public H264VideoStreamEncoder(Stream stream, int fps, Size frameSize)
        {
            _stream = stream;
            _frameSize = frameSize;

            var codecId = AVCodecID.AV_CODEC_ID_H264;
            _pCodec = ffmpeg.avcodec_find_encoder(codecId);
            if (_pCodec == null) throw new InvalidOperationException("Codec not found.");

            _pCodecContext = ffmpeg.avcodec_alloc_context3(_pCodec);
            _pCodecContext->width = frameSize.Width;
            _pCodecContext->height = frameSize.Height;
            _pCodecContext->time_base = new AVRational {num = 1, den = fps};
            _pCodecContext->pix_fmt = AVPixelFormat.AV_PIX_FMT_YUV420P;
            ffmpeg.av_opt_set(_pCodecContext->priv_data, "preset", "veryslow", 0);

            ffmpeg.avcodec_open2(_pCodecContext, _pCodec, null).ThrowExceptionIfError();

            _linesizeY = frameSize.Width;
            _linesizeU = frameSize.Width / 2;
            _linesizeV = frameSize.Width / 2;

            _ySize = _linesizeY * frameSize.Height;
            _uSize = _linesizeU * frameSize.Height / 2;
        }