Selection of lines through mouse click is not sharp in Qt

[tushu]

@JonB I understood the problem exactly.
While selecting any of the overlapping lines, the one with bounding rect on top is selected. And this is the exact my problem.
For that I need to override shape() ( you were right as usual ).
I googled and I got a few code samples.

QPainterPath Item::shape() const
{
    QPainterPath path;
    QPolygon polygon;
    polygon << QPoint(0, 0);
    polygon << QPoint(5, 5);
    polygon << QPoint(width, height);
    polygon << QPoint(width - 5, height - 5);
    path.addPolygon(polygon);

    return path;
}

QPainterPath Line::shape()
{
    QRectF rect(start_p, end_p).normalized();

    // increase the rect beyond the width of the line
    rect.adjust(-2, -2, 2, 2); 

    QPainterPath path;
    path.addRect(rect);

    return path;    // return the item's defined shape
}

So here the question is, how will I know start_p, end_p ( starting and ending points of a line ) width, height ?

[JonB]

@tushu
I'm not at all familiar, never done this sort of thing, so limited point asking me about it. You seem to draw it in DrawPolyline()? So isn't that the information you need for this Line::shape()?

[mchinand]

Have your sub-classed QGraphicsItem keep track of them. Store the points and/or polygon to a member variable. Then you can use them in your custom shape() method.

[tushu]

@mchinand I am following below approach. So not understanding how to keep remeber co-ordinates in a variables.

    QPolygon net;
    net << QPoint(67,202);
    net << QPoint(120,202);
    net<< QPoint(120,67);
    net <<QPoint(320,67);
    net <<QPoint(320,202);
    net <<QPoint(520,202);
    net <<QPoint(520,67);
    QPainterPath pPath;
    pPath.addPolygon(net);
    MyPoly* _poly = new MyPoly();
    _poly->DrawPolyline(pPath);
    scene->addItem(static_cast<QGraphicsPathItem*>(_poly));

[mchinand]

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

void MyPoly::DrawPolyline(QPolygon polygon)
{
    polygon_ = polygon;
    QPainterPath pPath;
    pPath.addPolygon(polygon);
    this->setPen(QPen(QColor("red"), 2));
    this->setPath(pPath);
    this->setFlag(QGraphicsItem::ItemIsSelectable);
    //this->setBoundingRegionGranularity(1.0); this has no effect

}

    QPolygon net;
    net << QPoint(67,202);
    net << QPoint(120,202);
    net<< QPoint(120,67);
    net <<QPoint(320,67);
    net <<QPoint(320,202);
    net <<QPoint(520,202);
    net <<QPoint(520,67);
    MyPoly* _poly = new MyPoly();
    _poly->DrawPolyline(net);

[tushu]

@mchinand Thank you for your reply.
I have a doubt :

1. QPolygon has many points ( in above example it has 7 points ) How the polygon gets drawn ? Segment by segment ? Means 1st 2 points are taken QPoint(67,202) and QPoint(120,202) and then line is drawn. Then next 2 points.
2. In shape() , how am I going to access all these points ?

[mchinand]

I thought your code already worked for drawing, is that not the case? I thought you just needed to figure out how to use the same points that are used for drawing the polyline in the custom shape() method as well.

[JonB]

@mchinand Yeah that's what I thought too....

[tushu]

@mchinand Yes, my code is working for drawing. But all these concept ( QPolygon, QPainterPath, QPainterPathStroker, shape() , boundingRect() are new to me. ) I just learned how to extract points from QPolygon. Qt is new to me. I am learning from taking help from people like you.
Sorry if you get irritated. But this is how the inexperienced people learn.

P.S. Just few days ago I learned the concept behind paint() from JonB.

[JoeCFD]

you may try to inherit QGraphicsPolygonItem and check some examples out.