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Game Programming using Qt 5 Beginner's Guide - Second Edition by Lorenz Haas, Witold Wysota, Pavel Strakhov

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Time for action – Transforming the viewport

Let's extend our code so that all future operations focus only on drawing within the border boundaries after the border is drawn. Use the window and viewport transformation, as follows:

void Widget::paintEvent(QPaintEvent *) {
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing, true);
    QPen pen(Qt::black);
    pen.setWidth(4);
    painter.setPen(pen);
    QRect r = rect().adjusted(10, 10, -10, -10);
    painter.drawRoundedRect(r, 20, 10);
    painter.save();
    r.adjust(2, 2, -2, -2);
    painter.setViewport(r);
    r.moveTo(0, -r.height() / 2);
    painter.setWindow(r);
    drawChart(&painter, r);
    painter.restore();
} 

Also, create a protected method called drawChart():

void Widget::drawChart(QPainter *painter, const ...

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