ffmpeg # AV_TIME_BASE units

看到ffmpeg中很多时间单位都是AV_TIME_BASE units, 这个是个什么单位呢?

ffmpeg中的内部计时单位（时间基），ffmepg中的所有时间都是于它为一个单位，比如AVStream中的duration即以为着这个流的长度为duration个AV_TIME_BASE。

/**
 * Internal time base represented as integer
 */

#define AV_TIME_BASE            1000000

/**
 * Internal time base represented as fractional value
 */

#define AV_TIME_BASE_Q          (AVRational){1, AV_TIME_BASE}

可见ffmpeg内部时间单位为微秒 (μs)， 即AV_TIME_BASE units单位是1us。
AV_TIME_BASE_Q是AV_TIME_BASE的倒数。

    int hours, mins, secs, us;
    ...
    secs = FFABS(pts) / AV_TIME_BASE;
    us = FFABS(pts) % AV_TIME_BASE;

举一些以AV_TIME_BASE units为单位的例子:

• InputStream

typedef struct InputStream {
    int file_index;
    AVStream *st;
   
    ...

    int64_t       start;     /* time when read started */
    /* predicted dts of the next packet read for this stream or (when there are
     * several frames in a packet) of the next frame in current packet (in AV_TIME_BASE units) */
    int64_t       next_dts;
    int64_t       dts;       ///< dts of the last packet read for this stream (in AV_TIME_BASE units)

    int64_t       next_pts;  ///< synthetic pts for the next decode frame (in AV_TIME_BASE units)
    int64_t       pts;       ///< current pts of the decoded frame  (in AV_TIME_BASE units)

    ...
}

• OutputFile

typedef struct OutputFile {
    ...

    int64_t recording_time;  ///< desired length of the resulting file in microseconds == AV_TIME_BASE units
    int64_t start_time;      ///< start time in microseconds == AV_TIME_BASE units
    
    ...
} OutputFile;

image.png

时间基转换公式

timestamp(ffmpeg内部时间戳) = AV_TIME_BASE * time(秒)
time(秒) = AV_TIME_BASE_Q * timestamp(ffmpeg内部时间戳)
所以当需要把视频跳转到N秒的时候可以使用下面的方法：

int64_t timestamp = N * AV_TIME_BASE; 
av_seek_frame(fmtctx, index_of_video, timestamp, AVSEEK_FLAG_BACKWARD);

av_rescale_q

ffmpeg同样为我们提供了不同时间基之间的转换函数

int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq)

这个函数的作用是计算a * bq / cq，来把时间戳从一个时基调整到另外一个时基。在进行时基转换的时候，我们应该首选这个函数，因为它可以避免溢出的情况发生。

References:

https://www.cnblogs.com/yinxiangpei/articles/3892982.html
https://blog.csdn.net/matrix_laboratory/article/details/53159511
https://www.suninf.net/2018/02/ffmpeg-timestamps.html
https://www.jianshu.com/p/bf323cee3b8e