FFmpeg很龐大，可以參考上面的連結，csdn上雷霄驊的部落格，稍微熟悉點後可以看官方自帶的例子，ffmpeg-3.3.8/doc/examples,通過例子熟悉FFmpeg的流程和函式．不過看了很多例子都是FFmpeg+SDL的,實際上Android開發者真的需要用SDL嗎，渲染部分用Android平臺自身的功能還更容易上手，SDL等熟悉FFmpeg了再進行研究吧．

這個例子說明了怎樣使用ffmpeg進行多媒體檔案的解封裝，解碼成Yuv420p，並儲存為檔案．然後可以用Yuv播放器播放該檔案． 流程圖如下，圖片來源： 函式說明(注意隨著ffmpeg的版本不同，流程圖上的函式名可能不同了):

/**
 * Initialize libavformat and register all the muxers, demuxers and
 * protocols. If you do not call this function, then you can select
 * exactly which formats you want to support.
 *
 * @see av_register_input_format()
 * @see av_register_output_format()
 */
void av_register_all(void);

註冊了所有的檔案格式和編解碼器的庫,
也可以只註冊特定的格式和編解碼器:av_register_input_format(),  av_register_output_format()
 

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, AVInputFormat *fmt, AVDictionary **options);
讀取檔案的頭部並且把資訊儲存到第一個引數AVFormatContext結構體中。
最後三個引數用來指定特殊的檔案格式，緩衝大小和格式引數，但如果把它們設定為空 NULL 或者 0，將自動檢測這些引數。
 

/**
 * Read packets of a media file to get stream information. This
 * is useful for file formats with no headers such as MPEG. This
 * function also computes the real framerate in case of MPEG-2 repeat
 * frame mode.
 * The logical file position is not changed by this function;
 * examined packets may be buffered for later processing.
 *
 * @param ic media file handle
 * @param options  If non-NULL, an ic.nb_streams long array of pointers to
 *                 dictionaries, where i-th member contains options for
 *                 codec corresponding to i-th stream.
 *                 On return each dictionary will be filled with options that were not found.
 * @return >=0 if OK, AVERROR_xxx on error
 *
 * @note this function isn't guaranteed to open all the codecs, so
 *       options being non-empty at return is a perfectly normal behavior.
 *
 * @todo Let the user decide somehow what information is needed so that
 *       we do not waste time getting stuff the user does not need.
 */
int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options);
avformat_open_input函式只是檢測了檔案的頭部，這個函式檢查在檔案中的流的資訊

 

/**
 * Find the "best" stream in the file.
 * The best stream is determined according to various heuristics as the most
 * likely to be what the user expects.
 * If the decoder parameter is non-NULL, av_find_best_stream will find the
 * default decoder for the stream's codec; streams for which no decoder can
 * be found are ignored.
 *
 * @param ic                media file handle
 * @param type              stream type: video, audio, subtitles, etc.
 * @param wanted_stream_nb  user-requested stream number,
 *                          or -1 for automatic selection
 * @param related_stream    try to find a stream related (eg. in the same
 *                          program) to this one, or -1 if none
 * @param decoder_ret       if non-NULL, returns the decoder for the
 *                          selected stream
 * @param flags             flags; none are currently defined
 * @return  the non-negative stream number in case of success,
 *          AVERROR_STREAM_NOT_FOUND if no stream with the requested type
 *          could be found,
 *          AVERROR_DECODER_NOT_FOUND if streams were found but no decoder
 * @note  If av_find_best_stream returns successfully and decoder_ret is not
 *        NULL, then *decoder_ret is guaranteed to be set to a valid AVCodec.
 */
int av_find_best_stream(AVFormatContext *ic,
                        enum AVMediaType type,
                        int wanted_stream_nb,
                        int related_stream,
                        AVCodec **decoder_ret,
                        int flags);
 找到視訊流或音訊流的索引,其中包含編解碼器的資訊

/**
 * Find a registered decoder with a matching codec ID.
 *
 * @param id AVCodecID of the requested decoder
 * @return A decoder if one was found, NULL otherwise.
 */
AVCodec *avcodec_find_decoder(enum AVCodecID id);
找到編解碼器

/**
 * Allocate an AVCodecContext and set its fields to default values. The
 * resulting struct should be freed with avcodec_free_context().
 *
 * @param codec if non-NULL, allocate private data and initialize defaults
 *              for the given codec. It is illegal to then call avcodec_open2()
 *              with a different codec.
 *              If NULL, then the codec-specific defaults won't be initialized,
 *              which may result in suboptimal default settings (this is
 *              important mainly for encoders, e.g. libx264).
 *
 * @return An AVCodecContext filled with default values or NULL on failure.
 */
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
流中關於編解碼器的資訊就是被我們叫做“編解碼器上下文”（codec context）的東西。這裡麵包含了流中所
使用的關於編解碼器的所有資訊，現在我們有了一個指向它的指標.

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);
開啟編碼器

/**
 * Allocate an AVFrame and set its fields to default values.  The resulting
 * struct must be freed using av_frame_free().
 *
 * @return An AVFrame filled with default values or NULL on failure.
 *
 * @note this only allocates the AVFrame itself, not the data buffers. Those
 * must be allocated through other means, e.g. with av_frame_get_buffer() or
 * manually.
 */
AVFrame *av_frame_alloc(void);
分配一個AVFrame並設為預設值，用來存放幀資料

int av_read_frame(AVFormatContext *s, AVPacket *pkt);
從流中讀取一幀完整資料或者多幀完整(ffmpeg內部保證了完整性)

int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,
                         int *got_picture_ptr,
                         const AVPacket *avpkt);
 解碼原始流中的一幀資料(輸出yuv資料)

/**
 * Copy image in src_data to dst_data.
 *
 * @param dst_linesizes linesizes for the image in dst_data
 * @param src_linesizes linesizes for the image in src_data
 */
void av_image_copy(uint8_t *dst_data[4], int dst_linesizes[4],
                   const uint8_t *src_data[4], const int src_linesizes[4],
                   enum AVPixelFormat pix_fmt, int width, int height);
 拷貝圖片資料