音视频开发基础（七）理解FFMpeg重要结构体及常用成员
AVStream
1.简介
AVStream在FFmpeg使用过程中关于编解码至关重要的结构体之一，是对流（Stream）的封装和抽象，描述了视频、音频等流的编码格式等基本流信息。此外也是音频、视频、字母数据流的重要载体。
2.源码解读
typedef struct AVStream {
int index; /**< 在AVFormatContext中的stream索引 */
/**
* 特定格式的stream id。
* 解码: 由libavformat设定
* 编码: 如果未设置，则由用户通过libavformat设置
*/
int id;
#if FF_API_LAVF_AVCTX
/**
* @deprecated use the codecpar struct instead
* 不推荐使用该字段
*/
attribute_deprecated
//在老版本中此处为struct AVFrac pts;
AVCodecContext *codec;
#endif
/**
* 这是表示帧时间戳的基本时间单位（以秒为单位）。
*
* 解码: libavformat设置
* 编码: 可以在avformat_write_header（）之前由调用者设置，以向混流器提供关于所需单位时间的
* 提示。在avformat_write_header（）中，混流器将用实际用于写入文件的时间戳（根据格式可能与
* 用户提供的时间戳相关或不相关）的单位时间覆盖该字段。
*/
AVRational time_base;
/**
* 解码: 流显示序列中的第一帧pts时间，基于流时间（in stream time base.）。
* 只有当你百分百确定该值就是真实的第一帧的pts时间，才可以设置它
* 该值可能未定义(AV_NOPTS_VALUE).
* @note The ASF header does NOT contain a correct start_time the ASF
* 分流器禁止设置该值。
*/
int64_t start_time;
/**
* 解码: 流时长，基于流时间（in stream time base.）
* 如果一个源文件指定了比特率，而未指定流时长，该值将由比特率和文件大小估算。
*
* 编码: May be set by the caller before 用户可以在avformat_write_header()调用前设
* 置，提示混流器估算时长
*/
int64_t duration;
int64_t nb_frames; ///< 表示该流的已知帧数，或者为0
int disposition; /**< AV_DISPOSITION_* 推荐比特字段 */
enum AVDiscard discard; ///< 选择那些数据包可以被丢掉而不用被分流器分流。
/**
* 采样率(如果未知，该值为0)
* - 编码: 用户设置.
* - 解码: libavformat设置.
*/
AVRational sample_aspect_ratio;
AVDictionary *metadata;//原数据信息
/**
* 平均帧率
*
* - 分流: 在创建流时或者才函数avformat_find_stream_info()函数中可能被设置。
* - 混流: 可能在avformat_write_header()函数调用前被设置
*/
AVRational avg_frame_rate;
/**
* 对于设置有AV_DISPOSITION_ATTACHED_PIC标志的流, 该数据包会包含该附加图片（专辑图片什么的）
*
* 解码: libavformat设置, 不能被用户修改。
* 编码: 不使用
*/
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;
常用字段以及使用
AVCodecParameters * codecpar ：用于记录编码后的流信息，即通道中存储的流的编码信息。
AVRational time_base ：AVStream通道的时间基，时间基是个相当重要的概念。（可参考之后的关于ffmpeg时间的文章）
- 创建AVStream ：
AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c);
avformat_new_stream之后便在 AVFormatContext 里增加了 AVStream 通道（相关的index已经被设置了）。之后，我们就可以自行设置 AVStream 的一些参数信息。例如 : codec_id , format ,bit_rate ,width , height … …