QT之二维绘图：场景坐标

场景坐标的原点在场景中心

QGraphicsScene scene;
    scene.addLine(QLineF(0, 0, 50, 0));
    scene.addLine(QLineF(0, 0, 0, 50));
    scene.addLine(QLineF(0, 0, -50, 0));
    scene.addLine(QLineF(0, 0, 0, -50));
    scene.addEllipse(QRectF(0, 0, 50, 50));
    scene.addRect(QRectF(0, 0, 25, 25));
    
    QGraphicsView view(&scene);  //必须将scene放到view中，不然不能看到图形
    view.show();