 @example
183 ffmpeg -i movie.mpg -f image2 -vcodec mjpeg menu%d.jpg
184 @end example
186 Beware that there is no "jpeg" codec. Use "mjpeg" instead.
188 @section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
190 For multithreaded MPEG* encoding, the encoded slices must be independent,
191 otherwise thread n would practically have to wait for n-1 to finish, so it's
192 quite logical that there is a small reduction of quality. This is not a bug.
194 @section How can I read from the standard input or write to the standard output?
196 Use @file{-} as file name.
198 @section Why does the chrominance data seem to be sampled at a different time from the luminance data on bt8x8 captures on Linux?
200 This is a well-known bug in the bt8x8 driver. For 2.4.26 there is a patch at
201 (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-420-2.4.26.patch?view=co}). This may also
202 apply cleanly to other 2.4-series kernels.
204 @section How do I avoid the ugly aliasing artifacts in bt8x8 captures on Linux?
206 Pass 'combfilter=1 lumafilter=1' to the bttv driver. Note though that 'combfilter=1'
207 will cause somewhat too strong filtering. A fix is to apply (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.4.26.patch?view=co})
208 or (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.6.6.patch?view=co})
209 and pass 'combfilter=2'.
211 @section -f jpeg doesn't work.
213 Try '-f image2 test%d.jpg'.
215 @section Why can I not change the framerate?
217 Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.
218 Choose a different codec with the -vcodec command line option.
220 @section How do I encode Xvid or DivX video with ffmpeg?
222 Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
223 standard (note that there are many other coding formats that use this
224 same standard). Thus, use '-vcodec mpeg4' to encode in these formats. The
225 default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
226 a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
227 force the fourcc 'xvid' to be stored as the video fourcc rather than the
228 default.
230 @section How do I encode videos which play on the iPod?
232 @table @option
233 @item needed stuff
234 -acodec libfaac -vcodec mpeg4 width<=320 height<=240
235 @item working stuff
236 4mv, title
237 @item non-working stuff
238 B-frames
239 @item example command line
240 ffmpeg -i input -acodec libfaac -ab 128kb -vcodec mpeg4 -b 1200kb -mbd 2 -flags +4mv -trellis 2 -aic 2 -cmp 2 -subcmp 2 -s 320x180 -title X output.mp4
241 @end table
243 @section How do I encode videos which play on the PSP?
245 @table @option
246 @item needed stuff
247 -acodec libfaac -vcodec mpeg4 width*height<=76800 width%16=0 height%16=0 -ar 24000 -r 30000/1001 or 15000/1001 -f psp
248 @item working stuff
249 4mv, title
250 @item non-working stuff
251 B-frames
252 @item example command line
253 ffmpeg -i input -acodec libfaac -ab 128kb -vcodec mpeg4 -b 1200kb -ar 24000 -mbd 2 -flags +4mv -trellis 2 -aic 2 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -title X -f psp output.mp4
254 @item needed stuff for H.264
255 -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
256 @item working stuff for H.264
257 title, loop filter
258 @item non-working stuff for H.264
260 @item example command line
261 ffmpeg -i input -acodec libfaac -ab 128kb -vcodec libx264 -b 1200kb -ar 48000 -mbd 2 -coder 1 -cmp 2 -subcmp 2 -s 368x192 -r 30000/1001 -title X -f psp -flags loop -trellis 2 -partitions parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 output.mp4
262 @item higher resolution for newer PSP firmwares, width<=480, height<=272
263 -vcodec libx264 -level 21 -coder 1 -f psp
264 @item example command line
265 ffmpeg -i input -acodec libfaac -ab 128kb -ac 2 -ar 48000 -vcodec libx264 -level 21 -b 640kb -coder 1 -f psp -flags +loop -trellis 2 -partitions +parti4x4+parti8x8+partp4x4+partp8x8+partb8x8 -g 250 -s 480x272 output.mp4
266 @end table
268 @section Which are good parameters for encoding high quality MPEG-4?
270 '-mbd rd -flags +4mv+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
271 things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.
273 @section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
275 '-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
276 but beware the '-g 100' might cause problems with some decoders.
277 Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.
279 @section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?
281 You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
282 material, and try '-top 0/1' if the result looks really messed-up.
284 @section How can I read DirectShow files?
286 If you have built FFmpeg with @code{./configure --enable-avisynth}
287 (only possible on MinGW/Cygwin platforms),
288 then you may use any file that DirectShow can read as input.
289 (Be aware that this feature has been recently added,
290 so you will need to help yourself in case of problems.)
292 Just create an "input.avs" text file with this single line ...
293 @example
294 DirectShowSource("C:\path to your file\yourfile.asf")
295 @end example
296 ... and then feed that text file to FFmpeg:
297 @example
298 ffmpeg -i input.avs
299 @end example
301 For ANY other help on Avisynth, please visit @url{http://www.avisynth.org/}.
303 @section How can I join video files?
305 A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by
306 merely concatenating them.
308 Hence you may concatenate your multimedia files by first transcoding them to
309 these privileged formats, then using the humble @code{cat} command (or the
310 equally humble @code{copy} under Windows), and finally transcoding back to your
311 format of choice.
313 @example
314 ffmpeg -i input1.avi -sameq intermediate1.mpg
315 ffmpeg -i input2.avi -sameq intermediate2.mpg
316 cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
317 ffmpeg -i intermediate_all.mpg -sameq output.avi
318 @end example
320 Notice that you should either use @code{-sameq} or set a reasonably high
321 bitrate for your intermediate and output files, if you want to preserve
322 video quality.
324 Also notice that you may avoid the huge intermediate files by taking advantage
325 of named pipes, should your platform support it:
327 @example
328 mkfifo intermediate1.mpg
329 mkfifo intermediate2.mpg
330 ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &
331 ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &
332 cat intermediate1.mpg intermediate2.mpg |\
333 ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi
334 @end example
336 Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
337 allow concatenation, and the transcoding step is almost lossless.
338 When using multiple yuv4mpegpipe(s), the first line needs to be discarded
339 from all but the first stream. This can be accomplished by piping through
340 @code{tail} as seen below. Note that when piping through @code{tail} you
341 must use command grouping, @code{@{ ;@}}, to background properly.
343 For example, let's say we want to join two FLV files into an output.flv file:
345 @example
346 mkfifo temp1.a
347 mkfifo temp1.v
348 mkfifo temp2.a
349 mkfifo temp2.v
350 mkfifo all.a
351 mkfifo all.v
352 ffmpeg -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
353 ffmpeg -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
354 ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
355 @{ ffmpeg -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &
356 cat temp1.a temp2.a > all.a &
357 cat temp1.v temp2.v > all.v &
358 ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
359 -f yuv4mpegpipe -i all.v \
360 -sameq -y output.flv
361 rm temp[12].[av] all.[av]
362 @end example
364 @section FFmpeg does not adhere to the -maxrate setting, some frames are bigger than maxrate/fps.
366 Read the MPEG spec about video buffer verifier.
368 @section I want CBR, but no matter what I do frame sizes differ.
370 You do not understand what CBR is, please read the MPEG spec.
371 Read about video buffer verifier and constant bitrate.
372 The one sentence summary is that there is a buffer and the input rate is
373 constant, the output can vary as needed.
375 @section How do I check if a stream is CBR?
377 To quote the MPEG-2 spec:
378 "There is no way to tell that a bitstream is constant bitrate without
379 examining all of the vbv_delay values and making complicated computations."
382 @chapter Development
384 @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
386 Yes. Read the Developers Guide of the FFmpeg documentation. Alternatively,
387 examine the source code for one of the many open source projects that
388 already incorporate FFmpeg at (@url{projects.html}).
390 @section Can you support my C compiler XXX?
392 It depends. If your compiler is C99-compliant, then patches to support
393 it are likely to be welcome if they do not pollute the source code
394 with @code{#ifdef}s related to the compiler.
396 @section Is Microsoft Visual C++ supported?
398 No. Microsoft Visual C++ is not compliant to the C99 standard and does
399 not - among other things - support the inline assembly used in FFmpeg.
400 If you wish to use MSVC++ for your
401 project then you can link the MSVC++ code with libav* as long as
402 you compile the latter with a working C compiler. For more information, see
403 the @emph{Microsoft Visual C++ compatibility} section in the FFmpeg
404 documentation.
406 There have been efforts to make FFmpeg compatible with MSVC++ in the
407 past. However, they have all been rejected as too intrusive, especially
408 since MinGW does the job adequately. None of the core developers
409 work with MSVC++ and thus this item is low priority. Should you find
410 the silver bullet that solves this problem, feel free to shoot it at us.
412 We strongly recommend you to move over from MSVC++ to MinGW tools.
414 @section Can I use FFmpeg or libavcodec under Windows?
416 Yes, but the Cygwin or MinGW tools @emph{must} be used to compile FFmpeg.
417 Read the @emph{Windows} section in the FFmpeg documentation to find more
418 information.
420 To get help and instructions for building FFmpeg under Windows, check out
421 the FFmpeg Windows Help Forum at
422 @url{http://ffmpeg.arrozcru.org/}.
424 @section Can you add automake, libtool or autoconf support?
426 No. These tools are too bloated and they complicate the build.
428 @section Why not rewrite ffmpeg in object-oriented C++?
430 FFmpeg is already organized in a highly modular manner and does not need to
431 be rewritten in a formal object language. Further, many of the developers
432 favor straight C; it works for them. For more arguments on this matter,
433 read "Programming Religion" at (@url{http://www.tux.org/lkml/#s15}).
435 @section Why are the ffmpeg programs devoid of debugging symbols?
437 The build process creates ffmpeg_g, ffplay_g, etc. which contain full debug
438 information. Those binaries are stripped to create ffmpeg, ffplay, etc. If
439 you need the debug information, used the *_g versions.
441 @section I do not like the LGPL, can I contribute code under the GPL instead?
443 Yes, as long as the code is optional and can easily and cleanly be placed
444 under #if CONFIG_GPL without breaking anything. So for example a new codec
445 or filter would be OK under GPL while a bug fix to LGPL code would not.
447 @section I want to compile xyz.c alone but my compiler produced many errors.
449 Common code is in its own files in libav* and is used by the individual
450 codecs. They will not work without the common parts, you have to compile
451 the whole libav*. If you wish, disable some parts with configure switches.
452 You can also try to hack it and remove more, but if you had problems fixing
453 the compilation failure then you are probably not qualified for this.
455 @section I'm using libavcodec from within my C++ application but the linker complains about missing symbols which seem to be available.
457 FFmpeg is a pure C project, so to use the libraries within your C++ application
458 you need to explicitly state that you are using a C library. You can do this by
459 encompassing your FFmpeg includes using @code{extern "C"}.
461 See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
463 @section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
465 You have to implement a URLProtocol, see libavformat/file.c in FFmpeg
466 and libmpdemux/demux_lavf.c in MPlayer sources.
468 @section I get "No compatible shell script interpreter found." in MSys.
470 The standard MSys bash (2.04) is broken. You need to install 2.05 or later.
472 @section I get "./configure: line <xxx>: pr: command not found" in MSys.
474 The standard MSys install doesn't come with pr. You need to get it from the coreutils package.
476 @section I tried to pass RTP packets into a decoder, but it doesn't work.
478 RTP is a container format like any other, you must first depacketize the
479 codec frames/samples stored in RTP and then feed to the decoder.
481 @section Where can I find libav* headers for Pascal/Delphi?
483 see @url{http://www.iversenit.dk/dev/ffmpeg-headers/}
485 @section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?
487 see @url{http://svn.ffmpeg.org/michael/trunk/docs/}
489 @section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?
491 Even if peculiar since it is network oriented, RTP is a container like any
492 other. You have to @emph{demux} RTP before feeding the payload to libavcodec.
493 In this specific case please look at RFC 4629 to see how it should be done.
495 @section AVStream.r_frame_rate is wrong, it is much larger than the framerate.
497 r_frame_rate is NOT the average framerate, it is the smallest framerate
498 that can accurately represent all timestamps. So no, it is not
499 wrong if it is larger than the average!
500 For example, if you have mixed 25 and 30 fps content, then r_frame_rate
501 will be 150.
503 @bye