ffmpeg时间基与时间戳

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。

c 复制代码

typedef struct AVRational{
    int num; //numerator
    int 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

c 复制代码

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

c 复制代码

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

c 复制代码

#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

c 复制代码

/**
 * 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

c 复制代码

/**
 * 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

c 复制代码

/**
 * 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

c 复制代码

/**
 * 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