時間基、時間戳

時間基：表示時間單位的分數，用來定義視頻或音頻流中時間的精度。其形式是一個分數，分子通常為 1，而分母則表示每秒的單位數。
時間戳：代表在時間軸里占了多少個格子，是特定的時間點。
時間基是時間戳的單位，時間戳值乘以時間基，就可得到實際的時刻值。

typedef struct AVRational{int num; //numeratorint den; //denominator
} AVRational;

FFmpeg 中的幾種時間基

• tbn：視頻流的時間基，time base of stream，AVStream.time_base 的倒數

typedef struct AVStream {.../*** 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;
}

• tbr：幀率，time base of rate
• tbc：視頻解碼的時間基，time base of codec，AVCodecContext.time_base 的倒數

typedef struct AVCodecContext {
/*** This is the fundamental unit of time (in seconds) in terms* of which frame timestamps are represented. For fixed-fps content,* timebase should be 1/framerate and timestamp increments should be* identically 1.* This often, but not always is the inverse of the frame rate or field rate* for video. 1/time_base is not the average frame rate if the frame rate is not* constant.** Like containers, elementary streams also can store timestamps, 1/time_base* is the unit in which these timestamps are specified.* As example of such codec time base see ISO/IEC 14496-2:2001(E)* vop_time_increment_resolution and fixed_vop_rate* (fixed_vop_rate == 0 implies that it is different from the framerate)** - encoding: MUST be set by user.* - decoding: the use of this field for decoding is deprecated.*             Use framerate instead.*/AVRational time_base;
}

• FFmpeg 內部時間基 AV_TIME_BASE

#define         AV_TIME_BASE   1000000
#define         AV_TIME_BASE_Q   (AVRational){1, AV_TIME_BASE}

使用此時間基計算得到的時間值表示的單位是微秒。

時間值的形式轉換

/*** Convert an AVRational to a `double`.* @param a AVRational to convert* @return `a` in floating-point form* @see av_d2q()*/
static inline double av_q2d(AVRational a){return a.num / (double) a.den;
}

將時間從 AVRational 形式轉換為 double 形式。將不同時間基的值轉成按秒為單位的運算如下：
timestamp(秒) = pts * av_q2d(time_base)

時間基轉換函數

/*** Rescale a 64-bit integer by 2 rational numbers.** The operation is mathematically equivalent to `a * bq / cq`.** This function is equivalent to av_rescale_q_rnd() with #AV_ROUND_NEAR_INF.** @see av_rescale(), av_rescale_rnd(), av_rescale_q_rnd()*/
int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq) av_const;

將時間值從一種時間基轉換為另一種時間基，其計算公式為 a * bq / cq

/*** Rescale a 64-bit integer by 2 rational numbers with specified rounding.** The operation is mathematically equivalent to `a * bq / cq`.** @see av_rescale(), av_rescale_rnd(), av_rescale_q()*/
int64_t av_rescale_q_rnd(int64_t a, AVRational bq, AVRational cq,enum AVRounding rnd) av_const;

同 av_rescale_q 相比增加了一個指定舍入方式的參數

/*** Convert valid timing fields (timestamps / durations) in a packet from one* timebase to another. Timestamps with unknown values (AV_NOPTS_VALUE) will be* ignored.** @param pkt packet on which the conversion will be performed* @param tb_src source timebase, in which the timing fields in pkt are*               expressed* @param tb_dst destination timebase, to which the timing fields will be*               converted*/
void av_packet_rescale_ts(AVPacket *pkt, AVRational tb_src, AVRational tb_dst);

用于將 AVPacket 中各種時間值從一種時間基轉換為另一種時間基