Mac :artifacts around text painted onto a transparently filled QImage

#include <QApplication>
#include <QMainWindow>
#include <QMenuBar>
#include <QAction>
#include <QPainter>

class test: public QMainWindow
{

public:
    test() : QMainWindow()
    {
    }

protected:
    void paintEvent(QPaintEvent *event)
    {
        QRect r = rect();

            QRect r1 = QRect(0, 0, r.width(), r.height() / 2);

            QImage image(r1.size(), QImage::Format_ARGB32_Premultiplied);
            image.fill(Qt::transparent);
            QPainter pp(&image);
            QString testText = "Lorem ipsum summo lucilius an eos, eum liber tation tamquam ne, graeco laboramus has in. An nec quaeque habemus, timeam delenit perpetua in vel, eu sit unum mucius. Ad vix augue everti dissentias, duis omnesque incorrupte nam at, sale feugiat ex pro. Odio dicit laoreet ius cu, per in invenire hendrerit definiebas, vel erant aliquip delenit an. Augue postea nec ad, illud dolorem complectitur his ex. Cu malis apeirian nam, ut erat ferri latine usu, solum impedit euripidis mea at. Pro mutat graeci qualisque te, qui omnis munere feugiat cu.";
            pp.drawText(r1, "QImage(Raster):\n" + testText);
            pp.end();

            QRect r2 = QRect(0, r1.height(), r.width(), r.height() - r1.height());
            QPainter p(this);
            p.drawImage(0, 0, image);
            p.drawText(r2, "CoreGraphics:\n" + testText);
        }
};

int main(int argc, char *argv[])
{
    QApplication app (argc, argv);

        test *t = new test;
        t->show();

    return app.exec();
}