AVCodec* vcodec = avcodec_find_encoder_by_name("h264_nvenc");
  AVStream* vstrm = avformat_new_stream(outctx, vcodec);
  if (!vstrm) {
    std::cerr << "fail to avformat_new_stream";
    fclose(time_fp);
    return 2;
  }

  avcodec_get_context_defaults3(vstrm->codec, vcodec);
  vstrm->codec->width = dst_width;
  vstrm->codec->height = dst_height;
  vstrm->codec->pix_fmt = vcodec->pix_fmts[0];
  vstrm->codec->time_base = vstrm->time_base = av_inv_q(dst_fps);
  vstrm->r_frame_rate = vstrm->avg_frame_rate = dst_fps;
  if (outctx->oformat->flags & AVFMT_GLOBALHEADER)
    vstrm->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

  // open video encoder
  ret = avcodec_open2(vstrm->codec, vcodec, nullptr);
  if (ret < 0) {
    std::cerr << "fail to avcodec_open2: ret=" << ret;
    fclose(time_fp);
    return 2;
  }

  std::cout << "outfile: " << file_name.c_str() << "\n"
            << "format:  " << outctx->oformat->name << "\n"
            << "vcodec:  " << vcodec->name << "\n"
            << "size:    " << dst_width << 'x' << dst_height << "\n"
            << "fps:     " << av_q2d(dst_fps) << "\n"
            << "pixfmt:  " << av_get_pix_fmt_name(vstrm->codec->pix_fmt) << "\n"
            << std::flush;