对于一个h264码流，若想要改变I帧间隔，需要解码再编码

ffmpeg 版本4.3

static void openEncoder(int width, int height, AVCodecContext** enc_ctx)
{
    //使用libx264编码器
    AVCodec * pCodec = avcodec_find_encoder_by_name("libx264");
    if(nullptr == pCodec)
    {
        printf("avcodec_find_encoder_by_name fail.\n");
        return;
    }
    //获取编码器上下文
    *enc_ctx = avcodec_alloc_context3(pCodec);
    if(nullptr == enc_ctx)
    {
        printf("avcodec_alloc_context3(pCodec) fail.\n");
        return;
    }
    //sps/pps
    (*enc_ctx)->profile = FF_PROFILE_H264_HIGH;
    (*enc_ctx)->level = 50;//表示level是5.0
    //分辨率
    (*enc_ctx)->width = width;
    (*enc_ctx)->height = height;
    //gop
    (*enc_ctx)->gop_size = 20;
    (*enc_ctx)->keyint_min = 15;//设置最小自动插入i帧的间隔.OPTION
    //B帧
    (*enc_ctx)->max_b_frames = 0;//OPTION
    (*enc_ctx)->has_b_frames = 0;//OPTION
    //参考帧
    (*enc_ctx)->refs = 3;//OPTION
    //设置输入的yuv格式
    (*enc_ctx)->pix_fmt = AV_PIX_FMT_YUV420P;
    //设置码率
    (*enc_ctx)->bit_rate = 1000000;
    //设置帧率
    (*enc_ctx)->time_base = (AVRational){1,25};//帧与帧之间的间隔
    (*enc_ctx)->framerate = (AVRational){25,1};//帧率 25帧每秒
    if(avcodec_open2((*enc_ctx),pCodec,nullptr) < 0)
    {
        printf("avcodec_open2 fail.\n");
    }
    return;

}
static AVFrame* creat_frame(int width,int height)
{
    AVFrame *frame = av_frame_alloc();
    frame->width = width;
    frame->height = height;
    frame->format = AV_PIX_FMT_YUV420P;
    //分配frame中buffer的大小
    av_frame_get_buffer(frame, 32);//按32位对齐
    return  frame;

}
static void encode(AVCodecContext *encCtx,AVFrame *frame,AVPacket *newpkt,FILE *outPutFile)
{
    int nRet = 0;
    if (frame)
        printf("send frame to encoder ,pts=%lld \n",frame->pts);
    //送原始数据给编码器进行编码
    nRet = avcodec_send_frame(encCtx,frame);
    if(nRet < 0)
    {
        printf("avcodec_send_frame fail.\n");
        return;
    }
    //从编码器获取编号的数据
    while(nRet >= 0)
    {
        nRet = avcodec_receive_packet(encCtx,newpkt);
        if(nRet < 0)
        {
            printf("avcodec_receive_packet fail.\n");
            return;
        }
        fwrite(newpkt->data,1,newpkt->size,outPutFile);
        av_packet_unref(newpkt);
    }
}
#define  YUVFORMAT_YUV420P AV_PIX_FMT_YUV420P
static void test()
{

    int nRet = 0;
    const char *pInFileName = "D:/videos/gop30.h264";
    const char *pOutFileName = "D:/output/gop20.h264";
    FILE *fp = fopen(pOutFileName, "wb+");
    AVDictionary *pDic = nullptr;
    AVFormatContext *pInFmtCtx = nullptr;
    nRet = avformat_open_input(&pInFmtCtx,pInFileName,nullptr,&pDic);
    if( nRet < 0)
    {
        printf("Could not open input file.");
        return;
    }
    avformat_find_stream_info(pInFmtCtx, nullptr);
    printf("===========Input Information==========\n");
    av_dump_format(pInFmtCtx, 0, pInFileName, 0);
    printf("======================================\n");
    //查找解码器
    int vudioStreamIndex = -1;
    for(int i = 0; i < pInFmtCtx->nb_streams; ++i)
    {
        if(AVMEDIA_TYPE_VIDEO == pInFmtCtx->streams[i]->codecpar->codec_type)
        {
            vudioStreamIndex = i;
            break;
        }
    }
    AVStream * in_stream = pInFmtCtx->streams[vudioStreamIndex];
    AVCodec *pInCodec = avcodec_find_decoder(in_stream->codecpar->codec_id);
    if(nullptr == pInCodec)
    {
        printf("avcodec_find_decoder fail.");
        return;
    }
    AVCodecContext* pInCodecCtx = avcodec_alloc_context3(pInCodec); //??????????????
    nRet = avcodec_parameters_to_context(pInCodecCtx, in_stream->codecpar);
    if(nRet < 0)
    {
        printf("avcodec_parameters_to_context fail.");
        return;
    }
    //打开解码器
    if(avcodec_open2(pInCodecCtx, pInCodec, nullptr) < 0)
    {
        printf("Error: Can't open codec!\n");
        return ;
    }
    printf("width = %d\n", pInCodecCtx->width);
    printf("height = %d\n", pInCodecCtx->height);
    AVFrame *pFrameYUV = av_frame_alloc();
    AVPacket *packet = av_packet_alloc();
    av_init_packet(packet);
    //计算这个格式的图片，需要多少字节来存储
    int bytes_num = av_image_get_buffer_size(YUVFORMAT_YUV420P, pInCodecCtx->width, pInCodecCtx->height,1);
    //申请空间来存放图片数据。包含源数据和目标数据
    uint8_t* out_buffer = (uint8_t*)av_malloc(bytes_num);
    //前面的av_frame_alloc函数，只是为这个AVFrame结构体分配了内存，
    //而该类型的指针指向的内存还没分配。这里把av_malloc得到的内存和AVFrame关联起来。
    //当然，其还会设置AVFrame的其他成员
    avpicture_fill((AVPicture*)pFrameYUV, out_buffer, YUVFORMAT_YUV420P,pInCodecCtx->width, pInCodecCtx->height);
    int got_picture = 0;
    AVFrame *pFrame = av_frame_alloc();
    AVPacket *newpkt = av_packet_alloc();
    int64_t pts = 0;
    AVCodecContext *encCtx = nullptr;
    //打开编码器
    openEncoder(1920,1080,&encCtx);
    while(av_read_frame(pInFmtCtx, packet) >= 0)
    {
        if( vudioStreamIndex == packet->stream_index)
        {
            //avcodec_send_packet送原始数据给编码器进行编码
            //avcodec_receive_frame
            if(avcodec_send_packet(pInCodecCtx, packet)<0 || (got_picture =avcodec_receive_frame(pInCodecCtx, pFrame))<0)
            {
                std::cout<<"changeGOP  end";
                goto __end;
            }
            if(!got_picture)//
            {
                pts += 40;
                // 设置pts
                pFrame->pts = pts;
                encode(encCtx, pFrame, newpkt, fp);
            }
            av_packet_unref(packet);
            av_packet_unref(newpkt);
        }
    }
    fflush(fp);
    //flush decoder
    //当av_read_frame 退出循环的时候，实际上解码器中可能还包含
    //剩余的几帧数据。直接调用avcodec_decode_video2获得AVFrame ,
    //而不再向解码器传递AVPacket
    while(1)
    {
        if(avcodec_send_packet(pInCodecCtx, packet)<0 || (got_picture =avcodec_receive_frame(pInCodecCtx, pFrame))<0)
        {
            std::cout<<"changeGOP  end";
            goto __end;
        }
        if(!got_picture)//
        {
            pts += 40;
            // 设置pts
            pFrame->pts = pts;
            encode(encCtx, pFrame, newpkt, fp);
        }
        av_packet_unref(packet);
        av_packet_unref(newpkt);
    }
__end:
    fflush(fp);
    fclose(fp);
    av_frame_free(&pFrame);
    av_frame_free(&pFrameYUV);
    avcodec_close(pInCodecCtx);
    avformat_close_input(&pInFmtCtx);
}


void main()
{
    test();
    std::cout<<"changeGOP end";
}