时间基、时间戳

时间基：表示时间单位的分数，用来定义视频或音频流中时间的精度。其形式是一个分数，分子通常为 1，而分母则表示每秒的单位数。
时间戳：代表在时间轴里占了多少个格子，是特定的时间点。
时间基是时间戳的单位，时间戳值乘以时间基，就可得到实际的时刻值。

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

FFmpeg 中的几种时间基

• tbn：视频流的时间基，time base of stream，AVStream.time_base 的倒数

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：帧率，time base of rate
• tbc：视频解码的时间基，time base of codec，AVCodecContext.time_base 的倒数

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 内部时间基 AV_TIME_BASE

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

使用此时间基计算得到的时间值表示的单位是微秒。

时间值的形式转换

/*** 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;
}

将时间从 AVRational 形式转换为 double 形式。将不同时间基的值转成按秒为单位的运算如下：
timestamp(秒) = pts * av_q2d(time_base)

时间基转换函数

/*** 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;

将时间值从一种时间基转换为另一种时间基，其计算公式为 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 相比增加了一个指定舍入方式的参数

/*** 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);

用于将 AVPacket 中各种时间值从一种时间基转换为另一种时间基