Draw a cosmetic filled ellipse in QT - qt

Question

I want to draw a filled ellipse in QT that would not change its size when zooming in and out. For now I have the following:
QPen pen = painter->pen();
pen.setCosmetic(true);
pen.setWidth(5);
painter->setPen(pen);
QBrush brush = painter->brush();
brush.setStyle(Qt::SolidPattern);
painter->setBrush(brush);
painter->drawEllipse(p, 2, 2);
When I zoom out a gap between the boundary and the filling appear. So it looks like 2 concentric circles. And when I zoom in the filling overgrows the boundary and the disk gets bigger and bigger. Any idea how to fix this? Thanks!

Answer 1

I would instead look to the ItemIgnoresTransformations flag, which will make the item itself "cosmetic", rather than just the pen. Here's a working example:
#include <QtGui>
class NonScalingItem : public QGraphicsItem
{
public:
NonScalingItem()
{ setFlag(ItemIgnoresTransformations, true); }
QRectF boundingRect() const
{ return QRectF(-5, -5, 10, 10); }
void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
QPen pen = painter->pen();
pen.setCosmetic(true);
pen.setWidth(5);
pen.setColor(QColor(Qt::red));
painter->setPen(pen);
QBrush brush = painter->brush();
brush.setStyle(Qt::SolidPattern);
painter->setBrush(brush);
painter->drawEllipse(QPointF(0, 0), 10, 10);
}
};
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QGraphicsScene *scene = new QGraphicsScene;
QGraphicsView *view = new QGraphicsView;
NonScalingItem *item = new NonScalingItem;
scene->addItem(item);
view->setScene(scene);
/* The item will remain unchanged regardless of whether
or not you comment out the following line: */
view->scale(2000, 2000);
view->show();
return app.exec();
}