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;
  }