Position of QGraphicsItem on Scene.

[Andre008]

Hello,

im new in progamming wie Qt, so i had try some tutorials.
One of is:

http://www.bogotobogo.com/Qt/Qt5_QGr...phicsScene.php

This is the basic of my code.

What i want to do:

On the graphics view i add two ellipse (ellipse1, ellipse2) and a line.

I change the flags of the ellipses to moveable.

So now i want that the start point of the line (x1,y2) is on the center of ellipse1 and the end point of the line (x2,y2) is at the center of ellipse2.
If one ellipse move per mouse, the line should move too.

If i add the line in this way:

line = scene->addLine(ellipse1->scenePos().x(),ellipse1->scenePos().y(),
                               ellipse2->scenePos().x(),ellipse2->scenePos().y(),outlinePen);

line = scene->addLine(ellipse1->scenePos().x(),ellipse1->scenePos().y(),
                               ellipse2->scenePos().x(),ellipse2->scenePos().y(),outlinePen);

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For the scenPos() function i get 0. But i dont know why.

Did anyone has an idee ?

Thx

dialog.hdialog.cppdialog.hdialog.cpp

[anda_skoa]

Derive a custom class from QGraphicsEllipseItem.

Add a setter method that takes a line element and an indicator to which end of the line your item is connected to.

Then in the itemChange() method of your item class, you move "your" end of the line when ever the item moves.

Cheers,
_

[Andre008]

thx for the answer:

i had try the modification, on basic of:

http://www.bogotobogo.com/Qt/Qt5_QGr...phicsItems.php

A additional class customItem is inherit from QGraphicsItem.

With this approach i get now the positions on scene from ellipse1 and ellipse2.

But drawing a line is not possible until now.

Now there are different options. The line can be member of the Dialog class or is a member of the customItem class.
For the second option, it should be drawn two lines.
One from ellipses1 to ellipses2, and one from ellipses2 to ellipses1


customitem.cppcustomitem.hdialog.cppdialog.h

[anda_skoa]

My suggestion was to use a QGraphicsLineItem and adjusting its end points

Cheers,
_

[Andre008]

ok, i had tried it with:

dialog.cpp
Here i send the line pointer .

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
 
 
    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);
 
    // create our object and add it to the scene
   ellipse1 = new customItem();
   ellipse2 = new customItem();
   line = new QGraphicsLineItem(100,100,450,300);
 
 
   qDebug() << "ellipse1->scenePos().x(): " << ellipse1->scenePos().x();
 
   ellipse1->setLine(line,false);
   ellipse1->setLine(line,true);
 
   scene->addItem(ellipse1);
   scene->addItem(ellipse2);
   scene->addItem(line);
 
}

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);


    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);

    // create our object and add it to the scene
   ellipse1 = new customItem();
   ellipse2 = new customItem();
   line = new QGraphicsLineItem(100,100,450,300);


   qDebug() << "ellipse1->scenePos().x(): " << ellipse1->scenePos().x();

   ellipse1->setLine(line,false);
   ellipse1->setLine(line,true);

   scene->addItem(ellipse1);
   scene->addItem(ellipse2);
   scene->addItem(line);

}

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dialog.h

class Dialog : public QDialog
{
    Q_OBJECT
 
public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();
 
private:
    Ui::Dialog *ui;
 
    QGraphicsScene *scene;
 
    customItem *ellipse1;
    customItem *ellipse2;
 
    QGraphicsLineItem *line;
 
};
 
#endif // DIALOG_H

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    Ui::Dialog *ui;

    QGraphicsScene *scene;

    customItem *ellipse1;
    customItem *ellipse2;

    QGraphicsLineItem *line;

};

#endif // DIALOG_H

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customItem.cpp

void customItem::setLine(QGraphicsLineItem* newLine, bool side)
{
    line = newLine;
    startOrEnd = side;
}
 
void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;
    update();
    QGraphicsItem::mouseReleaseEvent(event);
 
    if(startOrEnd == false)
    {
        line->line().setLine(this->scenePos().x(),this->scenePos().y(),
                             line->line().x2(),line->line().y2() );
    }
 
    if(startOrEnd == true)
    {
        line->line().setLine(line->line().x1(),line->line().y1(),
                             this->scenePos().x(),this->scenePos().y());
 
    }
 
    qDebug()<<"xPos: " << this->scenePos().x();
    qDebug()<<"yPos: " << this->scenePos().y();
}

void customItem::setLine(QGraphicsLineItem* newLine, bool side)
{
    line = newLine;
    startOrEnd = side;
}

void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;
    update();
    QGraphicsItem::mouseReleaseEvent(event);

    if(startOrEnd == false)
    {
        line->line().setLine(this->scenePos().x(),this->scenePos().y(),
                             line->line().x2(),line->line().y2() );
    }

    if(startOrEnd == true)
    {
        line->line().setLine(line->line().x1(),line->line().y1(),
                             this->scenePos().x(),this->scenePos().y());

    }

    qDebug()<<"xPos: " << this->scenePos().x();
    qDebug()<<"yPos: " << this->scenePos().y();
}

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class customItem : public QGraphicsItem
{
public:
    customItem();
.....
 
    void setLine(QGraphicsLineItem line, bool side);
 
.....
 
public:
 
    bool Pressed;
 
    QGraphicsLineItem *line;
    bool startOrEnd;
 
};

class customItem : public QGraphicsItem
{
public:
    customItem();
.....

    void setLine(QGraphicsLineItem line, bool side);

.....

public:

    bool Pressed;

    QGraphicsLineItem *line;
    bool startOrEnd;

};

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But it dont work.

[anda_skoa]

First, I thought you wanted the line to be between the two centers, so why are you using x/y?
Also why use scenePos() instead of just the position?

The center would be just

QPointF center = rect().center();

QPointF center = rect().center();

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It would also be easier to get the updated position after it has changed, instead of manually reacting to mouse events.

Cheers,
_

[Andre008]

youre right:

i had change it to:

customItem.cpp
here ist the member variable for the center: mCenter for the ellipse

void customItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    QRectF ellipse = boundingRect();
 
    mCenter = ellipse.center();
 
    qDebug()  << "mCenter: " << mCenter;
 
    if(Pressed)
    {
        QPen pen(Qt::red, 3);
        painter->setPen(pen);
        painter->drawEllipse(ellipse);
    }
    else
    {
        QPen pen(Qt::black, 3);
        painter->setPen(pen);
        painter->drawEllipse(ellipse);
    }
}
 
 
void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;
 
 
 
    if(startOrEnd == false)
    {
       line->line().setP1(mCenter);
    }
 
 
    if(startOrEnd == true)
    {
       line->line().setP2(mCenter);
    }
 
 
    update();
    QGraphicsItem::mouseReleaseEvent(event);
}

void customItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    QRectF ellipse = boundingRect();

    mCenter = ellipse.center();

    qDebug()  << "mCenter: " << mCenter;

    if(Pressed)
    {
        QPen pen(Qt::red, 3);
        painter->setPen(pen);
        painter->drawEllipse(ellipse);
    }
    else
    {
        QPen pen(Qt::black, 3);
        painter->setPen(pen);
        painter->drawEllipse(ellipse);
    }
}


void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;



    if(startOrEnd == false)
    {
       line->line().setP1(mCenter);
    }


    if(startOrEnd == true)
    {
       line->line().setP2(mCenter);
    }


    update();
    QGraphicsItem::mouseReleaseEvent(event);
}

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but the line do not move

[anda_skoa]

If look at the method signature of QGraphicsLineItem::line(), which is generally a good idea, you will see that it returns a QLineF object.
Not a pointer, not a reference, an object.

You are modifying that object but are not storing or using it in any way.
So you are modifying a temporary object.

How would that change anything for the line item?

Cheers,
_

[Andre008]

If look at the method signature of QGraphicsLineItem::line(), which is generally a good idea, you will see that it returns a QLineF object.
Not a pointer, not a reference, an object.

You are modifying that object but are not storing or using it in any way.
So you are modifying a temporary object.

Cheers,
_

Ok, now i change the specific parts to QLineF:

customItem.cpp

I thought the storing could be made with the function setLine or how can i implement it, to assign a line to an ellipses?.

void customItem::setLine(QLineF newLine, bool side)
{
    line = newLine;
    startOrEnd = side;
}
 
void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;
 
    if(startOrEnd == false)
    {
       line.setP1(mCenter);
    }
 
 
    if(startOrEnd == true)
    {
       line.setP2(mCenter);
    }
 
    update();
    QGraphicsItem::mouseReleaseEvent(event);
}

void customItem::setLine(QLineF newLine, bool side)
{
    line = newLine;
    startOrEnd = side;
}

void customItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    Pressed = false;

    if(startOrEnd == false)
    {
       line.setP1(mCenter);
    }


    if(startOrEnd == true)
    {
       line.setP2(mCenter);
    }

    update();
    QGraphicsItem::mouseReleaseEvent(event);
}

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customItem.h

class customItem : public QGraphicsItem
{
public:
    customItem();
 
    QRectF boundingRect() const;
 
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
 
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
 
    void setLine(QLineF newLine, bool side);
 
    qreal getXPosEllipse();
    qreal getYPosEllipse();
public:
 
    bool Pressed;
 
    bool startOrEnd;
    //QGraphicsLineItem *line;
 
    QLineF line;
    QPointF mCenter;
};

class customItem : public QGraphicsItem
{
public:
    customItem();

    QRectF boundingRect() const;

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);

    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);

    void setLine(QLineF newLine, bool side);

    qreal getXPosEllipse();
    qreal getYPosEllipse();
public:

    bool Pressed;

    bool startOrEnd;
    //QGraphicsLineItem *line;

    QLineF line;
    QPointF mCenter;
};

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Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
 
 
    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);
 
    QBrush grayBrush(Qt::gray);
    QPen outlinePen(Qt::black);
    outlinePen.setWidth(2);
 
    // create our object and add it to the scene
 
 
    ellipse1 = new customItem();
    ellipse2 = new customItem();
    line = new QGraphicsLineItem(100,100,450,300);
 
    ellipse1->setLine(line->line(),false);
    ellipse2->setLine(line->line(),true);
 
 
   qDebug() << "ellipse1->getXPosEllipse(): " << ellipse1->getXPosEllipse();
   qDebug() << "ellipse1->getYPosEllipse(): " << ellipse1->getYPosEllipse();
   scene->addItem(ellipse1);
   scene->addItem(ellipse2);
   scene->addItem(line);
 
 
 
   qDebug() << "ellipse1->getXPosEllipse(): " << ellipse1->getXPosEllipse();
   qDebug() << "ellipse1->getYPosEllipse(): " << ellipse1->getYPosEllipse();
 
}

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);


    scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(scene);

    QBrush grayBrush(Qt::gray);
    QPen outlinePen(Qt::black);
    outlinePen.setWidth(2);

    // create our object and add it to the scene


    ellipse1 = new customItem();
    ellipse2 = new customItem();
    line = new QGraphicsLineItem(100,100,450,300);

    ellipse1->setLine(line->line(),false);
    ellipse2->setLine(line->line(),true);


   qDebug() << "ellipse1->getXPosEllipse(): " << ellipse1->getXPosEllipse();
   qDebug() << "ellipse1->getYPosEllipse(): " << ellipse1->getYPosEllipse();
   scene->addItem(ellipse1);
   scene->addItem(ellipse2);
   scene->addItem(line);



   qDebug() << "ellipse1->getXPosEllipse(): " << ellipse1->getXPosEllipse();
   qDebug() << "ellipse1->getYPosEllipse(): " << ellipse1->getYPosEllipse();

}

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[anda_skoa]

That doesn't even make sense. Now you not even attempting to update the line item anymore.

The ellipse items need to update the position of the line item, so they need the line item.

But instead of your previous attempt of changing some random temporary line value, you need to update the line item's value.

Lets have a look at some C++ basics

int a = 5;
int b = a;
b = 3;
// which value does a have?

int a = 5;
int b = a;
b = 3;
// which value does a have?

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Cheers,
_

[Andre008]

That doesn't even make sense. Now you not even attempting to update the line item anymore.

The ellipse items need to update the position of the line item, so they need the line item.

But instead of your previous attempt of changing some random temporary line value, you need to update the line item's value.

Lets have a look at some C++ basics

int a = 5;
int b = a;
b = 3;
// which value does a have?

int a = 5;
int b = a;
b = 3;
// which value does a have?

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Cheers,
_

My idee was to make something like this:

int a=5;
int *b;
 
b= &a;
*b = 3;

int a=5;
int *b;

b= &a;
*b = 3;

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For this reason i create a local variable line in the class custonItem.