Loading... 我们在分析AVFormatContext结构的时候提到其中的一个成员streams。使用该成员我们就可以得到AVStream对象。 > unsigned int nb_streams; > > AVStream** streams; nb_streams是当前轨道数，就是流数量，streams是轨道的指针数组。一般而言一个视频文件会有一个视频流和一个音频流，那就是两个AVStream结构。  对于AVStream，我们常用的成员有： ```cpp //(1)标识该视频/音频流 int index; //(2)指向该音频/视频流的AVCodecContext（已被废弃，不推荐使用） AVCodecContext* codec; //(3)获得AVCodecParameters对象，用于替代AVCodecContext（推荐使用） AVCodecParameters* codecpar; //(4)该视频长度； int64_t duration; //(5)视频平均帧率 AVRational avg_frame_rate; //其中，AVRational表示有理数，他有两个参数 //avg_frame_rate.num : 分子 //avg_frame_rate.den : 母 //平均帧率 = avg_frame_rate.num / avg_frame_rate.den ``` AVStream详细定义： ```cpp /** * Stream structure. * New fields can be added to the end with minor version bumps. * Removal, reordering and changes to existing fields require a major * version bump. * sizeof(AVStream) must not be used outside libav*. */ typedef struct AVStream { int index; /**< stream index in AVFormatContext */ /** * Format-specific stream ID. * decoding: set by libavformat * encoding: set by the user, replaced by libavformat if left unset */ int id; #if FF_API_LAVF_AVCTX /** * @deprecated use the codecpar struct instead */ attribute_deprecated AVCodecContext *codec; #endif void *priv_data; /** * This is the fundamental unit of time (in seconds) in terms * of which frame timestamps are represented. * * decoding: set by libavformat * encoding: May be set by the caller before avformat_write_header() to * provide a hint to the muxer about the desired timebase. In * avformat_write_header(), the muxer will overwrite this field * with the timebase that will actually be used for the timestamps * written into the file (which may or may not be related to the * user-provided one, depending on the format). */ AVRational time_base; /** * Decoding: pts of the first frame of the stream in presentation order, in stream time base. * Only set this if you are absolutely 100% sure that the value you set * it to really is the pts of the first frame. * This may be undefined (AV_NOPTS_VALUE). * @note The ASF header does NOT contain a correct start_time the ASF * demuxer must NOT set this. */ int64_t start_time; /** * Decoding: duration of the stream, in stream time base. * If a source file does not specify a duration, but does specify * a bitrate, this value will be estimated from bitrate and file size. * * Encoding: May be set by the caller before avformat_write_header() to * provide a hint to the muxer about the estimated duration. */ int64_t duration; int64_t nb_frames; ///< number of frames in this stream if known or 0 int disposition; /**< AV_DISPOSITION_* bit field */ enum AVDiscard discard; ///< Selects which packets can be discarded at will and do not need to be demuxed. /** * sample aspect ratio (0 if unknown) * - encoding: Set by user. * - decoding: Set by libavformat. */ AVRational sample_aspect_ratio; AVDictionary *metadata; /** * Average framerate * * - demuxing: May be set by libavformat when creating the stream or in * avformat_find_stream_info(). * - muxing: May be set by the caller before avformat_write_header(). */ AVRational avg_frame_rate; /** * For streams with AV_DISPOSITION_ATTACHED_PIC disposition, this packet * will contain the attached picture. * * decoding: set by libavformat, must not be modified by the caller. * encoding: unused */ AVPacket attached_pic; /** * An array of side data that applies to the whole stream (i.e. the * container does not allow it to change between packets). * * There may be no overlap between the side data in this array and side data * in the packets. I.e. a given side data is either exported by the muxer * (demuxing) / set by the caller (muxing) in this array, then it never * appears in the packets, or the side data is exported / sent through * the packets (always in the first packet where the value becomes known or * changes), then it does not appear in this array. * * - demuxing: Set by libavformat when the stream is created. * - muxing: May be set by the caller before avformat_write_header(). * * Freed by libavformat in avformat_free_context(). * * @see av_format_inject_global_side_data() */ AVPacketSideData *side_data; /** * The number of elements in the AVStream.side_data array. */ int nb_side_data; /** * Flags for the user to detect events happening on the stream. Flags must * be cleared by the user once the event has been handled. * A combination of AVSTREAM_EVENT_FLAG_*. */ int event_flags; #define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata. /** * Real base framerate of the stream. * This is the lowest framerate with which all timestamps can be * represented accurately (it is the least common multiple of all * framerates in the stream). Note, this value is just a guess! * For example, if the time base is 1/90000 and all frames have either * approximately 3600 or 1800 timer ticks, then r_frame_rate will be 50/1. */ AVRational r_frame_rate; #if FF_API_LAVF_FFSERVER /** * String containing pairs of key and values describing recommended encoder configuration. * Pairs are separated by ','. * Keys are separated from values by '='. * * @deprecated unused */ attribute_deprecated char *recommended_encoder_configuration; #endif /** * Codec parameters associated with this stream. Allocated and freed by * libavformat in avformat_new_stream() and avformat_free_context() * respectively. * * - demuxing: filled by libavformat on stream creation or in * avformat_find_stream_info() * - muxing: filled by the caller before avformat_write_header() */ AVCodecParameters *codecpar; /***************************************************************** * All fields below this line are not part of the public API. They * may not be used outside of libavformat and can be changed and * removed at will. * Internal note: be aware that physically removing these fields * will break ABI. Replace removed fields with dummy fields, and * add new fields to AVStreamInternal. ***************************************************************** */ #define MAX_STD_TIMEBASES (30*12+30+3+6) /** * Stream information used internally by avformat_find_stream_info() */ struct { int64_t last_dts; int64_t duration_gcd; int duration_count; int64_t rfps_duration_sum; double (*duration_error)[2][MAX_STD_TIMEBASES]; int64_t codec_info_duration; int64_t codec_info_duration_fields; int frame_delay_evidence; /** * 0 -> decoder has not been searched for yet. * >0 -> decoder found * <0 -> decoder with codec_id == -found_decoder has not been found */ int found_decoder; int64_t last_duration; /** * Those are used for average framerate estimation. */ int64_t fps_first_dts; int fps_first_dts_idx; int64_t fps_last_dts; int fps_last_dts_idx; } *info; int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */ // Timestamp generation support: /** * Timestamp corresponding to the last dts sync point. * * Initialized when AVCodecParserContext.dts_sync_point >= 0 and * a DTS is received from the underlying container. Otherwise set to * AV_NOPTS_VALUE by default. */ int64_t first_dts; int64_t cur_dts; int64_t last_IP_pts; int last_IP_duration; /** * Number of packets to buffer for codec probing */ int probe_packets; /** * Number of frames that have been demuxed during avformat_find_stream_info() */ int codec_info_nb_frames; /* av_read_frame() support */ enum AVStreamParseType need_parsing; struct AVCodecParserContext *parser; /** * last packet in packet_buffer for this stream when muxing. */ struct AVPacketList *last_in_packet_buffer; AVProbeData probe_data; #define MAX_REORDER_DELAY 16 int64_t pts_buffer[MAX_REORDER_DELAY+1]; AVIndexEntry *index_entries; /**< Only used if the format does not support seeking natively. */ int nb_index_entries; unsigned int index_entries_allocated_size; /** * Stream Identifier * This is the MPEG-TS stream identifier +1 * 0 means unknown */ int stream_identifier; /** * Details of the MPEG-TS program which created this stream. */ int program_num; int pmt_version; int pmt_stream_idx; int64_t interleaver_chunk_size; int64_t interleaver_chunk_duration; /** * stream probing state * -1 -> probing finished * 0 -> no probing requested * rest -> perform probing with request_probe being the minimum score to accept. * NOT PART OF PUBLIC API */ int request_probe; /** * Indicates that everything up to the next keyframe * should be discarded. */ int skip_to_keyframe; /** * Number of samples to skip at the start of the frame decoded from the next packet. */ int skip_samples; /** * If not 0, the number of samples that should be skipped from the start of * the stream (the samples are removed from packets with pts==0, which also * assumes negative timestamps do not happen). * Intended for use with formats such as mp3 with ad-hoc gapless audio * support. */ int64_t start_skip_samples; /** * If not 0, the first audio sample that should be discarded from the stream. * This is broken by design (needs global sample count), but can't be * avoided for broken by design formats such as mp3 with ad-hoc gapless * audio support. */ int64_t first_discard_sample; /** * The sample after last sample that is intended to be discarded after * first_discard_sample. Works on frame boundaries only. Used to prevent * early EOF if the gapless info is broken (considered concatenated mp3s). */ int64_t last_discard_sample; /** * Number of internally decoded frames, used internally in libavformat, do not access * its lifetime differs from info which is why it is not in that structure. */ int nb_decoded_frames; /** * Timestamp offset added to timestamps before muxing * NOT PART OF PUBLIC API */ int64_t mux_ts_offset; /** * Internal data to check for wrapping of the time stamp */ int64_t pts_wrap_reference; /** * Options for behavior, when a wrap is detected. * * Defined by AV_PTS_WRAP_ values. * * If correction is enabled, there are two possibilities: * If the first time stamp is near the wrap point, the wrap offset * will be subtracted, which will create negative time stamps. * Otherwise the offset will be added. */ int pts_wrap_behavior; /** * Internal data to prevent doing update_initial_durations() twice */ int update_initial_durations_done; /** * Internal data to generate dts from pts */ int64_t pts_reorder_error[MAX_REORDER_DELAY+1]; uint8_t pts_reorder_error_count[MAX_REORDER_DELAY+1]; /** * Internal data to analyze DTS and detect faulty mpeg streams */ int64_t last_dts_for_order_check; uint8_t dts_ordered; uint8_t dts_misordered; /** * Internal data to inject global side data */ int inject_global_side_data; /** * display aspect ratio (0 if unknown) * - encoding: unused * - decoding: Set by libavformat to calculate sample_aspect_ratio internally */ AVRational display_aspect_ratio; /** * An opaque field for libavformat internal usage. * Must not be accessed in any way by callers. */ AVStreamInternal *internal; } AVStream; ``` Last modification：June 15th, 2021 at 06:06 pm © 允许规范转载