Selection of lines through mouse click is not sharp in Qt

Question

In my view there are 3 lines. 1 Polyline, and 2 straight lines. I have named them as P1 and s1 and s2

I am not able to select s1 line. If I try to select it , P1 gets selected ( though I have not clicked on P1 )

Selection of p1 is also not sharp. If I clicked somewhere around P1 ( not on P1 ) still P1 gets selected.

 void Widget::on_designButoon_clicked()
  {
      // For straight line S2
      QPolygon net0;
      net0 << QPoint(50,180);
      net0 << QPoint(600,180);
      MyPoly* _poly0 = new MyPoly();
      _poly0->DrawPolyline(net0,scene);
      scene->addItem(static_cast<QGraphicsPathItem*>(_poly0));
            // Same logic for Line S1 and P1
        }
    

MyPoly.h

class MyPoly : public QGraphicsPathItem
{
    //Q_OBJECT
public:
    explicit MyPoly();
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
    void DrawPolyline(QPolygon polygon);
private:
 QPolygon polygon_;
};

MyPoly.cpp

MyPoly::MyPoly()
{}
void MyPoly::DrawPolyline(QPolygon polygon)
{
    this->polygon_ = polygon;
    QPainterPath pPath;
    pPath.addPolygon(polygon);
    this->setPen(QPen(QColor("blue"), 2));
    this->setPath(pPath);
    this->setFlag(QGraphicsItem::ItemIsSelectable);
}

void MyPoly::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
                             QWidget *widget)
{
    auto copied_option = *option;
    copied_option.state &= ~QStyle::State_Selected;
    auto selected = option->state & QStyle::State_Selected;
    QGraphicsPathItem::paint(painter, &copied_option, widget);
    if (selected) {
        painter->save();
        painter->setBrush(Qt::NoBrush);
        painter->setPen(QPen(option->palette.windowText(), 0, Qt::SolidLine));
        if(contains(currentCursorPos))
                 painter->drawPath(shape());
        painter->restore();
    }
}

bool MyPoly::contains(const QPointF &point) const
{
    for(int i=0;i < polygon_.count() - 1; i++)
    {
        QPointF firstPoint = polygon_.at(i);
        QPointF lastPoint = polygon_.at(i+1);

        if(firstPoint.x() == lastPoint.x())
        {
            qDebug()<<"Inside vertical line ";
            //It is a vertical line
            if(firstPoint.x() == point.x() &&
                    point.y() >= firstPoint.y() && point.y() <= lastPoint.y())
                return true;
        }
        else
        {
            // it is a horizontal line
            if(point.x() >= firstPoint.x() && point.x() <= lastPoint.x() &&
                    (firstPoint.y() - 3 <= point.y()) && (point.y() <= lastPoint.y() + 3 ))
                return true;
        }
    }
    return false;
 }

Answer 1

From looking at the Qt source code it appears that QGraphicsView determines which QGraphicsItem(s) are located at the clicked-on point by calling their contains() method.

Assuming you haven't re-implemented MyPoly::contains(const QPointF &) const to do something more specific, that means that Qt is calling the default QGraphicsItem::contains(const QPointF &) const method, which is implemented like this:

bool QGraphicsItem::contains(const QPointF &point) const
{
   return isClipped() ? clipPath().contains(point) : shape().contains(point);
}

... which in turn calls QPainterPath::contains(const QPointF &) const, which uses either the Non-zero winding rule or the Even-Odd rule (depending on the QPainterPath's fillRule setting) to determine if the point is in the path. In either case it looks like the algorithm is designed to detect whether the point is in the interior of a polygon defined by the path and not just "directly on the line", so if you want a more accurate algorithm for your use-case, you'll probably want to override the contains method of your subclass and implement some more precise logic there.