FFmpeg使用太复杂，一个重要的方面是包含的结构体太多，太复杂。我们来分析分析其中的几个重要的结构体。 AVFormatContext：该结构体在使用FFmpeg的过程中会一直使用到，它的定义如下
/** * 格式化输入输出上下文. * 使用avformat_alloc_context() 创建一个 AVFormatContext. */typedef struct AVFormatContext { /** * AVClass最主要的作用就是给结构体（如AVFormatContext等）增加AVOption功能的 *支持。换句话说就是AVClass是AVOption和目标结构体之间的桥梁，AVClass要求必须 *为目标结构体声明的第一个变量。 *AVOption是用来设置ffmpeg中变量的结构体。后续会具体介绍 */ const AVClass *av_class; /** * 输入文件的格式 * 在解复用是出现，由avformat_open_input()设置. */ struct AVInputFormat *iformat; /** * 输出文件的格式 * 复用的时候出现, 必须在avformat_write_header()之前由用户设置. */ struct AVOutputFormat *oformat; /** * 指向具体某个文件格式的私有数据. This is an AVOptions-enabled struct * if and only if iformat/oformat.PRiv_class is not NULL. * * - 复用时由avformat_write_header()设置 * - 解复用时由avformat_open_input()设置 */ void *priv_data; /** * 管理输入输出数据的结构体 * 通过对该变量的赋值可以改变输入源或输出目的地 * - 解复用: 在使用avformat_open_input()或者使avformat_open_input(). * 之前使用 * - 复用: 使用avformat_write_header()之前使用. 调用的时候需要注意释放IO * 上下文 * * Do NOT set this field if AVFMT_NOFILE flag is set in * iformat/oformat.flags. In such a case, the (de)muxer will handle * I/O in some other way and this field will be NULL. */ AVIOContext *pb; /* 码流信息 */ /** * 标志位又来表示码流的性能. A combination of AVFMTCTX_*. * Set by libavformat. */ int ctx_flags; /** * AVFormatContext.streams的个数. * * Set by avformat_new_stream(),不能被随意修改. */ unsigned int nb_streams; /** * 输入文件的所有码流信息.新的码流信息可由 * avformat_new_stream()生成。 * * - demuxing: streams are created by libavformat in avformat_open_input(). * If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams may also * appear in av_read_frame(). * - muxing: streams are created by the user before avformat_write_header(). * * 通过avformat_free_context()释放内存. */ AVStream **streams; /** * 输入输出文件名 * * - demuxing: set by avformat_open_input() * - muxing: may be set by the caller before avformat_write_header() */ char filename[1024]; /** * 定位第一帧的位置,利用 * AV_TIME_BASE fractional seconds. 不能直接设定: * 可以从 AVStream values中推导得出. * * Demuxing only, set by libavformat. */ int64_t start_time; /** * 码流的时长, 也是利用 AV_TIME_BASE fractional * seconds表征. 一般情况下由 * AVStream values 推导得出. * * Demuxing only, set by libavformat. */ int64_t duration; /** * 这个stream的码率, 0 if not * available. ffmpeg可以根据file_size和duration自动计算出来，所以无需指定. */ int64_t bit_rate; unsigned int packet_size; int max_delay; /** * 标志位用来修改解/复用行为. A combination of AVFMT_FLAG_*. * Set by the user before avformat_open_input() / avformat_write_header(). */ int flags;#define AVFMT_FLAG_GENPTS 0x0001 ///< Generate missing pts even if it requires parsing future frames.#define AVFMT_FLAG_IGNIDX 0x0002 ///< Ignore index.#define AVFMT_FLAG_NONBLOCK 0x0004 ///< Do not block when reading packets from input.#define AVFMT_FLAG_IGNDTS 0x0008 ///< Ignore DTS on frames that contain both DTS & PTS#define AVFMT_FLAG_NOFILLIN 0x0010 ///< Do not infer any values from other values, just return what is stored in the container#define AVFMT_FLAG_NOPARSE 0x0020 ///< Do not use AVParsers, you also must set AVFMT_FLAG_NOFILLIN as the fillin code works on frames and no parsing -> no frames. Also seeking to frames can not work if parsing to find frame boundaries has been disabled#define AVFMT_FLAG_NOBUFFER 0x0040 ///< Do not buffer frames when possible#define AVFMT_FLAG_CUSTOM_IO 0x0080 ///< The caller has supplied a custom AVIOContext, don't avio_close() it.#define AVFMT_FLAG_DISCARD_CORRUPT 0x0100 ///< Discard frames marked corrupted#define AVFMT_FLAG_FLUSH_PACKETS 0x0200 ///< Flush the AVIOContext every packet./** * When muxing, try to avoid writing any random/volatile data to the output. * This includes any random IDs, real-time timestamps/dates, muxer version, etc. * * This flag is mainly intended for testing. */#define AVFMT_FLAG_BITEXACT 0x0400#define AVFMT_FLAG_MP4A_LATM 0x8000 ///< Enable RTP MP4A-LATM payload#define AVFMT_FLAG_SORT_DTS 0x10000 ///< try to interleave outputted packets by dts (using this flag can slow demuxing down)#define AVFMT_FLAG_PRIV_OPT 0x20000 ///< Enable use of private options by delaying codec open (this could be made default once all code is converted)#define AVFMT_FLAG_KEEP_SIDE_DATA 0x40000 ///< Don't merge side data but keep it separate.#define AVFMT_FLAG_FAST_SEEK 0x80000 ///< Enable fast, but inaccurate seeks for some formats#define AVFMT_FLAG_SHORTEST 0x100000 ///< Stop muxing when the shortest stream stops.#define AVFMT_FLAG_AUTO_BSF 0x200000 ///< Wait for packet data before writing a header, and add bitstream filters as requested by the muxer /** * 从输入文件中读取数据的最大size， for determining * the input container format. * Demuxing only, set by the caller before avformat_open_input(). */ int64_t probesize; /** * Maximum duration (in AV_TIME_BASE units) of the data read * from input in avformat_find_stream_info(). * Demuxing only, set by the caller before avformat_find_stream_info(). * Can be set to 0 to let avformat choose using a heuristic. */ int64_t max_analyze_duration; const uint8_t *key; int keylen; unsigned int nb_programs; AVProgram **programs; /** * Forced video（视频） codec_id. * Demuxing: Set by user. */ enum AVCodecID video_codec_id; /** * Forced audio（音频） codec_id. * Demuxing: Set by user. */ enum AVCodecID audio_codec_id; /** * Forced subtitle（字幕） codec_id. * Demuxing: Set by user. */ enum AVCodecID subtitle_codec_id; /** * 每个stream的index所需要的内存字节数. * 如果index尺寸超过了这个值, entries will be discarded as * needed to maintain a smaller size. This can lead to slower or less * accurate seeking (depends on demuxer). * Demuxers for which a full in-memory index is mandatory will ignore * this. * - muxing: unused * - demuxing: set by user */ unsigned int max_index_size; /** * Maximum amount of memory in bytes to use for buffering frames * obtained from realtime capture devices. */ unsigned int max_picture_buffer; /** * Number of chapters in AVChapter array. * When muxing, chapters are normally written in the file header, * so nb_chapters should normally be initialized before write_header * is called. Some muxers (e.g. mov and mkv) can also write chapters * in the trailer. To write chapters in the trailer, nb_chapters * must be zero when write_header is called and non-zero when * write_trailer is called. * - muxing: set by user * - demuxing: set by libavformat */ unsigned int nb_chapters; AVChapter **chapters; /** * Metadata that applies to the whole file. * * - demuxing: set by libavformat in avformat_open_input() * - muxing: may be set by the caller before avformat_write_header() * * Freed by libavformat in avformat_free_context(). */ AVDictionary *metadata; /** * Start time of the stream in real world time, in microseconds * since the Unix epoch (00:00 1st January 1970). That is, pts=0 in the * stream was captured at this real world time. * - muxing: Set by the caller before avformat_write_header(). If set to * either 0 or AV_NOPTS_VALUE, then the current wall-time will * be used. * - demuxing: Set by libavformat. AV_NOPTS_VALUE if unknown. Note that * the value may become known after some number of frames * have been received. */ int64_t start_time_realtime; /** * 帧率 in * avformat_find_stream_info(). * Demuxing only, set by the caller before avformat_find_stream_info(). */ int fps_probe_size; /** * Error recognition; higher values will detect more errors but may * misdetect some more or less valid parts as errors. * Demuxing only, set by the caller before avformat_open_input(). */ int error_recognition; /** * Custom interrupt callbacks for the I/O layer. * * demuxing: set by the user before avformat_open_input(). * muxing: set by the user before avformat_write_header() * (mainly useful for AVFMT_NOFILE formats). The callback * should also be passed to avio_open2() if it's used to * open the file. */ AVIOInterruptCB interrupt_callback; /** * Flags to enable debugging. */ int debug;#define FF_FDEBUG_TS 0x0001 /** * 交叉存取的最大buffer. * * 为了保证所有streams交叉存取正确, * av_interleaved_write_frame() will wait until it has at least one packet * for each stream before actually writing any packets to the output file. * When some streams are "sparse（稀疏）" (i.e. there are large gaps between * successive packets), this can result in excessive（过大的） buffering. * * This field specifies the maximum difference between the timestamps（时间戳） of the * first and the last packet in the muxing queue, above which libavformat * will output a packet regardless of whether it has queued a packet for all * the streams. * * Muxing only, set by the caller before avformat_write_header(). */ int64_t max_interleave_delta; /** * Allow non-standard and experimental extension * @see AVCodecContext.strict_std_compliance */ int strict_std_compliance; /** * Flags for the user to detect events happening on the file. Flags must * be cleared by the user once the event has been handled. * A combination of AVFMT_EVENT_FLAG_*. */ int event_flags;#define AVFMT_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata. /** * Maximum number of packets to read while waiting for the first timestamp. * Decoding only. */ int max_ts_probe; /** * 复用时为了避免出现负的时间戳 * Any value of the AVFMT_AVOID_NEG_TS_* constants. * Note, this only works when using av_interleaved_write_frame. (interleave_packet_per_dts is in use) * - muxing: Set by user * - demuxing: unused */ int avoid_negative_ts;#define AVFMT_AVOID_NEG_TS_AUTO -1 ///< Enabled when required by target format#define AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE 1 ///< Shift timestamps so they are non negative#define AVFMT_AVOID_NEG_TS_MAKE_ZERO 2 ///< Shift timestamps so that they start at 0 /** * Transport stream id. * This will be moved into demuxer private options. Thus no API/ABI compatibility */ int ts_id; /** * Audio preload in microseconds. * Note, not all formats support this and unpredictable things may happen if it is used when not supported. * - encoding: Set by user via AVOptions (NO direct access) * - decoding: unused */ int audio_preload; /** * Max chunk time in microseconds. * Note, not all formats support this and unpredictable things may happen if it is used when not supported. * - encoding: Set by user via AVOptions (NO direct access) * - decoding: unused */ int max_chunk_duration; /** * Max chunk size in bytes * Note, not all formats support this and unpredictable things may happen if it is used when not supported. * - encoding: Set by user via AVOptions (NO direct access) * - decoding: unused */ int max_chunk_size; /** * forces the use of wallclock timestamps as pts/dts of packets * This has undefined results in the presence of B frames. * - encoding: unused * - decoding: Set by user via AVOptions (NO direct access) */ int use_wallclock_as_timestamps; /** * avio flags, used to force AVIO_FLAG_DIRECT. * - encoding: unused * - decoding: Set by user via AVOptions (NO direct access) */ int avio_flags; /** * The duration field can be estimated through various ways, and this field can be used * to know how the duration was estimated. * - encoding: unused * - decoding: Read by user via AVOptions (NO direct access) */ enum AVDurationEstimationMethod duration_estimation_method; /** * Skip initial bytes when opening stream * - encoding: unused * - decoding: Set by user via AVOptions (NO direct access) */ int64_t skip_initial_bytes; /** * Correct single timestamp overflows * - encoding: unused * - decoding: Set by user via AVOptions (NO direct access) */ unsigned int correct_ts_overflow; /** * Force seeking to any (also non key) frames. * - encoding: unused * - decoding: Set by user via AVOptions (NO direct access) */ int seek2any; /** * Flush the I/O context after each packet. * - encoding: Set by user via AVOptions (NO direct access) * - decoding: unused */ int flush_packets; /** * format probing score. * The maximal score is AVPROBE_SCORE_MAX, its set when the demuxer probes * the format. * - encoding: unused * - decoding: set by avformat, read by user via av_format_get_probe_score() (NO direct access) */ int probe_score; /** * number of bytes to read maximally to identify format. * - encoding: unused * - decoding: set by user through AVOPtions (NO direct access) */ int format_probesize; /** * ',' separated list of allowed decoders. * If NULL then all are allowed * - encoding: unused * - decoding: set by user through AVOptions (NO direct access) */ char *codec_whitelist; /** * ',' separated list of allowed demuxers. * If NULL then all are allowed * - encoding: unused * - decoding: set by user through AVOptions (NO direct access) */ char *format_whitelist; /** * An opaque field for libavformat internal usage. * Must not be accessed in any way by callers. */ AVFormatInternal *internal; /** * IO repositioned flag. * This is set by avformat when the underlaying IO context read pointer * is repositioned, for example when doing byte based seeking. * Demuxers can use the flag to detect such changes. */ int io_repositioned; /** * Forced video codec. * This allows forcing a specific decoder, even when there are multiple with * the same codec_id. * Demuxing: Set by user via av_format_set_video_codec (NO direct access). */ AVCodec *video_codec; /** * Forced audio codec. * This allows forcing a specific decoder, even when there are multiple with * the same codec_id. * Demuxing: Set by user via av_format_set_audio_codec (NO direct access). */ AVCodec *audio_codec; /** * Forced subtitle codec. * This allows forcing a specific decoder, even when there are multiple with * the same codec_id. * Demuxing: Set by user via av_format_set_subtitle_codec (NO direct access). */ AVCodec *subtitle_codec; /** * Forced data codec. * This allows forcing a specific decoder, even when there are multiple with * the same codec_id. * Demuxing: Set by user via av_format_set_data_codec (NO direct access). */ AVCodec *data_codec; /** * Number of bytes to be written as padding in a metadata header. * Demuxing: Unused. * Muxing: Set by user via av_format_set_metadata_header_padding. */ int metadata_header_padding; /** * User data. * This is a place for some private data of the user. */ void *opaque; /** * Callback used by devices to communicate with application. */ av_format_control_message control_message_cb; /** * Output timestamp offset, in microseconds. * Muxing: set by user via AVOptions (NO direct access) */ int64_t output_ts_offset; /** * dump format separator. * can be ", " or "/n " or anything else * Code outside libavformat should access this field using AVOptions * (NO direct access). * - muxing: Set by user. * - demuxing: Set by user. */ uint8_t *dump_separator; /** * Forced Data codec_id. * Demuxing: Set by user. */ enum AVCodecID data_codec_id;#if FF_API_OLD_OPEN_CALLBACKS /** * Called to open further IO contexts when needed for demuxing. * * This can be set by the user application to perform security checks on * the URLs before opening them. * The function should behave like avio_open2(), AVFormatContext is provided * as contextual information and to reach AVFormatContext.opaque. * * If NULL then some simple checks are used together with avio_open2(). * * Must not be accessed directly from outside avformat. * @See av_format_set_open_cb() * * Demuxing: Set by user. * * @deprecated Use io_open and io_close. */ attribute_deprecated int (*open_cb)(struct AVFormatContext *s, AVIOContext **p, const char *url, int flags, const AVIOInterruptCB *int_cb, AVDictionary **options);#endif /** * ',' separated list of allowed protocols. * - encoding: unused * - decoding: set by user through AVOptions (NO direct access) */ char *protocol_whitelist; /* * A callback for opening new IO streams. * * Whenever a muxer or a demuxer needs to open an IO stream (typically from * avformat_open_input() for demuxers, but for certain formats can happen at * other times as well), it will call this callback to obtain an IO context. * * @param s the format context * @param pb on success, the newly opened IO context should be returned here * @param url the url to open * @param flags a combination of AVIO_FLAG_* * @param options a dictionary of additional options, with the same * semantics as in avio_open2() * @return 0 on success, a negative AVERROR code on failure * * @note Certain muxers and demuxers do nesting, i.e. they open one or more * additional internal format contexts. Thus the AVFormatContext pointer * passed to this callback may be different from the one facing the caller. * It will, however, have the same 'opaque' field. */ int (*io_open)(struct AVFormatContext *s, AVIOContext **pb, const char *url, int flags, AVDictionary **options); /** * A callback for closing the streams opened with AVFormatContext.io_open(). */ void (*io_close)(struct AVFormatContext *s, AVIOContext *pb); /** * ',' separated list of disallowed protocols. * - encoding: unused * - decoding: set by user through AVOptions (NO direct access) */ char *protocol_blacklist;} AVFormatContext;视频的原数据（metadata）信息可以通过AVDictionary获取。元数据存储在AVDictionaryEntry结构体中，如下所示
typedef struct AVDictionaryEntry { char *key; char *value; } AVDictionaryEntry;每一条元数据分为key和value两个属性。 在ffmpeg中通过av_dict_get()函数获得视频的原数据。
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