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   引言

    在视频录制中，有时会碰到这样一个需求，将不同摄像头的画面写入到一个视频文件，这个叫法很多，有的厂家叫合流模式，有的叫多画面多流模式。无论如何，它们的实质都是在一个视频文件上实现多路不同分辨率视频的保存。

     经过调查，支持这种需求封装格式的有MP4、MOV、MKV 等，这里因为MP4 格式应用最广泛。

   原理

     ffmpeg 有一个map命令，可以将多路视频轨封装在一个视频容器，掰ffmpeg源码发现其实新建一个新的AVStream，修改stream->index，就可以实现多流录制的目的。

ffmpeg -i input.mp4 -i  test.mp4  -map 0:v:0 -map 1:v -map 0:a -map 1:a -c copy  -y mix.mp4

#include <iostream>
#include <string>extern "C"
{#include <libavutil/timestamp.h>#include <libavformat/avformat.h>
}typedef struct
{char* file_name;AVFormatContext* fmt_ctx;int video_index;int audio_index;int source_index;double last_pts[2];double last_dts[2];AVRational video_time_base;AVRational audio_time_base;bool is_end;
}InputStream;int create_stream(InputStream *input_stream, AVFormatContext* out_fmt_ctx, int &stream_num)
{int i = 0;int ret = 0;if ((ret = avformat_open_input(&input_stream->fmt_ctx, input_stream->file_name, 0, 0)) < 0) //打开输出文件{fprintf(stderr, "Could not open input file '%s'", input_stream->file_name);goto end;}if ((ret = avformat_find_stream_info(input_stream->fmt_ctx, 0)) < 0) //打开输入{fprintf(stderr, "Failed to retrieve input stream information");goto end;}//av_dump_format(input_stream->fmt_ctx, 0, input_stream->file_name, 0);//打印信息for (size_t i = 0; i < input_stream->fmt_ctx->nb_streams; i++){AVStream* in_stream = input_stream->fmt_ctx->streams[i];AVStream* out_stream = avformat_new_stream(out_fmt_ctx, in_stream->codec->codec);if (in_stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO){input_stream->audio_index = i;input_stream->audio_time_base = in_stream->time_base;printf("流 %d 是音频 \n", input_stream->source_index + i);}if (in_stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){input_stream->video_index = i;input_stream->video_time_base = in_stream->time_base;printf("流 %d 是视频 \n", input_stream->source_index + i);}if (!out_stream){fprintf(stderr, "Failed allocating output stream\n");goto end;}ret = avcodec_copy_context(out_stream->codec, in_stream->codec);if (ret < 0){fprintf(stderr, "Failed to copy context from input to output stream codec context\n");goto end;}out_stream->codec->codec_tag = 0;if (out_fmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;stream_num++;}
end:return ret;
}int write_a_channel(InputStream *stream, AVFormatContext *out_fmt_ctx)
{int ret = -1;AVPacket packet;ret = av_read_frame(stream->fmt_ctx, &packet);if (ret < 0){if (ret == AVERROR_EOF){//printf("readFrame报错 %d\n", ret);av_free_packet(&packet);return ret;}}AVRational time_base = {0};AVStream* in_stream = stream->fmt_ctx->streams[packet.stream_index];AVStream* out_stream = out_fmt_ctx->streams[packet.stream_index];packet.stream_index = stream->source_index + packet.stream_index;if (in_stream->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){time_base = stream->video_time_base;}if (in_stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO){time_base = stream->audio_time_base;}packet.pts = av_rescale_q_rnd(packet.pts, time_base, out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));packet.dts = av_rescale_q_rnd(packet.dts, time_base, out_stream->time_base, (AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));if (packet.pts < 0){packet.pts = 0;}if (packet.dts < 0){packet.dts = 0;}//printf("%d, pts %d dts %d \n", packet.stream_index, packet.pts, packet.dts);ret = av_interleaved_write_frame(out_fmt_ctx, &packet);if (ret < 0){printf("写入错误\n");}av_free_packet(&packet);return ret;
}int main(int argc, char** argv)
{int ret = -1;char file_path[100][100] = {"D:\\素材\\test1.mp4", "D:\\素材\\test2.mp4", "D:\\素材\\test3.mp4", "D:\\素材\\test4.mp4"};int count = sizeof(file_path) / sizeof(char);const char *out_fileName = "my_muxing.mp4";av_register_all();AVOutputFormat* ofmt = NULL;AVFormatContext* out_fmt_ctx = NULL;const int channel_num = 4;avformat_alloc_output_context2(&out_fmt_ctx, NULL, NULL, out_fileName);//创建streams	InputStream stream_arry[channel_num] = {0};int stream_index = 0;for (int i = 0; i < channel_num; i++){		int stream_num = 0;stream_arry[i].file_name = file_path[i];stream_arry[i].source_index = stream_index;ret = create_stream(&stream_arry[i], out_fmt_ctx, stream_index);stream_index += stream_num;}if (!out_fmt_ctx){return -1;}ofmt = out_fmt_ctx->oformat;if (!(ofmt->flags & AVFMT_NOFILE)){ret = avio_open(&out_fmt_ctx->pb, out_fileName, AVIO_FLAG_WRITE);if (ret < 0){fprintf(stderr, "Could not open output file '%s'", out_fileName);return -1;}}AVDictionary* opts = NULL;av_dict_set(&opts, "movflags", "frag_keyframe+empty_moov", 0);//fmp4输出ret = avformat_write_header(out_fmt_ctx, &opts);if (ret < 0){fprintf(stderr, "Error occurred when opening output file\n");return -1;}while (true){int finish_count = 0;for (int i = 0; i < channel_num; i++){if (!stream_arry[i].is_end){ret = write_a_channel(&stream_arry[i], out_fmt_ctx);if (ret == AVERROR_EOF){stream_arry[i].is_end = true;finish_count++;}else{continue;}}}if (finish_count >= channel_num - 1){break;}}//清理for (size_t i = 0; i < channel_num; i++){avformat_close_input(&stream_arry[i].fmt_ctx);}ret = av_write_trailer(out_fmt_ctx);printf("==============合并完毕，写文件尾部 %d=============\n", ret);return 0;
}

设置flags 避免播放器拖动的时候出现花屏。

 packet->flags = isKeyFrame ? packet->flags | AV_PKT_FLAG_KEY : packet->flags;