QGraphicsPixmapItem rotate/scale around center of pixmap

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MadBear

Beginner

Posts: 3

Tuesday, December 14th 2010, 8:52am

QGraphicsPixmapItem rotate/scale around center of pixmap

Greetings,
I have a problem with rotation and scale of QGraphicsPixmapItem. The center of rotation/scale is top left corner of pixmap. I would like to move the center of rotation/scale to center of pixmap. I have tried with translate(x,y).scale(sx,sy).translate(-x,-y), but this does not work well with scenePos. When I use setPos the new position is not correct (I saved scene position of the item into file and then I load it again). I have also tried to replace this with setTransformOriginPoint(x,y) but it works like I did nothing(center of scale is still top left corner). What am I doing wrong? Please help. It is driving me insane.
Thank you.

Regards,
MadBear

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Loko Luke

Beginner

Posts: 42

Location: Scotland

Occupation: Software Engineer

Tuesday, December 14th 2010, 11:51am

Hi MadBear,

Would you be able to put up the code of where you create the pixmap item and where your setting it's position.

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MadBear

Beginner

Posts: 3

Wednesday, December 15th 2010, 7:27am

Greetings,

Well it is prety simple. I will try to provide basic example of what I am trying to achieve.
1. Creation of item

	Source code
1 2 3	QGraphicsPixmap *pix=new QGraphicsPixmap(); pix->setPixmap(somePixmap); scene->addItem(pix);

2. Mouse wheel event on scene

Source code

1
2
3

resetTransform();
QRectF brc=boundingRect();
setTransform(QTransform::fromTranslate(-brc.width()/2.0, -brc.height()/2.0)*QTransform::fromScale(sx, sy).rotate(alpha)*QTransform::fromTranslate(brc.width()/2.0, brc.height()/2.0), true);

3. Do something with scene position of item

Source code

    QList<QGraphicsItem*> sel=selectedItems();
    int i;
    for(i=0;i<sel.count();i++)
    {
        QPointF xy=sel[i]->scenePos();
        sel[i]->setPos(xy.x(),xy.y()-tranStep);    //here the position should only change for tranStep but it does not (the change is much larger)
    }

I hope that this explains basic idea of what I would like to do and where is the problem. Please help. Like I said it is driving me insane.
Thank you.

Regards,
MadBear

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Loko Luke

Beginner

Posts: 42

Location: Scotland

Occupation: Software Engineer

Wednesday, December 15th 2010, 4:28pm

Hi MadBear,

Try setting the position of the pixmap when it is first created also.

QGraphicsPixmap *pix=new QGraphicsPixmap();
pix->setPixmap(somePixmap);
pix->setPos(100,100); // or pix->setPos(event.scenePos()) if you want to use a mouse click event
scene->addItem(pix);

I think the problem is that your pixmap does not have a position set orignally.

Also try use translate(0, - tranStep) instead of setpos again.

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