QGraphicsScene draw a non transformable circle - qt

Question

I am trying to draw a 10 pixel radius circle on a QGraphicsScene that is anchored on the center and doesn't transform (change size) when the scene is zoomed.
I have tried using the QGraphicsEllipseItem with the QGraphicsItem::ItemIgnoresTransformations flag, but it still transforms when the scene is zoomed.
Is this possible with standard graphics items, or do I need to create my own and do the paint event manually?

Answer 1

The following code does what I think you're looking for. It creates a single 10 pixel immutable circle that anchors itself to the centre of the scene as well as 10 larger, movable, circles that obey the scaling transform applied...
#include <iostream>
#include <QApplication>
#include <QGraphicsEllipseItem>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPainter>
int main (int argc, char **argv)
{
QApplication app(argc, argv);
QGraphicsView view;
QGraphicsScene scene;
view.setScene(&scene);
QGraphicsEllipseItem immutable(0, 0, 20, 20);
immutable.setFlag(QGraphicsItem::ItemIgnoresTransformations);
immutable.setPen(Qt::NoPen);
immutable.setBrush(Qt::blue);
scene.addItem(&immutable);
QObject::connect(&scene, &QGraphicsScene::sceneRectChanged,
[&](const QRectF &rect)
{
immutable.setPos(rect.center() - 0.5 * immutable.boundingRect().center());
});
for (int i = 0; i < 10; ++i) {
auto *e = new QGraphicsEllipseItem(20, 20, 50, 50);
e->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsSelectable);
scene.addItem(e);
}
view.show();
view.setTransform(QTransform::fromScale(5, 2));
return app.exec();
}