FFmpeg源码：avpriv_set_pts_info函数分析

一、avpriv_set_pts_info函数的声明

avpriv_set_pts_info函数声明在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的头文件libavformat/internal.h中：

/*** Set the time base and wrapping info for a given stream. This will be used* to interpret the stream's timestamps. If the new time base is invalid* (numerator or denominator are non-positive), it leaves the stream* unchanged.** @param st stream* @param pts_wrap_bits number of bits effectively used by the pts*        (used for wrap control)* @param pts_num time base numerator* @param pts_den time base denominator*/
void avpriv_set_pts_info(AVStream *st, int pts_wrap_bits,unsigned int pts_num, unsigned int pts_den);

该函数的作用是：设置AVStream的time_base。

形参st：输出型参数，指向一个AVStream对象。执行avpriv_set_pts_info函数后，st->time_base和st->pts_wrap_bits会被设置。

形参pts_wrap_bits：输入型参数，pts有效使用的位数。执行avpriv_set_pts_info函数后，st->pts_wrap_bits的值会被设置为pts_wrap_bits。

形参pts_num：输入型参数。time_base的分子。

形参pts_den：输入型参数。time_base的分母。

二、avpriv_set_pts_info函数的定义

avpriv_set_pts_info函数定义在源文件libavformat/avformat.c中：

void avpriv_set_pts_info(AVStream *st, int pts_wrap_bits,unsigned int pts_num, unsigned int pts_den)
{FFStream *const sti = ffstream(st);AVRational new_tb;if (av_reduce(&new_tb.num, &new_tb.den, pts_num, pts_den, INT_MAX)) {if (new_tb.num != pts_num)av_log(NULL, AV_LOG_DEBUG,"st:%d removing common factor %d from timebase\n",st->index, pts_num / new_tb.num);} elseav_log(NULL, AV_LOG_WARNING,"st:%d has too large timebase, reducing\n", st->index);if (new_tb.num <= 0 || new_tb.den <= 0) {av_log(NULL, AV_LOG_ERROR,"Ignoring attempt to set invalid timebase %d/%d for st:%d\n",new_tb.num, new_tb.den,st->index);return;}st->time_base     = new_tb;if (sti->avctx)sti->avctx->pkt_timebase = new_tb;st->pts_wrap_bits = pts_wrap_bits;
}

 关于av_reduce函数可以参考：《FFmpeg源码：av_reduce函数分析》。avpriv_set_pts_info函数的核心就是通过调用av_reduce函数对有理数进行化简：

av_reduce(&new_tb.num, &new_tb.den, pts_num, pts_den, INT_MAX)