faq.texi 25 KB

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  1. \input texinfo @c -*- texinfo -*-
  2. @documentencoding UTF-8
  3. @settitle FFmpeg FAQ
  4. @titlepage
  5. @center @titlefont{FFmpeg FAQ}
  6. @end titlepage
  7. @top
  8. @contents
  9. @chapter General Questions
  10. @section Why doesn't FFmpeg support feature [xyz]?
  11. Because no one has taken on that task yet. FFmpeg development is
  12. driven by the tasks that are important to the individual developers.
  13. If there is a feature that is important to you, the best way to get
  14. it implemented is to undertake the task yourself or sponsor a developer.
  15. @section FFmpeg does not support codec XXX. Can you include a Windows DLL loader to support it?
  16. No. Windows DLLs are not portable, bloated and often slow.
  17. Moreover FFmpeg strives to support all codecs natively.
  18. A DLL loader is not conducive to that goal.
  19. @section I cannot read this file although this format seems to be supported by ffmpeg.
  20. Even if ffmpeg can read the container format, it may not support all its
  21. codecs. Please consult the supported codec list in the ffmpeg
  22. documentation.
  23. @section Which codecs are supported by Windows?
  24. Windows does not support standard formats like MPEG very well, unless you
  25. install some additional codecs.
  26. The following list of video codecs should work on most Windows systems:
  27. @table @option
  28. @item msmpeg4v2
  29. .avi/.asf
  30. @item msmpeg4
  31. .asf only
  32. @item wmv1
  33. .asf only
  34. @item wmv2
  35. .asf only
  36. @item mpeg4
  37. Only if you have some MPEG-4 codec like ffdshow or Xvid installed.
  38. @item mpeg1video
  39. .mpg only
  40. @end table
  41. Note, ASF files often have .wmv or .wma extensions in Windows. It should also
  42. be mentioned that Microsoft claims a patent on the ASF format, and may sue
  43. or threaten users who create ASF files with non-Microsoft software. It is
  44. strongly advised to avoid ASF where possible.
  45. The following list of audio codecs should work on most Windows systems:
  46. @table @option
  47. @item adpcm_ima_wav
  48. @item adpcm_ms
  49. @item pcm_s16le
  50. always
  51. @item libmp3lame
  52. If some MP3 codec like LAME is installed.
  53. @end table
  54. @chapter Compilation
  55. @section @code{error: can't find a register in class 'GENERAL_REGS' while reloading 'asm'}
  56. This is a bug in gcc. Do not report it to us. Instead, please report it to
  57. the gcc developers. Note that we will not add workarounds for gcc bugs.
  58. Also note that (some of) the gcc developers believe this is not a bug or
  59. not a bug they should fix:
  60. @url{https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11203}.
  61. Then again, some of them do not know the difference between an undecidable
  62. problem and an NP-hard problem...
  63. @section I have installed this library with my distro's package manager. Why does @command{configure} not see it?
  64. Distributions usually split libraries in several packages. The main package
  65. contains the files necessary to run programs using the library. The
  66. development package contains the files necessary to build programs using the
  67. library. Sometimes, docs and/or data are in a separate package too.
  68. To build FFmpeg, you need to install the development package. It is usually
  69. called @file{libfoo-dev} or @file{libfoo-devel}. You can remove it after the
  70. build is finished, but be sure to keep the main package.
  71. @section How do I make @command{pkg-config} find my libraries?
  72. Somewhere along with your libraries, there is a @file{.pc} file (or several)
  73. in a @file{pkgconfig} directory. You need to set environment variables to
  74. point @command{pkg-config} to these files.
  75. If you need to @emph{add} directories to @command{pkg-config}'s search list
  76. (typical use case: library installed separately), add it to
  77. @code{$PKG_CONFIG_PATH}:
  78. @example
  79. export PKG_CONFIG_PATH=/opt/x264/lib/pkgconfig:/opt/opus/lib/pkgconfig
  80. @end example
  81. If you need to @emph{replace} @command{pkg-config}'s search list
  82. (typical use case: cross-compiling), set it in
  83. @code{$PKG_CONFIG_LIBDIR}:
  84. @example
  85. export PKG_CONFIG_LIBDIR=/home/me/cross/usr/lib/pkgconfig:/home/me/cross/usr/local/lib/pkgconfig
  86. @end example
  87. If you need to know the library's internal dependencies (typical use: static
  88. linking), add the @code{--static} option to @command{pkg-config}:
  89. @example
  90. ./configure --pkg-config-flags=--static
  91. @end example
  92. @section How do I use @command{pkg-config} when cross-compiling?
  93. The best way is to install @command{pkg-config} in your cross-compilation
  94. environment. It will automatically use the cross-compilation libraries.
  95. You can also use @command{pkg-config} from the host environment by
  96. specifying explicitly @code{--pkg-config=pkg-config} to @command{configure}.
  97. In that case, you must point @command{pkg-config} to the correct directories
  98. using the @code{PKG_CONFIG_LIBDIR}, as explained in the previous entry.
  99. As an intermediate solution, you can place in your cross-compilation
  100. environment a script that calls the host @command{pkg-config} with
  101. @code{PKG_CONFIG_LIBDIR} set. That script can look like that:
  102. @example
  103. #!/bin/sh
  104. PKG_CONFIG_LIBDIR=/path/to/cross/lib/pkgconfig
  105. export PKG_CONFIG_LIBDIR
  106. exec /usr/bin/pkg-config "$@@"
  107. @end example
  108. @chapter Usage
  109. @section ffmpeg does not work; what is wrong?
  110. Try a @code{make distclean} in the ffmpeg source directory before the build.
  111. If this does not help see
  112. (@url{https://ffmpeg.org/bugreports.html}).
  113. @section How do I encode single pictures into movies?
  114. First, rename your pictures to follow a numerical sequence.
  115. For example, img1.jpg, img2.jpg, img3.jpg,...
  116. Then you may run:
  117. @example
  118. ffmpeg -f image2 -i img%d.jpg /tmp/a.mpg
  119. @end example
  120. Notice that @samp{%d} is replaced by the image number.
  121. @file{img%03d.jpg} means the sequence @file{img001.jpg}, @file{img002.jpg}, etc.
  122. Use the @option{-start_number} option to declare a starting number for
  123. the sequence. This is useful if your sequence does not start with
  124. @file{img001.jpg} but is still in a numerical order. The following
  125. example will start with @file{img100.jpg}:
  126. @example
  127. ffmpeg -f image2 -start_number 100 -i img%d.jpg /tmp/a.mpg
  128. @end example
  129. If you have large number of pictures to rename, you can use the
  130. following command to ease the burden. The command, using the bourne
  131. shell syntax, symbolically links all files in the current directory
  132. that match @code{*jpg} to the @file{/tmp} directory in the sequence of
  133. @file{img001.jpg}, @file{img002.jpg} and so on.
  134. @example
  135. x=1; for i in *jpg; do counter=$(printf %03d $x); ln -s "$i" /tmp/img"$counter".jpg; x=$(($x+1)); done
  136. @end example
  137. If you want to sequence them by oldest modified first, substitute
  138. @code{$(ls -r -t *jpg)} in place of @code{*jpg}.
  139. Then run:
  140. @example
  141. ffmpeg -f image2 -i /tmp/img%03d.jpg /tmp/a.mpg
  142. @end example
  143. The same logic is used for any image format that ffmpeg reads.
  144. You can also use @command{cat} to pipe images to ffmpeg:
  145. @example
  146. cat *.jpg | ffmpeg -f image2pipe -c:v mjpeg -i - output.mpg
  147. @end example
  148. @section How do I encode movie to single pictures?
  149. Use:
  150. @example
  151. ffmpeg -i movie.mpg movie%d.jpg
  152. @end example
  153. The @file{movie.mpg} used as input will be converted to
  154. @file{movie1.jpg}, @file{movie2.jpg}, etc...
  155. Instead of relying on file format self-recognition, you may also use
  156. @table @option
  157. @item -c:v ppm
  158. @item -c:v png
  159. @item -c:v mjpeg
  160. @end table
  161. to force the encoding.
  162. Applying that to the previous example:
  163. @example
  164. ffmpeg -i movie.mpg -f image2 -c:v mjpeg menu%d.jpg
  165. @end example
  166. Beware that there is no "jpeg" codec. Use "mjpeg" instead.
  167. @section Why do I see a slight quality degradation with multithreaded MPEG* encoding?
  168. For multithreaded MPEG* encoding, the encoded slices must be independent,
  169. otherwise thread n would practically have to wait for n-1 to finish, so it's
  170. quite logical that there is a small reduction of quality. This is not a bug.
  171. @section How can I read from the standard input or write to the standard output?
  172. Use @file{-} as file name.
  173. @section -f jpeg doesn't work.
  174. Try '-f image2 test%d.jpg'.
  175. @section Why can I not change the frame rate?
  176. Some codecs, like MPEG-1/2, only allow a small number of fixed frame rates.
  177. Choose a different codec with the -c:v command line option.
  178. @section How do I encode Xvid or DivX video with ffmpeg?
  179. Both Xvid and DivX (version 4+) are implementations of the ISO MPEG-4
  180. standard (note that there are many other coding formats that use this
  181. same standard). Thus, use '-c:v mpeg4' to encode in these formats. The
  182. default fourcc stored in an MPEG-4-coded file will be 'FMP4'. If you want
  183. a different fourcc, use the '-vtag' option. E.g., '-vtag xvid' will
  184. force the fourcc 'xvid' to be stored as the video fourcc rather than the
  185. default.
  186. @section Which are good parameters for encoding high quality MPEG-4?
  187. '-mbd rd -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -g 300 -pass 1/2',
  188. things to try: '-bf 2', '-mpv_flags qp_rd', '-mpv_flags mv0', '-mpv_flags skip_rd'.
  189. @section Which are good parameters for encoding high quality MPEG-1/MPEG-2?
  190. '-mbd rd -trellis 2 -cmp 2 -subcmp 2 -g 100 -pass 1/2'
  191. but beware the '-g 100' might cause problems with some decoders.
  192. Things to try: '-bf 2', '-mpv_flags qp_rd', '-mpv_flags mv0', '-mpv_flags skip_rd'.
  193. @section Interlaced video looks very bad when encoded with ffmpeg, what is wrong?
  194. You should use '-flags +ilme+ildct' and maybe '-flags +alt' for interlaced
  195. material, and try '-top 0/1' if the result looks really messed-up.
  196. @section How can I read DirectShow files?
  197. If you have built FFmpeg with @code{./configure --enable-avisynth}
  198. (only possible on MinGW/Cygwin platforms),
  199. then you may use any file that DirectShow can read as input.
  200. Just create an "input.avs" text file with this single line ...
  201. @example
  202. DirectShowSource("C:\path to your file\yourfile.asf")
  203. @end example
  204. ... and then feed that text file to ffmpeg:
  205. @example
  206. ffmpeg -i input.avs
  207. @end example
  208. For ANY other help on AviSynth, please visit the
  209. @uref{http://www.avisynth.org/, AviSynth homepage}.
  210. @section How can I join video files?
  211. To "join" video files is quite ambiguous. The following list explains the
  212. different kinds of "joining" and points out how those are addressed in
  213. FFmpeg. To join video files may mean:
  214. @itemize
  215. @item
  216. To put them one after the other: this is called to @emph{concatenate} them
  217. (in short: concat) and is addressed
  218. @ref{How can I concatenate video files, in this very faq}.
  219. @item
  220. To put them together in the same file, to let the user choose between the
  221. different versions (example: different audio languages): this is called to
  222. @emph{multiplex} them together (in short: mux), and is done by simply
  223. invoking ffmpeg with several @option{-i} options.
  224. @item
  225. For audio, to put all channels together in a single stream (example: two
  226. mono streams into one stereo stream): this is sometimes called to
  227. @emph{merge} them, and can be done using the
  228. @url{ffmpeg-filters.html#amerge, @code{amerge}} filter.
  229. @item
  230. For audio, to play one on top of the other: this is called to @emph{mix}
  231. them, and can be done by first merging them into a single stream and then
  232. using the @url{ffmpeg-filters.html#pan, @code{pan}} filter to mix
  233. the channels at will.
  234. @item
  235. For video, to display both together, side by side or one on top of a part of
  236. the other; it can be done using the
  237. @url{ffmpeg-filters.html#overlay, @code{overlay}} video filter.
  238. @end itemize
  239. @anchor{How can I concatenate video files}
  240. @section How can I concatenate video files?
  241. There are several solutions, depending on the exact circumstances.
  242. @subsection Concatenating using the concat @emph{filter}
  243. FFmpeg has a @url{ffmpeg-filters.html#concat,
  244. @code{concat}} filter designed specifically for that, with examples in the
  245. documentation. This operation is recommended if you need to re-encode.
  246. @subsection Concatenating using the concat @emph{demuxer}
  247. FFmpeg has a @url{ffmpeg-formats.html#concat,
  248. @code{concat}} demuxer which you can use when you want to avoid a re-encode and
  249. your format doesn't support file level concatenation.
  250. @subsection Concatenating using the concat @emph{protocol} (file level)
  251. FFmpeg has a @url{ffmpeg-protocols.html#concat,
  252. @code{concat}} protocol designed specifically for that, with examples in the
  253. documentation.
  254. A few multimedia containers (MPEG-1, MPEG-2 PS, DV) allow one to concatenate
  255. video by merely concatenating the files containing them.
  256. Hence you may concatenate your multimedia files by first transcoding them to
  257. these privileged formats, then using the humble @code{cat} command (or the
  258. equally humble @code{copy} under Windows), and finally transcoding back to your
  259. format of choice.
  260. @example
  261. ffmpeg -i input1.avi -qscale:v 1 intermediate1.mpg
  262. ffmpeg -i input2.avi -qscale:v 1 intermediate2.mpg
  263. cat intermediate1.mpg intermediate2.mpg > intermediate_all.mpg
  264. ffmpeg -i intermediate_all.mpg -qscale:v 2 output.avi
  265. @end example
  266. Additionally, you can use the @code{concat} protocol instead of @code{cat} or
  267. @code{copy} which will avoid creation of a potentially huge intermediate file.
  268. @example
  269. ffmpeg -i input1.avi -qscale:v 1 intermediate1.mpg
  270. ffmpeg -i input2.avi -qscale:v 1 intermediate2.mpg
  271. ffmpeg -i concat:"intermediate1.mpg|intermediate2.mpg" -c copy intermediate_all.mpg
  272. ffmpeg -i intermediate_all.mpg -qscale:v 2 output.avi
  273. @end example
  274. Note that you may need to escape the character "|" which is special for many
  275. shells.
  276. Another option is usage of named pipes, should your platform support it:
  277. @example
  278. mkfifo intermediate1.mpg
  279. mkfifo intermediate2.mpg
  280. ffmpeg -i input1.avi -qscale:v 1 -y intermediate1.mpg < /dev/null &
  281. ffmpeg -i input2.avi -qscale:v 1 -y intermediate2.mpg < /dev/null &
  282. cat intermediate1.mpg intermediate2.mpg |\
  283. ffmpeg -f mpeg -i - -c:v mpeg4 -c:a libmp3lame output.avi
  284. @end example
  285. @subsection Concatenating using raw audio and video
  286. Similarly, the yuv4mpegpipe format, and the raw video, raw audio codecs also
  287. allow concatenation, and the transcoding step is almost lossless.
  288. When using multiple yuv4mpegpipe(s), the first line needs to be discarded
  289. from all but the first stream. This can be accomplished by piping through
  290. @code{tail} as seen below. Note that when piping through @code{tail} you
  291. must use command grouping, @code{@{ ;@}}, to background properly.
  292. For example, let's say we want to concatenate two FLV files into an
  293. output.flv file:
  294. @example
  295. mkfifo temp1.a
  296. mkfifo temp1.v
  297. mkfifo temp2.a
  298. mkfifo temp2.v
  299. mkfifo all.a
  300. mkfifo all.v
  301. ffmpeg -i input1.flv -vn -f u16le -c:a pcm_s16le -ac 2 -ar 44100 - > temp1.a < /dev/null &
  302. ffmpeg -i input2.flv -vn -f u16le -c:a pcm_s16le -ac 2 -ar 44100 - > temp2.a < /dev/null &
  303. ffmpeg -i input1.flv -an -f yuv4mpegpipe - > temp1.v < /dev/null &
  304. @{ ffmpeg -i input2.flv -an -f yuv4mpegpipe - < /dev/null | tail -n +2 > temp2.v ; @} &
  305. cat temp1.a temp2.a > all.a &
  306. cat temp1.v temp2.v > all.v &
  307. ffmpeg -f u16le -c:a pcm_s16le -ac 2 -ar 44100 -i all.a \
  308. -f yuv4mpegpipe -i all.v \
  309. -y output.flv
  310. rm temp[12].[av] all.[av]
  311. @end example
  312. @section Using @option{-f lavfi}, audio becomes mono for no apparent reason.
  313. Use @option{-dumpgraph -} to find out exactly where the channel layout is
  314. lost.
  315. Most likely, it is through @code{auto-inserted aresample}. Try to understand
  316. why the converting filter was needed at that place.
  317. Just before the output is a likely place, as @option{-f lavfi} currently
  318. only support packed S16.
  319. Then insert the correct @code{aformat} explicitly in the filtergraph,
  320. specifying the exact format.
  321. @example
  322. aformat=sample_fmts=s16:channel_layouts=stereo
  323. @end example
  324. @section Why does FFmpeg not see the subtitles in my VOB file?
  325. VOB and a few other formats do not have a global header that describes
  326. everything present in the file. Instead, applications are supposed to scan
  327. the file to see what it contains. Since VOB files are frequently large, only
  328. the beginning is scanned. If the subtitles happen only later in the file,
  329. they will not be initially detected.
  330. Some applications, including the @code{ffmpeg} command-line tool, can only
  331. work with streams that were detected during the initial scan; streams that
  332. are detected later are ignored.
  333. The size of the initial scan is controlled by two options: @code{probesize}
  334. (default ~5 Mo) and @code{analyzeduration} (default 5,000,000 µs = 5 s). For
  335. the subtitle stream to be detected, both values must be large enough.
  336. @section Why was the @command{ffmpeg} @option{-sameq} option removed? What to use instead?
  337. The @option{-sameq} option meant "same quantizer", and made sense only in a
  338. very limited set of cases. Unfortunately, a lot of people mistook it for
  339. "same quality" and used it in places where it did not make sense: it had
  340. roughly the expected visible effect, but achieved it in a very inefficient
  341. way.
  342. Each encoder has its own set of options to set the quality-vs-size balance,
  343. use the options for the encoder you are using to set the quality level to a
  344. point acceptable for your tastes. The most common options to do that are
  345. @option{-qscale} and @option{-qmax}, but you should peruse the documentation
  346. of the encoder you chose.
  347. @section I have a stretched video, why does scaling does not fix it?
  348. A lot of video codecs and formats can store the @emph{aspect ratio} of the
  349. video: this is the ratio between the width and the height of either the full
  350. image (DAR, display aspect ratio) or individual pixels (SAR, sample aspect
  351. ratio). For example, EGA screens at resolution 640×350 had 4:3 DAR and 35:48
  352. SAR.
  353. Most still image processing work with square pixels, i.e. 1:1 SAR, but a lot
  354. of video standards, especially from the analogic-numeric transition era, use
  355. non-square pixels.
  356. Most processing filters in FFmpeg handle the aspect ratio to avoid
  357. stretching the image: cropping adjusts the DAR to keep the SAR constant,
  358. scaling adjusts the SAR to keep the DAR constant.
  359. If you want to stretch, or “unstretch”, the image, you need to override the
  360. information with the
  361. @url{ffmpeg-filters.html#setdar_002c-setsar, @code{setdar or setsar filters}}.
  362. Do not forget to examine carefully the original video to check whether the
  363. stretching comes from the image or from the aspect ratio information.
  364. For example, to fix a badly encoded EGA capture, use the following commands,
  365. either the first one to upscale to square pixels or the second one to set
  366. the correct aspect ratio or the third one to avoid transcoding (may not work
  367. depending on the format / codec / player / phase of the moon):
  368. @example
  369. ffmpeg -i ega_screen.nut -vf scale=640:480,setsar=1 ega_screen_scaled.nut
  370. ffmpeg -i ega_screen.nut -vf setdar=4/3 ega_screen_anamorphic.nut
  371. ffmpeg -i ega_screen.nut -aspect 4/3 -c copy ega_screen_overridden.nut
  372. @end example
  373. @anchor{background task}
  374. @section How do I run ffmpeg as a background task?
  375. ffmpeg normally checks the console input, for entries like "q" to stop
  376. and "?" to give help, while performing operations. ffmpeg does not have a way of
  377. detecting when it is running as a background task.
  378. When it checks the console input, that can cause the process running ffmpeg
  379. in the background to suspend.
  380. To prevent those input checks, allowing ffmpeg to run as a background task,
  381. use the @url{ffmpeg.html#stdin-option, @code{-nostdin} option}
  382. in the ffmpeg invocation. This is effective whether you run ffmpeg in a shell
  383. or invoke ffmpeg in its own process via an operating system API.
  384. As an alternative, when you are running ffmpeg in a shell, you can redirect
  385. standard input to @code{/dev/null} (on Linux and macOS)
  386. or @code{NUL} (on Windows). You can do this redirect either
  387. on the ffmpeg invocation, or from a shell script which calls ffmpeg.
  388. For example:
  389. @example
  390. ffmpeg -nostdin -i INPUT OUTPUT
  391. @end example
  392. or (on Linux, macOS, and other UNIX-like shells):
  393. @example
  394. ffmpeg -i INPUT OUTPUT </dev/null
  395. @end example
  396. or (on Windows):
  397. @example
  398. ffmpeg -i INPUT OUTPUT <NUL
  399. @end example
  400. @section How do I prevent ffmpeg from suspending with a message like @emph{suspended (tty output)}?
  401. If you run ffmpeg in the background, you may find that its process suspends.
  402. There may be a message like @emph{suspended (tty output)}. The question is how
  403. to prevent the process from being suspended.
  404. For example:
  405. @example
  406. % ffmpeg -i INPUT OUTPUT &> ~/tmp/log.txt &
  407. [1] 93352
  408. %
  409. [1] + suspended (tty output) ffmpeg -i INPUT OUTPUT &>
  410. @end example
  411. The message "tty output" notwithstanding, the problem here is that
  412. ffmpeg normally checks the console input when it runs. The operating system
  413. detects this, and suspends the process until you can bring it to the
  414. foreground and attend to it.
  415. The solution is to use the right techniques to tell ffmpeg not to consult
  416. console input. You can use the
  417. @url{ffmpeg.html#stdin-option, @code{-nostdin} option},
  418. or redirect standard input with @code{< /dev/null}.
  419. See FAQ
  420. @ref{background task, @emph{How do I run ffmpeg as a background task?}}
  421. for details.
  422. @chapter Development
  423. @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
  424. Yes. Check the @file{doc/examples} directory in the source
  425. repository, also available online at:
  426. @url{https://github.com/FFmpeg/FFmpeg/tree/master/doc/examples}.
  427. Examples are also installed by default, usually in
  428. @code{$PREFIX/share/ffmpeg/examples}.
  429. Also you may read the Developers Guide of the FFmpeg documentation. Alternatively,
  430. examine the source code for one of the many open source projects that
  431. already incorporate FFmpeg at (@url{projects.html}).
  432. @section Can you support my C compiler XXX?
  433. It depends. If your compiler is C99-compliant, then patches to support
  434. it are likely to be welcome if they do not pollute the source code
  435. with @code{#ifdef}s related to the compiler.
  436. @section Is Microsoft Visual C++ supported?
  437. Yes. Please see the @uref{platform.html, Microsoft Visual C++}
  438. section in the FFmpeg documentation.
  439. @section Can you add automake, libtool or autoconf support?
  440. No. These tools are too bloated and they complicate the build.
  441. @section Why not rewrite FFmpeg in object-oriented C++?
  442. FFmpeg is already organized in a highly modular manner and does not need to
  443. be rewritten in a formal object language. Further, many of the developers
  444. favor straight C; it works for them. For more arguments on this matter,
  445. read @uref{https://web.archive.org/web/20111004021423/http://kernel.org/pub/linux/docs/lkml/#s15, "Programming Religion"}.
  446. @section Why are the ffmpeg programs devoid of debugging symbols?
  447. The build process creates @command{ffmpeg_g}, @command{ffplay_g}, etc. which
  448. contain full debug information. Those binaries are stripped to create
  449. @command{ffmpeg}, @command{ffplay}, etc. If you need the debug information, use
  450. the *_g versions.
  451. @section I do not like the LGPL, can I contribute code under the GPL instead?
  452. Yes, as long as the code is optional and can easily and cleanly be placed
  453. under #if CONFIG_GPL without breaking anything. So, for example, a new codec
  454. or filter would be OK under GPL while a bug fix to LGPL code would not.
  455. @section I'm using FFmpeg from within my C application but the linker complains about missing symbols from the libraries themselves.
  456. FFmpeg builds static libraries by default. In static libraries, dependencies
  457. are not handled. That has two consequences. First, you must specify the
  458. libraries in dependency order: @code{-lavdevice} must come before
  459. @code{-lavformat}, @code{-lavutil} must come after everything else, etc.
  460. Second, external libraries that are used in FFmpeg have to be specified too.
  461. An easy way to get the full list of required libraries in dependency order
  462. is to use @code{pkg-config}.
  463. @example
  464. c99 -o program program.c $(pkg-config --cflags --libs libavformat libavcodec)
  465. @end example
  466. See @file{doc/example/Makefile} and @file{doc/example/pc-uninstalled} for
  467. more details.
  468. @section I'm using FFmpeg from within my C++ application but the linker complains about missing symbols which seem to be available.
  469. FFmpeg is a pure C project, so to use the libraries within your C++ application
  470. you need to explicitly state that you are using a C library. You can do this by
  471. encompassing your FFmpeg includes using @code{extern "C"}.
  472. See @url{http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3}
  473. @section I'm using libavutil from within my C++ application but the compiler complains about 'UINT64_C' was not declared in this scope
  474. FFmpeg is a pure C project using C99 math features, in order to enable C++
  475. to use them you have to append -D__STDC_CONSTANT_MACROS to your CXXFLAGS
  476. @section I have a file in memory / a API different from *open/*read/ libc how do I use it with libavformat?
  477. You have to create a custom AVIOContext using @code{avio_alloc_context},
  478. see @file{libavformat/aviobuf.c} in FFmpeg and @file{libmpdemux/demux_lavf.c} in MPlayer or MPlayer2 sources.
  479. @section Where is the documentation about ffv1, msmpeg4, asv1, 4xm?
  480. see @url{https://www.ffmpeg.org/~michael/}
  481. @section How do I feed H.263-RTP (and other codecs in RTP) to libavcodec?
  482. Even if peculiar since it is network oriented, RTP is a container like any
  483. other. You have to @emph{demux} RTP before feeding the payload to libavcodec.
  484. In this specific case please look at RFC 4629 to see how it should be done.
  485. @section AVStream.r_frame_rate is wrong, it is much larger than the frame rate.
  486. @code{r_frame_rate} is NOT the average frame rate, it is the smallest frame rate
  487. that can accurately represent all timestamps. So no, it is not
  488. wrong if it is larger than the average!
  489. For example, if you have mixed 25 and 30 fps content, then @code{r_frame_rate}
  490. will be 150 (it is the least common multiple).
  491. If you are looking for the average frame rate, see @code{AVStream.avg_frame_rate}.
  492. @section Why is @code{make fate} not running all tests?
  493. Make sure you have the fate-suite samples and the @code{SAMPLES} Make variable
  494. or @code{FATE_SAMPLES} environment variable or the @code{--samples}
  495. @command{configure} option is set to the right path.
  496. @section Why is @code{make fate} not finding the samples?
  497. Do you happen to have a @code{~} character in the samples path to indicate a
  498. home directory? The value is used in ways where the shell cannot expand it,
  499. causing FATE to not find files. Just replace @code{~} by the full path.
  500. @bye