Does QFontMetricsF ::width() equals painter.drawText() width?

[Mozzie]

Hi,
I want to use QPainter to draw text and make sure the text center at the origin of the coordinate axis

QFontMetricsF fm(m_graduateFont);
qDebug() << "m_graduateFont" << m_graduateFont;
qDebug() << "fm" << fm.size(Qt::TextSingleLine, "0");
qDebug() << "fm" << fm.size(Qt::TextSingleLine, "11");
qDebug() << "fm" << fm.size(Qt::TextSingleLine, "100");
qDebug() << "fm" << fm.boundingRect(QRect(), Qt::AlignCenter, "100");
double fontHeigth = fm.height();
double textWidth = fm.width(num);
painter.save();
painter.setFont(m_graduateFont);
painter.drawText(QPointF(- textWidth * .5, fontHeigth * .5), num);

painter.setPen(Qt::red);
painter.drawLine(-textWidth * .5, 0, textWidth * .5, 0);
painter.drawLine(0, -fontHeigth * .5, 0, fontHeigth * .5);
painter.restore();

but it seems they do not match, did I forget something?

thanks.