Question

I have a question about drawing specific arc on a scene. I have this information about arc:

Starting Koordinates,
Start Angle,
End Angle ,
Radius.

But I can’t use them efficently with QPainter. Actually I tried QPainterPath to use shape to show on QGraphicsScene with addPath("") but I can’t use function properly. My questions are about how to use this infortmation to draw arc and how to show it on my graphic scene.

Answer 1

You can use a QGraphicsEllipseItem to add ellipses, circles, and segments/arcs to a QGraphicsScene.

Try

QGraphicsEllipseItem* item = new QGraphicsEllipseItem(x, y, width, height);
item->setStartAngle(startAngle);
item->setSpanAngle(endAngle - startAngle);
scene->addItem(item);

Unfortunately, QGraphicsEllipseItem only supports QPainter::drawEllipse() and QPainter::drawPie() – the latter can be used to draw arcs, but has the side effect that there is always a line drawn from the start and the end of the arc to the center.

If you require a true arc, you can e.g. subclass QGraphicsEllipseItem and override the paint() method:

class QGraphicsArcItem : public QGraphicsEllipseItem {
public:
    QGraphicsArcItem ( qreal x, qreal y, qreal width, qreal height, QGraphicsItem * parent = 0 ) :
        QGraphicsEllipseItem(x, y, width, height, parent) {
    }

protected:
    void paint ( QPainter * painter, const QStyleOptionGraphicsItem * option, QWidget * widget) {
        painter->setPen(pen());
        painter->setBrush(brush());
        painter->drawArc(rect(), startAngle(), spanAngle());

//        if (option->state & QStyle::State_Selected)
//            qt_graphicsItem_highlightSelected(this, painter, option);
    }
};

You then still need to handle the item highlighting, unfortunately qt_graphicsItem_highlightSelected is a static function defined inside the Qt library.