[Qt-interest] QGraphicsItem::setTransformOriginPoint BUG

[Malyushytsky, Alex]

Please use mailing list to ask questions instead of sending e-mails directly.

>> Would I do this before or after I have transformed the item? I tried playing around with this some, but no good results so far.

I have not used QGraphicsItem for a while, so I suggested you to find and an answer - the ways I could suggest probably are obsolete.
For example scale() as I believe is obsolete in Qt 4.6.
More of that newer Qt versions might give you better way to achieve what you want.
For example:
 - setting transformOriginPoint to middle point of pixmap before scaling may be just what you need:
void QGraphicsItem::setTransformOriginPoint ( const QPointF & origin )

- or using transform specifying center of transformation instead of calling scale() as in your example:

 // Scale an item by 3x2 from (x, y)
 item->setTransform(QTransform().translate(x, y).scale(3, 2).translate(-x, -y));


Alex


From: Cole, Derek [mailto:dcole at integrity-apps.com] 
Sent: Tuesday, March 15, 2011 12:26 PM
To: Malyushytsky, Alex
Subject: RE: [Qt-interest] QGraphicsItem::setTransformOriginPoint BUG

Would I do this before or after I have transformed the item? I tried playing around with this some, but no good results so far.


-----Original Message-----
From: qt-interest-bounces+dcole=integrity-apps.com at qt.nokia.com on behalf of Malyushytsky, Alex
Sent: Fri 3/11/2011 7:50 PM
To: qt-interest at qt.nokia.com
Subject: Re: [Qt-interest] QGraphicsItem::setTransformOriginPoint BUG

QGraphicsItem is scaled relative to the item local coordinate system origin.
If it is in the top left corner (which I believe is normal for some of the item types like QPixmapGraphicsItem ) you get what you observe.
In case of the above you may try to play with setOffest()

Alex


From: qt-interest-bounces+alex=wai.com at qt.nokia.com [mailto:qt-interest-bounces+alex=wai.com at qt.nokia.com] On Behalf Of Cole, Derek
Sent: Thursday, March 10, 2011 2:19 PM
To: qt-interest at qt.nokia.com
Subject: [Qt-interest] QGraphicsItem::setTransformOriginPoint BUG



Hello,

I have created my own class by extending QGraphicsItem and I want to make it so that when someone does a wheel even while over this item, it scales.

This way, i can have multiple items in a scene, and scale each of them in and out as I please.

The problem is, I want the item to scale under the mouse cursor, much like google maps does. That is, a move forward will keep panning the image and scaling it, so taht the area in the vicinity around my mouse pointer is always in view.

void ImagePixmapItem::wheelEvent ( QGraphicsSceneWheelEvent * event ){

    update();
    qreal factor = 1.2;
    if (event->delta() < 0)
      factor = 1.0 / factor;

    scale(factor, factor);

    scaleFactor *=factor;
    this->scene()->setSceneRect(0,0,this->boundingRect().width(), this->boundingRect().height());

}

This is the code I am using to do the scale. The problem is, it always seems to be scaling from the top left corner. Well, this is undesirable, beacuse if I scale in or out too much, eventually my  area of interest around the mouse pointer has moved off the screen, and I have to either scroll manually or pan to the location, zoom, pan, etc, until i get to the desired level.

I tried to use the  QGraphicsItem::setTransformOriginPoint, but no matter what values I put in there, it still seems to scale and such from the top left.

What can I add to that code I posted to get the desired effect?


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