源文件：

#include <iostream>
#include <thread>
using namespace std;
extern "C" { //指定函数是c语言函数，函数名不包含重载标注
//引用ffmpeg头文件
#include <libavformat/avformat.h>
}
//预处理指令导入库
#pragma comment(lib,"avformat.lib")
#pragma comment(lib,"avutil.lib")
#pragma comment(lib,"avcodec.lib")
void PrintErr(int err)
{char buf[1024] = { 0 };av_strerror(err, buf, sizeof(buf) - 1);cerr << endl;
}
#define CERR(err) if(err!=0){ PrintErr(err);getchar();return -1;}int main(int argc, char* argv[])
{//打开媒体文件const char* url = "v1080.mp4";/// 解封装//解封装输入上下文AVFormatContext* ic = nullptr;//初始化格式上下文IC//*Open an input streamand read the header.The codecs are not opened.
//* The stream must be closed with avformat_close_input().auto re = avformat_open_input(&ic, url,NULL,       //封装器格式 null 自动探测 根据后缀名或者文件头NULL        //参数设置，rtsp需要设置);CERR(re);//获取媒体信息 无头部格式re = avformat_find_stream_info(ic, NULL);CERR(re);//打印封装信息av_dump_format(ic, 0, url,0 //0表示上下文是输入 1 输出);AVStream* as = nullptr; //音频流AVStream* vs = nullptr; //视频流for (int i = 0; i < ic->nb_streams; i++){//音频if (ic->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO){as = ic->streams[i];cout << "=====音频=====" << endl;cout << "sample_rate:" << as->codecpar->sample_rate << endl;}else if (ic->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){vs = ic->streams[i];cout << "=========视频=========" << endl;cout << "width:" << vs->codecpar->width << endl;cout << "height:" << vs->codecpar->height << endl;}}/// 解封装/开始编码到文件中//编码器上下文const char* out_url = "test_mux.mp4";AVFormatContext* ec = nullptr;re = avformat_alloc_output_context2(&ec, NULL, NULL,out_url         //根据文件名推测封装格式);CERR(re);//再output文件的上下文ec中添加视频流、音频流auto mvs = avformat_new_stream(ec, NULL);  //视频流auto mas = avformat_new_stream(ec, NULL);  //音频流//打开输出IOre = avio_open(&ec->pb, out_url, AVIO_FLAG_WRITE);CERR(re);//设置编码音视频流参数//ec->streams[0];//mvs->codecpar;//视频参数if (vs){mvs->time_base = vs->time_base;// 时间基数与原视频一致//从解封装复制参数avcodec_parameters_copy(mvs->codecpar, vs->codecpar);}if (as){mas->time_base = as->time_base;//从解封装复制参数avcodec_parameters_copy(mas->codecpar, as->codecpar);}//写入文件头re = avformat_write_header(ec, NULL);CERR(re);//打印输出上下文av_dump_format(ec, 0, out_url, 1);/// 截取10 ~ 20 秒之间的音频视频 取多不取少// 假定 9 11秒有关键帧 我们取第9秒double begin_sec = 10.0;    //截取开始时间double end_sec = 20.0;      //截取结束时间long long begin_pts = 0;long long begin_audio_pts = 0;  //音频的开始时间long long end_pts = 0;//换算成pts 换算成输入ic的pts，以视频流为准if (vs && vs->time_base.num > 0){//sec /timebase = pts// pts =  sec/(num/den) = sec* (den/num)  double t = (double)vs->time_base.den / (double)vs->time_base.num;//den分母/num分子begin_pts = begin_sec * t;end_pts = end_sec * t;}if (as && as->time_base.num > 0)begin_audio_pts = begin_sec * ((double)as->time_base.den / (double)as->time_base.num);//seek输入媒体 移动到第十秒的关键帧位置if (vs)re = av_seek_frame(ic, vs->index, begin_pts,AVSEEK_FLAG_FRAME | AVSEEK_FLAG_BACKWARD); //向后关键帧CERR(re);AVPacket pkt;for (;;){re = av_read_frame(ic, &pkt);if (re != 0){PrintErr(re);break;}AVStream* in_stream = ic->streams[pkt.stream_index];AVStream* out_stream = nullptr;long long offset_pts = 0; //偏移pts，用于截断的开头pts运算if (vs && pkt.stream_index == vs->index){cout << "视频:";//超过第20秒退出，只存10~20秒if (pkt.pts > end_pts){av_packet_unref(&pkt);break;}out_stream = ec->streams[0];offset_pts = begin_pts;}else if (as && pkt.stream_index == as->index){cout << "音频:";out_stream = ec->streams[1];offset_pts = begin_audio_pts;}cout << pkt.pts << " : " << pkt.dts << " :" << pkt.size << endl;//重新计算pts dts duration//`a * bq（输入basetime） / cq（输出basetime）`if (out_stream){pkt.pts = av_rescale_q_rnd(pkt.pts - offset_pts, in_stream->time_base,out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));pkt.dts = av_rescale_q_rnd(pkt.dts - offset_pts, in_stream->time_base,out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);}pkt.pos = -1;//写入音视频帧 会清理pktre = av_interleaved_write_frame(ec,&pkt);if (re != 0){PrintErr(re);}//av_packet_unref(&pkt);//this_thread::sleep_for(100ms);}//写入结尾 包含文件偏移索引re = av_write_trailer(ec);if (re != 0)PrintErr(re);avformat_close_input(&ic);avio_closep(&ec->pb);avformat_free_context(ec);ec = nullptr;return 0;
}

运行结果：

成功生产力另一个mp4文件，内容为我们重新封装的音视频流，和解封装之前的视频一样