FFmpeg 编码详细流程

介绍

1. FFmpeg的 libavcodec 模块完成音视频多媒体的编解码模块。
2. FFmpeg 本身不具有音视频编码的功能和底层能力，只是对各类第三方的编码器API 进行封装调用。
3. 老版本的 FFmpeg 将avcodec_encode_video2()作为视频的解码函数 API，将avcodec_encode_audio2()作为音频的解码函数 API；从 3.4版本开始已经将二者标记为废弃过时 API（attribute_deprecated）。
4. 新版本 FFmpeg 将 avcodec_send_frame() 与 avcodec_receive_packet() 作为音视频的解码函数 API，但同时仍然保留了对老接口的兼容，通过avcodec_encode_video2()、avcodec_encode_audio2()调用 compat_decode()完成对新 API 的封装。

//具体可以参考 FFmpeg 中 doc/APIchanges 中的记录.

2016-04-21 - 7fc329e - lavc 57.37.100 - avcodec.h
  Add a new audio/video encoding and decoding API with decoupled input
  and output -- avcodec_send_packet(), avcodec_receive_frame(),
  avcodec_send_frame() and avcodec_receive_packet().

4. 可以通过命令行ffmpeg configure -encoders查看支持哪些编码器格式；但是好像configure 里说明的和源码不完全一致，源码里有支持 kvazaar 编码器（一种符合h265编码标准的开源编码器），但 configure 并没有体现出来。

Encoders:
 V..... = Video
 A..... = Audio
 S..... = Subtitle
 .F.... = Frame-level multithreading
 ..S... = Slice-level multithreading
 ...X.. = Codec is experimental
 ....B. = Supports draw_horiz_band
 .....D = Supports direct rendering method 1
 ------
 V....D a64multi             Multicolor charset for Commodore 64 (codec a64_multi)
 V....D a64multi5            Multicolor charset for Commodore 64, extended with 5th color (colram) (codec a64_multi5)
 V....D alias_pix            Alias/Wavefront PIX image
 V..... amv                  AMV Video
 V....D apng                 APNG (Animated Portable Network Graphics) image
 V....D asv1                 ASUS V1
 V....D asv2                 ASUS V2
 V....D libaom-av1           libaom AV1 (codec av1)
 V....D librav1e             librav1e AV1 (codec av1)
 V..... libsvtav1            SVT-AV1(Scalable Video Technology for AV1) encoder (codec av1)
 V....D avrp                 Avid 1:1 10-bit RGB Packer
 V..X.D avui                 Avid Meridien Uncompressed
 V....D ayuv                 Uncompressed packed MS 4:4:4:4
 VF...D bitpacked            Bitpacked
 V....D bmp                  BMP (Windows and OS/2 bitmap)
 VF...D cfhd                 GoPro CineForm HD
 V....D cinepak              Cinepak
 V....D cljr                 Cirrus Logic AccuPak
 V.S..D vc2                  SMPTE VC-2 (codec dirac)
 VFS..D dnxhd                VC3/DNxHD
 V....D dpx                  DPX (Digital Picture Exchange) image
 VFS..D dvvideo              DV (Digital Video)
 VF...D exr                  OpenEXR image
 V.S..D ffv1                 FFmpeg video codec #1
 VF...D ffvhuff              Huffyuv FFmpeg variant
 V....D fits                 Flexible Image Transport System
 V....D flashsv              Flash Screen Video
 V....D flashsv2             Flash Screen Video Version 2
 V..... flv                  FLV / Sorenson Spark / Sorenson H.263 (Flash Video) (codec flv1)
 V....D gif                  GIF (Graphics Interchange Format)
 V..... h261                 H.261
 V..... h263                 H.263 / H.263-1996
 V.S... h263p                H.263+ / H.263-1998 / H.263 version 2
 V....D libx264              libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
 V....D libx264rgb           libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 RGB (codec h264)
 V....D h264_videotoolbox    VideoToolbox H.264 Encoder (codec h264)
 V.S..D hap                  Vidvox Hap
 VF...D hdr                  HDR (Radiance RGBE format) image
 V....D libx265              libx265 H.265 / HEVC (codec hevc)
 V....D hevc_videotoolbox    VideoToolbox H.265 Encoder (codec hevc)
 VF...D huffyuv              Huffyuv / HuffYUV
 V....D jpeg2000             JPEG 2000
 VF.... libopenjpeg          OpenJPEG JPEG 2000 (codec jpeg2000)
 VF...D jpegls               JPEG-LS
 VF...D ljpeg                Lossless JPEG
 VF...D magicyuv             MagicYUV video
 VFS... mjpeg                MJPEG (Motion JPEG)
 V.S... mpeg1video           MPEG-1 video
 V.S... mpeg2video           MPEG-2 video
 V.S... mpeg4                MPEG-4 part 2
 V....D libxvid              libxvidcore MPEG-4 part 2 (codec mpeg4)
 V..... msmpeg4v2            MPEG-4 part 2 Microsoft variant version 2
 V..... msmpeg4              MPEG-4 part 2 Microsoft variant version 3 (codec msmpeg4v3)
 V..... msvideo1             Microsoft Video-1
 V....D pam                  PAM (Portable AnyMap) image
 V....D pbm                  PBM (Portable BitMap) image
 V....D pcx                  PC Paintbrush PCX image
 V....D pfm                  PFM (Portable FloatMap) image
 V....D pgm                  PGM (Portable GrayMap) image
 V....D pgmyuv               PGMYUV (Portable GrayMap YUV) image
 V....D phm                  PHM (Portable HalfFloatMap) image
 VF...D png                  PNG (Portable Network Graphics) image
 V....D ppm                  PPM (Portable PixelMap) image
 VF...D prores               Apple ProRes
 VF...D prores_aw            Apple ProRes (codec prores)
 VFS... prores_ks            Apple ProRes (iCodec Pro) (codec prores)
 V....D prores_videotoolbox  VideoToolbox ProRes Encoder (codec prores)
 VF...D qoi                  QOI (Quite OK Image format) image
 V....D qtrle                QuickTime Animation (RLE) video
 V....D r10k                 AJA Kona 10-bit RGB Codec
 V....D r210                 Uncompressed RGB 10-bit
 VF...D rawvideo             raw video
 V....D roqvideo             id RoQ video (codec roq)
 V....D rpza                 QuickTime video (RPZA)
 V..... rv10                 RealVideo 1.0
 V..... rv20                 RealVideo 2.0
 V....D sgi                  SGI image
 V....D smc                  QuickTime Graphics (SMC)
 V....D snow                 Snow
 V..... speedhq              NewTek SpeedHQ
 V....D sunrast              Sun Rasterfile image
 V....D svq1                 Sorenson Vector Quantizer 1 / Sorenson Video 1 / SVQ1
 V....D targa                Truevision Targa image
 V....D libtheora            libtheora Theora (codec theora)
 VF...D tiff                 TIFF image
 VF...D utvideo              Ut Video
 VF...D v210                 Uncompressed 4:2:2 10-bit
 V....D v308                 Uncompressed packed 4:4:4
 V....D v408                 Uncompressed packed QT 4:4:4:4
 V....D v410                 Uncompressed 4:4:4 10-bit
 V.S..D vbn                  Vizrt Binary Image
 V..... vnull                null video
 V....D libvpx               libvpx VP8 (codec vp8)
 V....D libvpx-vp9           libvpx VP9 (codec vp9)
 VF...D wbmp                 WBMP (Wireless Application Protocol Bitmap) image
 V....D libwebp_anim         libwebp WebP image (codec webp)
 V....D libwebp              libwebp WebP image (codec webp)
 V..... wmv1                 Windows Media Video 7
 V..... wmv2                 Windows Media Video 8
 V..... wrapped_avframe      AVFrame to AVPacket passthrough
 V....D xbm                  XBM (X BitMap) image
 V....D xface                X-face image
 V....D xwd                  XWD (X Window Dump) image
 V....D y41p                 Uncompressed YUV 4:1:1 12-bit
 V....D yuv4                 Uncompressed packed 4:2:0
 VF...D zlib                 LCL (LossLess Codec Library) ZLIB
 V....D zmbv                 Zip Motion Blocks Video
 A....D aac                  AAC (Advanced Audio Coding)
 A..... aac_at               aac (AudioToolbox) (codec aac)
 A....D ac3                  ATSC A/52A (AC-3)
 A....D ac3_fixed            ATSC A/52A (AC-3) (codec ac3)
 A....D adpcm_adx            SEGA CRI ADX ADPCM
 A....D adpcm_argo           ADPCM Argonaut Games
 A....D g722                 G.722 ADPCM (codec adpcm_g722)
 A....D g726                 G.726 ADPCM (codec adpcm_g726)
 A....D g726le               G.726 little endian ADPCM ("right-justified") (codec adpcm_g726le)
 A....D adpcm_ima_alp        ADPCM IMA High Voltage Software ALP
 A....D adpcm_ima_amv        ADPCM IMA AMV
 A....D adpcm_ima_apm        ADPCM IMA Ubisoft APM
 A....D adpcm_ima_qt         ADPCM IMA QuickTime
 A....D adpcm_ima_ssi        ADPCM IMA Simon & Schuster Interactive
 A....D adpcm_ima_wav        ADPCM IMA WAV
 A....D adpcm_ima_ws         ADPCM IMA Westwood
 A....D adpcm_ms             ADPCM Microsoft
 A....D adpcm_swf            ADPCM Shockwave Flash
 A....D adpcm_yamaha         ADPCM Yamaha
 A....D alac                 ALAC (Apple Lossless Audio Codec)
 A..... alac_at              alac (AudioToolbox) (codec alac)
 A....D libopencore_amrnb    OpenCORE AMR-NB (Adaptive Multi-Rate Narrow-Band) (codec amr_nb)
 A..... anull                null audio
 A....D aptx                 aptX (Audio Processing Technology for Bluetooth)
 A....D aptx_hd              aptX HD (Audio Processing Technology for Bluetooth)
 A....D comfortnoise         RFC 3389 comfort noise generator
 A....D dfpwm                DFPWM1a audio
 A..X.D dca                  DCA (DTS Coherent Acoustics) (codec dts)
 A....D eac3                 ATSC A/52 E-AC-3
 A....D flac                 FLAC (Free Lossless Audio Codec)
 A....D g723_1               G.723.1
 A..... ilbc_at              ilbc (AudioToolbox) (codec ilbc)
 A..X.D mlp                  MLP (Meridian Lossless Packing)
 A....D mp2                  MP2 (MPEG audio layer 2)
 A....D mp2fixed             MP2 fixed point (MPEG audio layer 2) (codec mp2)
 A....D libmp3lame           libmp3lame MP3 (MPEG audio layer 3) (codec mp3)
 A....D nellymoser           Nellymoser Asao
 A..X.D opus                 Opus
 A....D libopus              libopus Opus (codec opus)
 A....D pcm_alaw             PCM A-law / G.711 A-law
 A..... pcm_alaw_at          pcm_alaw (AudioToolbox) (codec pcm_alaw)
 A....D pcm_bluray           PCM signed 16|20|24-bit big-endian for Blu-ray media
 A....D pcm_dvd              PCM signed 16|20|24-bit big-endian for DVD media
 A....D pcm_f32be            PCM 32-bit floating point big-endian
 A....D pcm_f32le            PCM 32-bit floating point little-endian
 A....D pcm_f64be            PCM 64-bit floating point big-endian
 A....D pcm_f64le            PCM 64-bit floating point little-endian
 A....D pcm_mulaw            PCM mu-law / G.711 mu-law
 A..... pcm_mulaw_at         pcm_mulaw (AudioToolbox) (codec pcm_mulaw)
 A....D pcm_s16be            PCM signed 16-bit big-endian
 A....D pcm_s16be_planar     PCM signed 16-bit big-endian planar
 A....D pcm_s16le            PCM signed 16-bit little-endian
 A....D pcm_s16le_planar     PCM signed 16-bit little-endian planar
 A....D pcm_s24be            PCM signed 24-bit big-endian
 A....D pcm_s24daud          PCM D-Cinema audio signed 24-bit
 A....D pcm_s24le            PCM signed 24-bit little-endian
 A....D pcm_s24le_planar     PCM signed 24-bit little-endian planar
 A....D pcm_s32be            PCM signed 32-bit big-endian
 A....D pcm_s32le            PCM signed 32-bit little-endian
 A....D pcm_s32le_planar     PCM signed 32-bit little-endian planar
 A....D pcm_s64be            PCM signed 64-bit big-endian
 A....D pcm_s64le            PCM signed 64-bit little-endian
 A....D pcm_s8               PCM signed 8-bit
 A....D pcm_s8_planar        PCM signed 8-bit planar
 A....D pcm_u16be            PCM unsigned 16-bit big-endian
 A....D pcm_u16le            PCM unsigned 16-bit little-endian
 A....D pcm_u24be            PCM unsigned 24-bit big-endian
 A....D pcm_u24le            PCM unsigned 24-bit little-endian
 A....D pcm_u32be            PCM unsigned 32-bit big-endian
 A....D pcm_u32le            PCM unsigned 32-bit little-endian
 A....D pcm_u8               PCM unsigned 8-bit
 A....D pcm_vidc             PCM Archimedes VIDC
 A....D real_144             RealAudio 1.0 (14.4K) (codec ra_144)
 A....D roq_dpcm             id RoQ DPCM
 A..X.D s302m                SMPTE 302M
 A....D sbc                  SBC (low-complexity subband codec)
 A..X.D sonic                Sonic
 A..X.D sonicls              Sonic lossless
 A....D libspeex             libspeex Speex (codec speex)
 A..X.D truehd               TrueHD
 A....D tta                  TTA (True Audio)
 A..X.D vorbis               Vorbis
 A....D libvorbis            libvorbis (codec vorbis)
 A....D wavpack              WavPack
 A....D wmav1                Windows Media Audio 1
 A....D wmav2                Windows Media Audio 2
 S..... ssa                  ASS (Advanced SubStation Alpha) subtitle (codec ass)
 S..... ass                  ASS (Advanced SubStation Alpha) subtitle
 S..... dvbsub               DVB subtitles (codec dvb_subtitle)
 S..... dvdsub               DVD subtitles (codec dvd_subtitle)
 S..... mov_text             3GPP Timed Text subtitle
 S..... srt                  SubRip subtitle (codec subrip)
 S..... subrip               SubRip subtitle
 S..... text                 Raw text subtitle
 S..... ttml                 TTML subtitle
 S..... webvtt               WebVTT subtitle
 S..... xsub                 DivX subtitles (XSUB)

视频编码详细流程

1. 从流程可以看到，完成编码的核心函数就是 do_video_out()；里面实现编码的核心 API 就是 avcodec_send_frame() 和 avcodec_receive_packet() 两个函数；两个函数有点类似状态机，收到帧frame 到输出包 packet，通过判断 buffer 中是否有数据完成编码。
   

编码核心 API 介绍

avcodec_send_frame()

1. API申明介绍

/**
 * Supply a raw video or audio frame to the encoder. Use avcodec_receive_packet()
 * to retrieve buffered output packets.
 *
 * @param avctx     codec context
 * @param[in] frame AVFrame containing the raw audio or video frame to be encoded.
 *                  Ownership of the frame remains with the caller, and the
 *                  encoder will not write to the frame. The encoder may create
 *                  a reference to the frame data (or copy it if the frame is
 *                  not reference-counted).
 *                  It can be NULL, in which case it is considered a flush
 *                  packet.  This signals the end of the stream. If the encoder
 *                  still has packets buffered, it will return them after this
 *                  call. Once flushing mode has been entered, additional flush
 *                  packets are ignored, and sending frames will return
 *                  AVERROR_EOF.
 *
 *                  For audio:
 *                  If AV_CODEC_CAP_VARIABLE_FRAME_SIZE is set, then each frame
 *                  can have any number of samples.
 *                  If it is not set, frame->nb_samples must be equal to
 *                  avctx->frame_size for all frames except the last.
 *                  The final frame may be smaller than avctx->frame_size.
 * @return 0 on success, otherwise negative error code:
 *      AVERROR(EAGAIN):   input is not accepted in the current state - user
 *                         must read output with avcodec_receive_packet() (once
 *                         all output is read, the packet should be resent, and
 *                         the call will not fail with EAGAIN).
 *      AVERROR_EOF:       the encoder has been flushed, and no new frames can
 *                         be sent to it
 *      AVERROR(EINVAL):   codec not opened, refcounted_frames not set, it is a
 *                         decoder, or requires flush
 *      AVERROR(ENOMEM):   failed to add packet to internal queue, or similar
 *      other errors: legitimate encoding errors
 */
int avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame);

2. 定义分析
   avcodec_send_frame()内部调用encode_send_frame_internal() 来检查 frame buffer 里是否有数据，当 buffer_pkt中没有数据时，调用encode_receive_packet_internal() 是完成编码的核心函数；
   在encode_receive_packet_internal()中主要调用encode_simple_receive_packet() 完成视频编码；
   在encode_simple_receive_packet()调用encode_simple_internal() 完成视频编码；
   在encode_simple_internal()主要利用函数指针 (*encode2)() 对各类第三方的编码器的调用；此外若涉及到多线程编码，则会通过ff_thread_video_encode_frame()来完成。

3. (*encode2)()
   通过该函数指针指向不同的编码器，完成具体的编码过程；比如 x264、openh264、x265、avs2、videotoolbox 等等；

AVCodec ff_libx264_encoder = {
    .name             = "libx264",
    .long_name        = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_H264,
    .priv_data_size   = sizeof(X264Context),
    .init             = X264_init,
    .encode2          = X264_frame,
    .close            = X264_close,
    .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS |
                        AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE,
    .priv_class       = &x264_class,
    .defaults         = x264_defaults,
    .init_static_data = X264_init_static,
#if X264_BUILD >= 158
    .caps_internal    = FF_CODEC_CAP_INIT_CLEANUP | FF_CODEC_CAP_INIT_THREADSAFE,
#else
    .caps_internal    = FF_CODEC_CAP_INIT_CLEANUP,
#endif
    .wrapper_name     = "libx264",
};

AVCodec ff_libx265_encoder = {
    .name             = "libx265",
    .long_name        = NULL_IF_CONFIG_SMALL("libx265 H.265 / HEVC"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_HEVC,
    .init             = libx265_encode_init,
    .init_static_data = libx265_encode_init_csp,
    .encode2          = libx265_encode_frame,
    .close            = libx265_encode_close,
    .priv_data_size   = sizeof(libx265Context),
    .priv_class       = &class,
    .defaults         = x265_defaults,
    .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS |
                        AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE,
    .wrapper_name     = "libx265",
};

AVCodec ff_libopenh264_encoder = {
    .name           = "libopenh264",
    .long_name      = NULL_IF_CONFIG_SMALL("OpenH264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_H264,
    .priv_data_size = sizeof(SVCContext),
    .init           = svc_encode_init,
    .encode2        = svc_encode_frame,
    .close          = svc_encode_close,
    .capabilities   = AV_CODEC_CAP_AUTO_THREADS,
    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
    .pix_fmts       = (const enum AVPixelFormat[]){ AV_PIX_FMT_YUV420P,
                                                    AV_PIX_FMT_NONE },
    .defaults       = svc_enc_defaults,
    .priv_class     = &class,
    .wrapper_name   = "libopenh264",
};

AVCodec ff_h264_videotoolbox_encoder = {
    .name             = "h264_videotoolbox",
    .long_name        = NULL_IF_CONFIG_SMALL("VideoToolbox H.264 Encoder"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_H264,
    .priv_data_size   = sizeof(VTEncContext),
    .pix_fmts         = avc_pix_fmts,
    .init             = vtenc_init,
    .encode2          = vtenc_frame,
    .close            = vtenc_close,
    .capabilities     = AV_CODEC_CAP_DELAY,
    .priv_class       = &h264_videotoolbox_class,
    .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE |
                        FF_CODEC_CAP_INIT_CLEANUP,
};

AVCodec ff_hevc_videotoolbox_encoder = {
    .name             = "hevc_videotoolbox",
    .long_name        = NULL_IF_CONFIG_SMALL("VideoToolbox H.265 Encoder"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_HEVC,
    .priv_data_size   = sizeof(VTEncContext),
    .pix_fmts         = hevc_pix_fmts,
    .init             = vtenc_init,
    .encode2          = vtenc_frame,
    .close            = vtenc_close,
    .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_HARDWARE,
    .priv_class       = &hevc_videotoolbox_class,
    .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE |
                        FF_CODEC_CAP_INIT_CLEANUP,
    .wrapper_name     = "videotoolbox",
};

AVCodec ff_libxavs_encoder = {
    .name           = "libxavs",
    .long_name      = NULL_IF_CONFIG_SMALL("libxavs Chinese AVS (Audio Video Standard)"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_CAVS,
    .priv_data_size = sizeof(XavsContext),
    .init           = XAVS_init,
    .encode2        = XAVS_frame,
    .close          = XAVS_close,
    .capabilities   = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
    .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE },
    .priv_class     = &xavs_class,
    .defaults       = xavs_defaults,
    .wrapper_name   = "libxavs",
};

AVCodec ff_libxavs2_encoder = {
    .name           = "libxavs2",
    .long_name      = NULL_IF_CONFIG_SMALL("libxavs2 AVS2-P2/IEEE1857.4"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_AVS2,
    .priv_data_size = sizeof(XAVS2EContext),
    .init           = xavs2_init,
    .encode2        = xavs2_encode_frame,
    .close          = xavs2_close,
    .capabilities   = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
    .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_YUV420P,
                                                     AV_PIX_FMT_NONE },
    .priv_class     = &libxavs2,
    .defaults       = xavs2_defaults,
    .wrapper_name   = "libxavs2",
} ;

AVCodec ff_libvpx_vp8_encoder = {
    .name           = "libvpx",
    .long_name      = NULL_IF_CONFIG_SMALL("libvpx VP8"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_VP8,
    .priv_data_size = sizeof(VPxContext),
    .init           = vp8_init,
    .encode2        = vpx_encode,
    .close          = vpx_free,
    .capabilities   = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
    .pix_fmts       = (const enum AVPixelFormat[]){ AV_PIX_FMT_YUV420P, AV_PIX_FMT_YUVA420P, AV_PIX_FMT_NONE },
    .priv_class     = &class_vp8,
    .defaults       = defaults,
    .wrapper_name   = "libvpx",
};

AVCodec ff_libvpx_vp9_encoder = {
    .name           = "libvpx-vp9",
    .long_name      = NULL_IF_CONFIG_SMALL("libvpx VP9"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_VP9,
    .priv_data_size = sizeof(VPxContext),
    .init           = vp9_init,
    .encode2        = vpx_encode,
    .close          = vpx_free,
    .capabilities   = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
    .profiles       = NULL_IF_CONFIG_SMALL(ff_vp9_profiles),
    .priv_class     = &class_vp9,
    .defaults       = defaults,
    .init_static_data = ff_vp9_init_static,
    .wrapper_name   = "libvpx",
};

AVCodec ff_libxvid_encoder = {
    .name           = "libxvid",
    .long_name      = NULL_IF_CONFIG_SMALL("libxvidcore MPEG-4 part 2"),
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_MPEG4,
    .priv_data_size = sizeof(struct xvid_context),
    .init           = xvid_encode_init,
    .encode2        = xvid_encode_frame,
    .close          = xvid_encode_close,
    .pix_fmts       = (const enum AVPixelFormat[]) { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE },
    .priv_class     = &xvid_class,
    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE |
                      FF_CODEC_CAP_INIT_CLEANUP,
    .wrapper_name   = "libxvid",
};

avcodec_receive_packet()

1. API 申明介绍

/**
 * Read encoded data from the encoder.
 *
 * @param avctx codec context
 * @param avpkt This will be set to a reference-counted packet allocated by the
 *              encoder. Note that the function will always call
 *              av_packet_unref(avpkt) before doing anything else.
 * @return 0 on success, otherwise negative error code:
 *      AVERROR(EAGAIN):   output is not available in the current state - user
 *                         must try to send input
 *      AVERROR_EOF:       the encoder has been fully flushed, and there will be
 *                         no more output packets
 *      AVERROR(EINVAL):   codec not opened, or it is a decoder
 *      other errors: legitimate encoding errors
 */
int avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt);

2. 定义分析
   从 avcode_receive_packet()函数内部实现逻辑可以看到，首先判断buffer_pkt是否有数据，如果有，则调用 av_packet_move_ref() 函数完成数据包 packet 的拷贝过程；如果buffer_pak里没有数据，则需要调用 encode_receive_packet_internal() 来完成编码，此过程跟 send frame 模块中相同。
   

官方编码实例

/*
 * Copyright (c) 2001 Fabrice Bellard
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * @file
 * video encoding with libavcodec API example
 *
 * @example encode_video.c
 */

#include 
#include 
#include 

#include 

#include 
#include 

static void encode(AVCodecContext *enc_ctx, AVFrame *frame, AVPacket *pkt,
                   FILE *outfile)
{
    int ret;

    /* send the frame to the encoder */
    if (frame)
        printf("Send frame %3"PRId64"\n", frame->pts);

    ret = avcodec_send_frame(enc_ctx, frame);
    if (ret < 0) {
        fprintf(stderr, "Error sending a frame for encoding\n");
        exit(1);
    }

    while (ret >= 0) {
        ret = avcodec_receive_packet(enc_ctx, pkt);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return;
        else if (ret < 0) {
            fprintf(stderr, "Error during encoding\n");
            exit(1);
        }

        printf("Write packet %3"PRId64" (size=%5d)\n", pkt->pts, pkt->size);
        fwrite(pkt->data, 1, pkt->size, outfile);
        av_packet_unref(pkt);
    }
}

int main(int argc, char **argv)
{
    const char *filename, *codec_name;
    const AVCodec *codec;
    AVCodecContext *c= NULL;
    int i, ret, x, y;
    FILE *f;
    AVFrame *frame;
    AVPacket *pkt;
    uint8_t endcode[] = { 0, 0, 1, 0xb7 };

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s 
 \n", argv[0]);
        exit(0);
    }
    filename = argv[1];
    codec_name = argv[2];

    /* find the mpeg1video encoder */
    codec = avcodec_find_encoder_by_name(codec_name);
    if (!codec) {
        fprintf(stderr, "Codec '%s' not found\n", codec_name);
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    pkt = av_packet_alloc();
    if (!pkt)
        exit(1);

    /* put sample parameters */
    c->bit_rate = 400000;
    /* resolution must be a multiple of two */
    c->width = 352;
    c->height = 288;
    /* frames per second */
    c->time_base = (AVRational){1, 25};
    c->framerate = (AVRational){25, 1};

    /* emit one intra frame every ten frames
     * check frame pict_type before passing frame
     * to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
     * then gop_size is ignored and the output of encoder
     * will always be I frame irrespective to gop_size
     */
    c->gop_size = 10;
    c->max_b_frames = 1;
    c->pix_fmt = AV_PIX_FMT_YUV420P;

    if (codec->id == AV_CODEC_ID_H264)
        av_opt_set(c->priv_data, "preset", "slow", 0);

    /* open it */
    ret = avcodec_open2(c, codec, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not open codec: %s\n", av_err2str(ret));
        exit(1);
    }

    f = fopen(filename, "wb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }
    frame->format = c->pix_fmt;
    frame->width  = c->width;
    frame->height = c->height;

    ret = av_frame_get_buffer(frame, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate the video frame data\n");
        exit(1);
    }

    /* encode 1 second of video */
    for (i = 0; i < 25; i++) {
        fflush(stdout);

        /* make sure the frame data is writable */
        ret = av_frame_make_writable(frame);
        if (ret < 0)
            exit(1);

        /* prepare a dummy image */
        /* Y */
        for (y = 0; y < c->height; y++) {
            for (x = 0; x < c->width; x++) {
                frame->data[0][y * frame->linesize[0] + x] = x + y + i * 3;
            }
        }

        /* Cb and Cr */
        for (y = 0; y < c->height/2; y++) {
            for (x = 0; x < c->width/2; x++) {
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + i * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + i * 5;
            }
        }

        frame->pts = i;

        /* encode the image */
        encode(c, frame, pkt, f);
    }

    /* flush the encoder */
    encode(c, NULL, pkt, f);

    /* add sequence end code to have a real MPEG file */
    if (codec->id == AV_CODEC_ID_MPEG1VIDEO || codec->id == AV_CODEC_ID_MPEG2VIDEO)
        fwrite(endcode, 1, sizeof(endcode), f);
    fclose(f);

    avcodec_free_context(&c);
    av_frame_free(&frame);
    av_packet_free(&pkt);

    return 0;
}

参考

1. http://ffmpeg.org/