[Qt-interest] mouse and polygons

[Malyushytsky, Alex]

The easiest way to find why you get segment  fault usually is to use debugger.
At  a glance you have unsafe implementation of  addPolygons()
which assumes QVector<QPointF> &points has at least 4 points.
Similar assumptions are made in other place.

With C++ you should be careful with your assumptions.
Also I would suggest you to review documentation on function before you are using them.
For example:
     removeItem(tempItem)  will not delete item, as I think you assume each time you do it you create memory leak.

As for keeping mouse inside polygon - you can't. User is able to move mouse where he wants.
What you can do is to change way you treat the movement.

For example you could register that user started movement on mount press, collect information you need about item in temporary storage and use it until you need it (in mouseMove for example).

Alex


From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Mahendra G.R
Sent: Wednesday, November 17, 2010 1:25 AM
To: qt-interest at trolltech.com
Subject: Re: [Qt-interest] mouse and polygons

Hi all,

anybody?, i just want to know how to contain the mouse pointer within the polygon, or avoid seg fault when mouse pointer goes out of my polygon,


Regards,

On Fri, Nov 12, 2010 at 5:01 PM, Mahendra G.R <mahendra at mahendragr.com> wrote:
Hello all,

I have implemented drawing polygons in a scene using QpolygonF, QGraphicsPolygonItem and QgraphicsScene.  When i click on the scene, i collect the points and create a polygon on the double click, until here its fine.  When i move/drag the polygon it does move.  But the problem is, i have a huge data set, and when i move the polygon with this dataset in the background, polygon moves slowly, and when the cursor goes out of the polygon, i get a segmentaion fault, how should i restrict cursor to be inside the polygon item or is there a better solution for this?


My function :


void CustomScene::addPolygons( QVector<QPointF> &points )
{

        QPolygonF polygon1(points), polygon2;
        MyItem *item1;
        QPointF p1,p2,p3,p4;

        p1 = points.at(0);
        p2 = points.at(1);
        p3 = points.at(2);
        p4 = points.at(3);

        item1 = new MyItem;
        item1->setPen(QPen(Qt::red));
        item1->setPolygon(polygon1);
        item1->setAcceptDrops(true);
        item1->setFlag(QGraphicsPolygonItem::ItemIsMovable);
        item1->setFlag(QGraphicsPolygonItem::ItemSendsGeometryChanges);


        this->addItem(item1);

        points.clear();
}

void CustomScene::mousePressEvent ( QGraphicsSceneMouseEvent * event )
{



    if (addPolygonFlag == true && event->button() == Qt::LeftButton)
    {

        points.append(event->scenePos());

    }
    else if (event->button() == Qt::RightButton)
    {
        QGraphicsPolygonItem *tempItem;
        tempItem = (QGraphicsPolygonItem*)this->itemAt(event->scenePos());
        removeItem(tempItem);

    }
    QGraphicsScene::mousePressEvent(event);

}
void CustomScene::mouseDoubleClickEvent(QGraphicsSceneMouseEvent *event)
{
    addPolygons(points);
    addPolygonFlag = true;
    QGraphicsScene::mouseDoubleClickEvent(event);

}
void CustomScene::mouseMoveEvent ( QGraphicsSceneMouseEvent * event )
{

        QTextStream out(stdout);
        MyItem *tempItem;

        tempItem = (MyItem*)this->itemAt(event->scenePos());


        if (tempItem == 0)
        {
            QGraphicsScene::mouseMoveEvent(event);
            return;

        }

        tempPoly = tempItem->polygon();
        //tempPoly1 = tempItem1->polygon();

        QPointF p1,p2,p3,p4;
        p1 = tempPoly.at(0);
        p1 = tempItem->mapToParent(p1);
        p2 = tempPoly.at(1);
        p2 = tempItem->mapToParent(p2);
        p3 = tempPoly.at(2);
        p3 = tempItem->mapToParent(p3);
        p4 = tempPoly.at(3);
        p4 = tempItem->mapToParent(p4);

        out <<"p1: "<<p1.x()<<" "<<p1.y()<<endl
            <<"p2 : "<<" "<<p2.x()<<" "<<p2.y()<<endl
            <<"p3 : "<<" "<<p3.x()<<" "<<p3.y()<<endl
            <<"p4 : "<<" "<<p4.x()<<" "<<p4.y()<<endl;

        QGraphicsScene::mouseMoveEvent(event);


}

--
http://www.mahendragr.com



--
http://www.mahendragr.com


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