Never assume the API of libav* to be stable unless at least 1 week has passed since
the last major version increase.
The last version increases were:
libavcodec:  ?
libavdevice: ?
libavfilter: 2009-10-18
libavformat: ?
libpostproc: ?
libswscale:  ?
libavutil:   2009-03-08


API changes, most recent first:

2010-04-01 - r22806 - lavf 52.61.0 - metadata API
  Add a flag for av_metadata_set2() to disable overwriting of
  existing tags.

2010-04-01 - r22753 - lavc 52.66.0
  Add avcodec_get_edge_width()

2010-03-31 - r22750 - lavc 52.65.0
  Add avcodec_copy_context().

2010-03-31 - r22748 - lavf 52.60.0 - av_match_ext()
  Make av_match_ext() public.

2010-03-31 - r22736 - lavu 50.14.0 - AVMediaType
  Move AVMediaType enum from libavcodec to libavutil.

2010-03-31 - r22735 - lavc 52.64.0 - AVMediaType
  Define AVMediaType enum, and use it instead of enum CodecType, which
  is deprecated and will be dropped at the next major bump.

2010-03-25 - r22684 - lavu 50.13.0 - av_strerror()
  Implement av_strerror().

2010-03-23 - r22649 - lavc 52.60.0 - av_dct_init()
  Support DCT-I and DST-I

2010-03-15 - r22540 - lavf 52.56.0 - AVFormatContext.start_time_realtime
  Add AVFormatContext.start_time_realtime field.

2010-03-13 - r22506 - lavfi 1.18.0 - AVFilterPicRef.pos
  Add AVFilterPicRef.pos field.

2010-03-13 - r22501 - lavu 50.12.0 - error.h
  Move error code definitions from libavcodec/avcodec.h to
  the new public header libavutil/error.h.

2010-03-07 - r22291 - lavc 52.56.0 - avfft.h
  Add public FFT interface.

2010-03-06 - r22251 - lavu 50.11.0 - av_stristr()
  Add av_stristr().

2010-03-03 - r22174 - lavu 50.10.0 - av_tree_enumerate()
  Add av_tree_enumerate().

2010-02-07 - r21673 - lavu 50.9.0 - av_compare_ts()
  Add av_compare_ts().

2010-02-05 - r30513 - lsws 0.10.0 - sws_getCoefficients()
  Add sws_getCoefficients().

2010-02-01 - r21587 - lavf 52.50.0 - metadata API
  Add a list of generic tag names, change 'author' -> 'artist',
  'year' -> 'date'.

2010-01-30 - r21545 - lavu 50.8.0 - av_get_pix_fmt()
  Add av_get_pix_fmt().

2010-01-21 - r30381 - lsws 0.9.0 - sws_scale
  Change constness attributes of sws_scale() parameters.

2010-01-10 - r21121 - lavfi 1.15.0 - avfilter_graph_config_links()
  Add a log_ctx parameter to avfilter_graph_config_links().

2010-01-07 - r30236 - lsws 0.8.0 - sws_isSupported{In,Out}put
  Add sws_isSupportedInput() and sws_isSupportedOutput() functions.

2010-01-06 - r21035 - lavfi 1.14.0 - avfilter_add_colorspace()
  Change the avfilter_add_colorspace() signature, make it accept an
  (AVFilterFormats **) rather than an (AVFilterFormats *) as before.

2010-01-03 - r21007 - lavfi 1.13.0 - avfilter_add_colorspace()
  Add avfilter_add_colorspace().

2010-01-02 - r20998 - lavf 52.46.0 - av_match_ext()
  Add av_match_ext(), it should be used in place of match_ext().

2010-01-01 - r20991 - lavf 52.45.0 - av_guess_format()
  Add av_guess_format(), it should be used in place of guess_format().

2009-12-13 - r20834 - lavf 52.43.0 - metadata API
  Add av_metadata_set2(), AV_METADATA_DONT_STRDUP_KEY and AV_METADATA_DONT_STRDUP_VAL.

2009-12-13 - r20829 - lavu 50.7.0 - avstring.h API
  Add av_d2str().

2009-12-13 - r20826 - lavc 52.42.0 - AVStream
  Add avg_frame_rate.

2009-12-12 - r20808 - lavu 50.6.0 - av_bmg_next()
  Introduce the av_bmg_next() function.

2009-12-05 - r20734 - lavfi 1.12.0 - avfilter_draw_slice()
  Add a slice_dir parameter to avfilter_draw_slice().

2009-11-26 - r20611 - lavfi 1.11.0 - AVFilter
  Remove the next field from AVFilter, this is not anymore required.

2009-11-25 - r20607 - lavfi 1.10.0 - avfilter_next()
  Introduce the avfilter_next() function.

2009-11-25 - r20605 - lavfi 1.9.0 - avfilter_register()
  Change the signature of avfilter_register() to make it return an
  int. This is required since now the registration operation may fail.

2009-11-25 - r20603 - lavu 50.5.0 - pixdesc.h API
  Make the pixdesc.h API public.

2009-10-27 - r20385 - lavfi 1.5.0 - AVFilter.next
  Add a next field to AVFilter, this is used for simplifying the
  registration and management of the registered filters.

2009-10-23 - r20356 - lavfi 1.4.1 - AVFilter.description
  Add a description field to AVFilter.

2009-10-19 - r20302 - lavfi 1.3.0 - avfilter_make_format_list()
  Change the interface of avfilter_make_format_list() from
  avfilter_make_format_list(int n, ...) to
  avfilter_make_format_list(enum PixelFormat *pix_fmts).

2009-10-18 - r20272 - lavfi 1.0.0 - avfilter_get_video_buffer()
  Make avfilter_get_video_buffer() recursive and add the w and h
  parameters to it.

2009-10-07 - r20189 - lavfi 0.5.1 - AVFilterPic
  Add w and h fields to AVFilterPic.

2009-06-22 - r19250 - lavf 52.34.1 - AVFormatContext.packet_size
  This is now an unsigned int instead of a signed int.

2009-06-19 - r19222 - lavc 52.32.0 - AVSubtitle.pts
  Add a pts field to AVSubtitle which gives the subtitle packet pts
  in AV_TIME_BASE. Some subtitle de-/encoders (e.g. XSUB) will
  not work right without this.

2009-06-03 - r19078 - lavc 52.30.2 - AV_PKT_FLAG_KEY
  PKT_FLAG_KEY has been deprecated and will be dropped at the next
  major version. Use AV_PKT_FLAG_KEY instead.

2009-06-01 - r19025 - lavc 52.30.0 - av_lockmgr_register()
  av_lockmgr_register() can be used to register a callback function
  that lavc (and in the future, libraries that depend on lavc) can use
  to implement mutexes. The application should provide a callback function
  that implements the AV_LOCK_* operations described in avcodec.h.
  When the lock manager is registered, FFmpeg is guaranteed to behave
  correctly in a multi-threaded application.

2009-04-30 - r18719 - lavc 52.28.0 - av_free_packet
  av_free_packet() is no longer an inline function. It is now exported.

2009-04-11 - r18431 - lavc 52.25.0 - deprecate av_destruct_packet_nofree
  Please use NULL instead. This has been supported since r16506
  (lavf > 52.23.1, lavc > 52.10.0).

2009-04-07 - r18351 - lavc 52.23.0 - avcodec_decode_video/audio/subtitle
  The old decoding functions are deprecated, all new code should use the
  new functions avcodec_decode_video2(), avcodec_decode_audio3() and
  avcodec_decode_subtitle2(). These new functions take an AVPacket *pkt
  argument instead of a const uint8_t *buf / int buf_size pair.

2009-04-03 - r18321 - lavu 50.3.0 - av_fifo_space
  Introduce the av_fifo_space() function.

2009-04-02 - r18317 - lavc 52.23.0 - AVPacket
  Move AVPacket declaration from libavformat/avformat.h to
  libavcodec/avcodec.h.

2009-03-22 - r18163 - lavu 50.2.0 - RGB32 pixel formats
  Convert the pixel formats PIX_FMT_ARGB, PIX_FMT_RGBA, PIX_FMT_ABGR,
  PIX_FMT_BGRA, which were defined as macros, into enum PixelFormat values.
  Conversely PIX_FMT_RGB32, PIX_FMT_RGB32_1, PIX_FMT_BGR32 and
  PIX_FMT_BGR32_1 are now macros.
  avcodec_get_pix_fmt() now recognizes the "rgb32" and "bgr32" aliases.
  Re-sort the enum PixelFormat list accordingly.
  This change breaks API/ABI backward compatibility.

2009-03-22 - r18133 - lavu 50.1.0 - PIX_FMT_RGB5X5 endian variants
  Add the enum PixelFormat values:
  PIX_FMT_RGB565BE, PIX_FMT_RGB565LE, PIX_FMT_RGB555BE, PIX_FMT_RGB555LE,
  PIX_FMT_BGR565BE, PIX_FMT_BGR565LE, PIX_FMT_BGR555BE, PIX_FMT_BGR555LE.

2009-03-21 - r18116 - lavu 50.0.0  - av_random*
  The Mersenne Twister PRNG implemented through the av_random* functions
  was removed. Use the lagged Fibonacci PRNG through the av_lfg* functions
  instead.

2009-03-08 - r17869 - lavu 50.0.0  - AVFifoBuffer
  av_fifo_init, av_fifo_read, av_fifo_write and av_fifo_realloc were dropped
  and replaced by av_fifo_alloc, av_fifo_generic_read, av_fifo_generic_write
  and av_fifo_realloc2.
  In addition, the order of the function arguments of av_fifo_generic_read
  was changed to match av_fifo_generic_write.
  The AVFifoBuffer/struct AVFifoBuffer may only be used in an opaque way by
  applications, they may not use sizeof() or directly access members.

2009-03-01 - r17682 - lavf 52.31.0 - Generic metadata API
  Introduce a new metadata API (see av_metadata_get() and friends).
  The old API is now deprecated and should not be used anymore. This especially
  includes the following structure fields:
    - AVFormatContext.title
    - AVFormatContext.author
    - AVFormatContext.copyright
    - AVFormatContext.comment
    - AVFormatContext.album
    - AVFormatContext.year
    - AVFormatContext.track
    - AVFormatContext.genre
    - AVStream.language
    - AVStream.filename
    - AVProgram.provider_name
    - AVProgram.name
    - AVChapter.title