FFmpeg Remux(重新封装)

此案例为FFmpeg的官方例子，用于学习FFmpeg的重新封装和格式转换的使用。

一、FFmpeg重新封装的流程

重新封装的原理就是打开文件 - 解封装 - 封装 - 输出文件，如下图：

二、代码示意

/**
*@Author：Magic Miao
*@Blog: http://www.hust-miao.top
*@date: 2022/08/30
*description:FFmpeg 重新封装 <br>
*/

//miao:引入FFmpeg库
extern "C"{
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>
}

static void log_packet(const AVFormatContext *fmt_ctx,const AVPacket *pkt,const char *tag)
{
    AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

    printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",
           tag,
           av_ts2str(pkt->pts), av_ts2timestr(pkt->pts,time_base),
           av_ts2str(pkt->dts), av_ts2timestr(pkt->dts,time_base),
           av_ts2str(pkt->duration),av_ts2timestr(pkt->duration,time_base),
           pkt->stream_index);
}

int main(int argc, char *argv[])
{
    const AVOutputFormat *out_fmt = NULL;
    AVFormatContext *fmt_ctx = NULL,*ofmt_ctx = NULL;
    AVPacket *pkt = NULL;

    const char *in_filename,*out_filename;

    int ret;
    int stream_index = 0;
    int *stream_mapping = NULL;
    int stream_mapping_size = 0;

    if(argc < 3)
    {
        printf("Usage: %s input output\n"
               "API example program to remux a media file with libavformat and libavcodec.\n"
               "The output format is guessed according to the file extension.\n"
               "\n",argv[0]);
        return 1;
    }

    in_filename = argv[1];
    out_filename = argv[2];

    pkt = av_packet_alloc();
    if(!pkt)
    {
        fprintf(stderr,"Could not allocate AVPacket\n");
        return 1;
    }

    //miao:打开输入文件
    if((ret = avformat_open_input(&fmt_ctx,in_filename,0,0)) < 0)
    {
        fprintf(stderr,"Could not open input file '%s'",in_filename);
        goto end;
    }

    if((ret = avformat_find_stream_info(fmt_ctx,0)) < 0)
    {
        fprintf(stderr,"Failed to retrieve input stream information");
        goto end;
    }

    av_dump_format(fmt_ctx,0,in_filename,0);

    avformat_alloc_output_context2(&ofmt_ctx,NULL,NULL,out_filename);
    if(!ofmt_ctx)
    {
        fprintf(stderr,"Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    stream_mapping_size = fmt_ctx->nb_streams;
    stream_mapping = av_calloc(stream_mapping_size,sizeof(*stream_mapping));
    if(!stream_mapping)
    {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    out_fmt = ofmt_ctx->oformat;

    for(int i = 0; i < fmt_ctx->nb_streams; i++)
    {
        AVStream *out_stream;
        AVStream *in_stream = ofmt_ctx->streams[i];
        AVCodecParameters *in_codecpar = in_stream->codecpar;

        if(in_codecpar->codec_type != AVMEDIA_TYPE_AUDIO &&
           in_codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&
           in_codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
        {
            stream_mapping[i] = -1;
            continue;
        }

        stream_mapping[i] = stream_index++;

        out_stream = avformat_new_stream(ofmt_ctx,NULL);
        if(!out_stream)
        {
            fprintf(stderr,"Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        ret = avcodec_parameters_copy(out_stream->codecpar,in_codecpar);
        if(ret < 0)
        {
            fprintf(stderr,"Failed to copy codec parameters\n");
            goto end;
        }

        out_stream->codecpar->codec_tag = 0;
    }

    av_dump_format(ofmt_ctx,0,out_filename,1);

    if(!(out_fmt->flags & AVFMT_NOFILE))
    {
        ret = avio_open(&ofmt_ctx->pb,out_filename,AVIO_FLAG_WRITE);
        if(ret < 0)
        {
            fprintf(stderr,"Could not open output file '%s'",out_filename);
            goto end;
        }
    }

    ret = avformat_write_header(ofmt_ctx,NULL);
    if(ret < 0)
    {
        fprintf(stderr,"Error occurred when opening output file\n");
        goto end;
    }

    while(1)
    {
        AVStream *in_stream,*out_stream;

        ret = av_read_frame(fmt_ctx,pkt);
        if(ret < 0) break;

        in_stream = fmt_ctx->streams[pkt->stream_index];
        if(pkt->stream_index >= stream_mapping_size || stream_mapping[pkt->stream_index] < 0)
        {
            av_packet_unref(pkt);
            continue;
        }

        pkt->stream_index = stream_mapping[pkt->stream_index];
        out_stream = ofmt_ctx->streams[pkt->stream_index];
        log_packet(fmt_ctx,pkt,"in");

        //miao:copy packet
        av_packet_rescale_ts(pkt,in_stream->time_base,out_stream->time_base);
        pkt->pos = -1;
        log_packet(ofmt_ctx,pkt,"out");

        ret = av_interleaved_write_frame(ofmt_ctx,pkt);
        if(ret < 0)
        {
            fprintf(stderr,"Error muxing packet\n");
            break;
        }
    }

    av_write_trailer(ofmt_ctx);

    end:
    av_packet_free(&pkt);
    avformat_close_input(&fmt_ctx);

    //miao:close output
    if(ofmt_ctx && !(out_fmt->flags & AVFMT_NOFILE))
    {
        avio_closep(&ofmt_ctx->pb);
    }

    avformat_free_context(ofmt_ctx);
    av_freep(&stream_mapping);

    if(ret < 0 && ret != AVERROR_EOF)
    {
        fprintf(stderr,"Error occurred: %s\n",av_err2str(ret));
        return 1;
    }

    return 0;
}