Я не понимаю Qt

void FormQR::paintQR(QPainter &painter, const QSize sz, const QString &data, QColor fg) {
    // NOTE: At this point you will use the API to get the encoding and format you want, instead of my hardcoded stuff:
    qrcodegen::QrCode qr = qrcodegen::QrCode::encodeText(data.toUtf8().constData(), qrcodegen::QrCode::Ecc::LOW);
    const int s=qr.getSize()>0?qr.getSize():1;
    const double w=sz.width();
    const double h=sz.height();
    const double aspect=w/h;
    const double size=((aspect>1.0)?h:w);
    const double scale=size/(s+2);
    // NOTE: For performance reasons my implementation only draws the foreground parts in supplied color.
    // It expects background to be prepared already (in white or whatever is preferred).
    painter.setPen(Qt::NoPen);
    painter.setBrush(fg);
    for(int y=0; y<s; y++) {
        for(int x=0; x<s; x++) {
            const int color=1; //qr.getModule(x, y);  // 0 for white, 1 for black
            if(0!=color) {
                const double rx1=(x+1)*scale, ry1=(y+1)*scale;
                QRectF r(rx1, ry1, scale, scale);
                painter.drawRects(&r,1);
            }
        }
    }
    QPixmap *pix = new QPixmap(sz.width(), sz.height());
    painter.drawPixmap(0, 0, *pix);
    painter.end();
    ui->label_qr->setPixmap(*pix);
}

void FormQR::updateQRImage()
{
    QPixmap *pix = new QPixmap(QSize(200,200));
    QPainter *painter = new QPainter(pix);
    QSize sz = QSize(200, 200);
    paintQR(*painter, sz, tr("Test"), QColor::fromRgb(Qt::red));
}