How to convert an image to binary with a custom color table so that the quality of the result does not suffer?

Question

Qt

Hello. There is this code:

QApplication a(argc, argv);

    //Адрес ресурса: :/<prefix>/<alias>
    const QString file_name = ":/ferrari.jpeg";
    QImage mono_img(file_name);

    QVector<QRgb> color_table(2);
    color_table[0] = QColor(Qt::white).rgb();
    color_table[1] = QColor(Qt::red).rgb();
    mono_img = mono_img.convertToFormat(QImage::Format_Mono, color_table);

    QLabel label;
    label.setPixmap(QPixmap::fromImage(mono_img));
    label.show();

    return a.exec();

Everything works, but the result of binarization is noticeably worse than when calling mono_img.convertToFormat(QImage::Format_Mono);Tell me how you can fix this.