ffmpeg封装和解封装介绍-（6）完成重封装mp4文件并截断10s~20s的视频并生成10s视频文件

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IO
    re = 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pkt
        re = 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