In several situations QPainter::drawEllipse paints an invalid ellipse for thick pens. The situation depends on the paint engine or attributes like antialiasing and alpha value ... . You can see the mismatch easily with the code below on a X11 system. (screenshot is attached). The first circle is painted correctly while the second one is wrong. ( Started with -graphicssystem raster both circles are wrong. ) The bug happens with Qt 4.5.3 and Qt 4.6-beta1..

#include <QApplication>
#include <QWidget>
#include <QPainter>

class MyWidget: public QWidget
{
public:
MyWidget(QWidget *parent = NULL):
QWidget(parent)

{ const int m = 20; setContentsMargins(m, m, m, m); }

protected:
virtual void paintEvent(QPaintEvent *)

{ double w = qMin(contentsRect().width(), contentsRect().height()); const double pw = w * 0.45; w -= pw; QRectF r(0, 0, w, w); r.moveCenter(contentsRect().center()); QPainter painter(this); // paint circle painter.setPen(QPen(Qt::black, pw)); painter.drawEllipse(r); // paint bad circle QColor c(Qt::white); c.setAlpha(100); QPen pen(c, pw); painter.setPen(pen); painter.drawEllipse(r); }

};

int main(int argc, char **argv)
{
QApplication a(argc, argv);

MyWidget w;
w.resize(600, 600);
w.show();

return a.exec();
}