Update after the last API change.
[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 The same logic is used for any image format that ffmpeg reads.
143 @section How do I encode movie to single pictures?
145 Use:
147 @example
148 ffmpeg -i movie.mpg movie%d.jpg
149 @end example
151 The @file{movie.mpg} used as input will be converted to
152 @file{movie1.jpg}, @file{movie2.jpg}, etc...
154 Instead of relying on file format self-recognition, you may also use
155 @table @option
156 @item -vcodec ppm
157 @item -vcodec png
158 @item -vcodec mjpeg
159 @end table
160 to force the encoding.
162 Applying that to the previous example:
163 @example
164 ffmpeg -i movie.mpg -f image2 -vcodec mjpeg menu%d.jpg
165 @end example
167 Beware that there is no "jpeg" codec. Use "mjpeg" instead.
169 @section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
171 For multithreaded MPEG* encoding, the encoded slices must be independent,
172 otherwise thread n would practically have to wait for n-1 to finish, so it's
173 quite logical that there is a small reduction of quality. This is not a bug.
175 @section How can I read from the standard input or write to the standard output?
177 Use @file{-} as file name.
179 @section Why does FFmpeg not decode audio in VOB files?
181 The audio is AC-3 (a.k.a. A/52). AC-3 decoding is an optional component in FFmpeg
182 as the component that handles AC-3 decoding is currently released under the GPL.
183 Enable AC-3 decoding with @code{./configure --enable-gpl}. Take care: By
184 enabling AC-3, you automatically change the license of libavcodec from
185 LGPL to GPL.
187 @section Why does the chrominance data seem to be sampled at a different time from the luminance data on bt8x8 captures on Linux?
189 This is a well-known bug in the bt8x8 driver. For 2.4.26 there is a patch at
190 (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-420-2.4.26.patch?view=co}). This may also
191 apply cleanly to other 2.4-series kernels.
193 @section How do I avoid the ugly aliasing artifacts in bt8x8 captures on Linux?
195 Pass 'combfilter=1 lumafilter=1' to the bttv driver. Note though that 'combfilter=1'
196 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})
197 or (@url{http://svn.ffmpeg.org/michael/trunk/patches/bttv-comb-2.6.6.patch?view=co})
198 and pass 'combfilter=2'.
200 @section -f jpeg doesn't work.
202 Try '-f image2 test%d.jpg'.
204 @section Why can I not change the framerate?
206 Some codecs, like MPEG-1/2, only allow a small number of fixed framerates.
207 Choose a different codec with the -vcodec command line option.
209 @section How do I encode Xvid or DivX video with ffmpeg?
211 Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
212 standard (note that there are many other coding formats that use this
213 same standard). Thus, use '-vcodec mpeg4' to encode in these formats. The
214 default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
215 a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
216 force the fourcc 'xvid' to be stored as the video fourcc rather than the
217 default.
219 @section How do I encode videos which play on the iPod?
221 @table @option
222 @item needed stuff
223 -acodec libfaac -vcodec mpeg4 width<=320 height<=240
224 @item working stuff
225 4mv, title
226 @item non-working stuff
227 B-frames
228 @item example command line
229 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
230 @end table
232 @section How do I encode videos which play on the PSP?
234 @table @option
235 @item needed stuff
236 -acodec libfaac -vcodec mpeg4 width*height<=76800 width%16=0 height%16=0 -ar 24000 -r 30000/1001 or 15000/1001 -f psp
237 @item working stuff
238 4mv, title
239 @item non-working stuff
240 B-frames
241 @item example command line
242 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
243 @item needed stuff for H.264
244 -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
245 @item working stuff for H.264
246 title, loop filter
247 @item non-working stuff for H.264
249 @item example command line
250 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
251 @item higher resolution for newer PSP firmwares, width<=480, height<=272
252 -vcodec libx264 -level 21 -coder 1 -f psp
253 @item example command line
254 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
255 @end table
257 @section Which are good parameters for encoding high quality MPEG-4?
259 '-mbd rd -flags +4mv+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
260 things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd'.
262 @section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
264 '-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
265 but beware the '-g 100' might cause problems with some decoders.
266 Things to try: '-bf 2', '-flags qprd', '-flags mv0', '-flags skiprd.
268 @section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?
270 You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
271 material, and try '-top 0/1' if the result looks really messed-up.
273 @section How can I read DirectShow files?
275 If you have built FFmpeg with @code{./configure --enable-avisynth}
276 (only possible on MinGW/Cygwin platforms),
277 then you may use any file that DirectShow can read as input.
278 (Be aware that this feature has been recently added,
279 so you will need to help yourself in case of problems.)
281 Just create an "input.avs" text file with this single line ...
282 @example
283 DirectShowSource("C:\path to your file\yourfile.asf")
284 @end example
285 ... and then feed that text file to FFmpeg:
286 @example
287 ffmpeg -i input.avs
288 @end example
290 For ANY other help on Avisynth, please visit @url{http://www.avisynth.org/}.
292 @section How can I join video files?
294 A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow to join video files by
295 merely concatenating them.
297 Hence you may concatenate your multimedia files by first transcoding them to
298 these privileged formats, then using the humble @code{cat} command (or the
299 equally humble @code{copy} under Windows), and finally transcoding back to your
300 format of choice.
302 @example
303 ffmpeg -i input1.avi -sameq intermediate1.mpg
304 ffmpeg -i input2.avi -sameq intermediate2.mpg
305 cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
306 ffmpeg -i intermediate_all.mpg -sameq output.avi
307 @end example
309 Notice that you should either use @code{-sameq} or set a reasonably high
310 bitrate for your intermediate and output files, if you want to preserve
311 video quality.
313 Also notice that you may avoid the huge intermediate files by taking advantage
314 of named pipes, should your platform support it:
316 @example
317 mkfifo intermediate1.mpg
318 mkfifo intermediate2.mpg
319 ffmpeg -i input1.avi -sameq -y intermediate1.mpg < /dev/null &
320 ffmpeg -i input2.avi -sameq -y intermediate2.mpg < /dev/null &
321 cat intermediate1.mpg intermediate2.mpg |\
322 ffmpeg -f mpeg -i - -sameq -vcodec mpeg4 -acodec libmp3lame output.avi
323 @end example
325 Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
326 allow concatenation, and the transcoding step is almost lossless.
328 For example, let's say we want to join two FLV files into an output.flv file:
330 @example
331 mkfifo temp1.a
332 mkfifo temp1.v
333 mkfifo temp2.a
334 mkfifo temp2.v
335 mkfifo all.a
336 mkfifo all.v
337 ffmpeg -i input1.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
338 ffmpeg -i input2.flv -vn -f u16le -acodec pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
339 ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
340 ffmpeg -i input2.flv -an -f yuv4mpegpipe - > temp2.v < /dev/null &
341 cat temp1.a temp2.a > all.a &
342 cat temp1.v temp2.v > all.v &
343 ffmpeg -f u16le -acodec pcm_s16le -ac 2 -ar 44100 -i all.a \
344 -f yuv4mpegpipe -i all.v \
345 -sameq -y output.flv
346 rm temp[12].[av] all.[av]
347 @end example
349 @section FFmpeg does not adhere to the -maxrate setting, some frames are bigger than maxrate/fps.
351 Read the MPEG spec about video buffer verifier.
353 @section I want CBR, but no matter what I do frame sizes differ.
355 You do not understand what CBR is, please read the MPEG spec.
356 Read about video buffer verifier and constant bitrate.
357 The one sentence summary is that there is a buffer and the input rate is
358 constant, the output can vary as needed.
360 @section How do I check if a stream is CBR?
362 To quote the MPEG-2 spec:
363 "There is no way to tell that a bitstream is constant bitrate without
364 examining all of the vbv_delay values and making complicated computations."
367 @chapter Development
369 @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
371 Yes. Read the Developers Guide of the FFmpeg documentation. Alternatively,
372 examine the source code for one of the many open source projects that
373 already incorporate FFmpeg at (@url{projects.html}).
375 @section Can you support my C compiler XXX?
377 It depends. If your compiler is C99-compliant, then patches to support
378 it are likely to be welcome if they do not pollute the source code
379 with @code{#ifdef}s related to the compiler.
381 @section Is Microsoft Visual C++ supported?
383 No. Microsoft Visual C++ is not compliant to the C99 standard and does
384 not - among other things - support the inline assembly used in FFmpeg.
385 If you wish to use MSVC++ for your
386 project then you can link the MSVC++ code with libav* as long as
387 you compile the latter with a working C compiler. For more information, see
388 the @emph{Microsoft Visual C++ compatibility} section in the FFmpeg
389 documentation.
391 There have been efforts to make FFmpeg compatible with MSVC++ in the
392 past. However, they have all been rejected as too intrusive, especially
393 since MinGW does the job adequately. None of the core developers
394 work with MSVC++ and thus this item is low priority. Should you find
395 the silver bullet that solves this problem, feel free to shoot it at us.
397 We strongly recommend you to move over from MSVC++ to MinGW tools.
399 @section Can I use FFmpeg or libavcodec under Windows?
401 Yes, but the Cygwin or MinGW tools @emph{must} be used to compile FFmpeg.
402 Read the @emph{Windows} section in the FFmpeg documentation to find more
403 information.
405 To get help and instructions for building FFmpeg under Windows, check out
406 the FFmpeg Windows Help Forum at
407 @url{http://ffmpeg.arrozcru.org/}.
409 @section Can you add automake, libtool or autoconf support?
411 No. These tools are too bloated and they complicate the build.
413 @section Why not rewrite ffmpeg in object-oriented C++?
415 FFmpeg is already organized in a highly modular manner and does not need to
416 be rewritten in a formal object language. Further, many of the developers
417 favor straight C; it works for them. For more arguments on this matter,
418 read "Programming Religion" at (@url{http://www.tux.org/lkml/#s15}).
420 @section Why are the ffmpeg programs devoid of debugging symbols?
422 The build process creates ffmpeg_g, ffplay_g, etc. which contain full debug
423 information. Those binaries are stripped to create ffmpeg, ffplay, etc. If
424 you need the debug information, used the *_g versions.
426 @section I do not like the LGPL, can I contribute code under the GPL instead?
428 Yes, as long as the code is optional and can easily and cleanly be placed
429 under #if CONFIG_GPL without breaking anything. So for example a new codec
430 or filter would be OK under GPL while a bug fix to LGPL code would not.
432 @section I want to compile xyz.c alone but my compiler produced many errors.
434 Common code is in its own files in libav* and is used by the individual
435 codecs. They will not work without the common parts, you have to compile
436 the whole libav*. If you wish, disable some parts with configure switches.
437 You can also try to hack it and remove more, but if you had problems fixing
438 the compilation failure then you are probably not qualified for this.
440 @section I'm using libavcodec from within my C++ application but the linker complains about missing symbols which seem to be available.
442 FFmpeg is a pure C project, so to use the libraries within your C++ application
443 you need to explicitly state that you are using a C library. You can do this by
444 encompassing your FFmpeg includes using @code{extern "C"}.
446 See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
448 @section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
450 You have to implement a URLProtocol, see libavformat/file.c in FFmpeg
451 and libmpdemux/demux_lavf.c in MPlayer sources.
453 @section I get "No compatible shell script interpreter found." in MSys.
455 The standard MSys bash (2.04) is broken. You need to install 2.05 or later.
457 @section I get "./configure: line <xxx>: pr: command not found" in MSys.
459 The standard MSys install doesn't come with pr. You need to get it from the coreutils package.
461 @section I tried to pass RTP packets into a decoder, but it doesn't work.
463 RTP is a container format like any other, you must first depacketize the
464 codec frames/samples stored in RTP and then feed to the decoder.
466 @section Where can I find libav* headers for Pascal/Delphi?
468 see @url{http://www.iversenit.dk/dev/ffmpeg-headers/}
470 @section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?
472 see @url{http://svn.ffmpeg.org/michael/trunk/docs/}
474 @section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?
476 Even if peculiar since it is network oriented, RTP is a container like any
477 other. You have to @emph{demux} RTP before feeding the payload to libavcodec.
478 In this specific case please look at RFC 4629 to see how it should be done.
480 @section AVStream.r_frame_rate is wrong, it is much larger than the framerate.
482 r_frame_rate is NOT the average framerate, it is the smallest framerate
483 that can accurately represent all timestamps. So no, it is not
484 wrong if it is larger than the average!
485 For example, if you have mixed 25 and 30 fps content, then r_frame_rate
486 will be 150.
488 @bye