QGraphicsView scale question.

[hickscorp]

Hello to everyone,

Here is what i have:
- a QDialog, containing a QGraphicsView.
- The View is "containing" a Scene, in which i only put a QImage.
- In the QDialog, there is a Zoom out button which scale(.5, .5) on the View, and another Zoom in button which scale(2, 2) the View.

Now i wish to add another widget on the graphics view, which is on top of all others (The QImage), but which doesnt "scales". What is the best thing to do to achieve this?
In other words: do i have to only scale the QImage (So the other widget doesnt scales)? If yes, how?

Thanks a lot!
Pierre.

[hickscorp]

Hello again,

I just figured that i am able to scale directly on a QGraphicsItem... which is very good for me, because it solves my problem.

However i have a related question. Let's take for instance a QGraphicsItem, which originaly was containing a QImage. After i retrieve the QGraphicsItem with QGraphicsView::items(), how can i get a pointer to the QImage it contains, so i can change it's contents without putting a new QPicture each time?

Thanks,
Pierre.

[marcel]

Do you use QGraphicsPixmapItem to display the pixmap?
If you do, you can use the pixmap() and setPixmap() to get/set the pixmap...

QGraphicsView::items() will return a list of QGraphicsItem. You will have to do a cast to QGrasphicsPixmapItem.

Regards.

[hickscorp]

Hello Marcel, and thanks for answering.

When adding the item to my scene, i simply do a addItem(myQImage);
Do you confirm that this QImage is then put in a QGraphicsPixmapItem?

Pierre.

[marcel]

Yes, it will add a QGraphicsPixmapItem since it is the only item that can hold a pixmap.

Why do you use a QImage and why not use addPixmap() ?

[hickscorp]

Ok, so it's impossible that if i change the QImage, i see the changes in the scene right?
Or does the QGraphicsPixmap work with a pointer to my QImage? oO

[marcel]

No, it passes a reference to the constructor of QGraphicsPixmapItem and then the operator= of QPixmap will copy your original pixmap into a QGraphicsPixmapItem.

Just checked it in the sources..

[marcel]

Hey, have you actually implemented this?

Because:

QImage img;
scene->addItem( img );

QImage img;
scene->addItem( img );

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will not even compile.

Try with QPixmap and addPixmap. Or just construct a QPixmapGraphicsItem from a QPixmap and just use addItem to add it to the scene.

Regards

[hickscorp]

Ouch

So all this means, if i just constructed a QImage of 1600x1200, then the QGraphicsPixmapItem implicitely created when calling addItem(myQImage) will hold another copy of my image, i'm glad i have 1Gb ram then

[EDIT:]You're right, what i'm actually doing is grphScToFill->addPixmap(QPixmap::fromImage(cImage, Qt::ColorOnly | Qt::ThresholdDither));

Now another simple question since you seem to know pretty much about the QGraphics framework:
Previously, when the user pressed the zoom +/- buttons, i was scaling the scene. It was very nice, since the point where the view were focused was staying on the center after the scale process. Also, when a zoom level like 0.5 was hit, the scene content was centered within the rectangle of my view...

Now what i am doing, is to scale directly the graphic item instead of the whole scene... But i have a problem doing this: when i scale to a width / height lower than the visible rect, the item is put on the top left corner, and the scene content is still "draggable" to much more top left...

So i have tried to setPos on my item each time i resize it, it doesnt moves in scene. I also tried to change the sceneRect to fit the item, but i wasnt able to get the item size...

What am i doing / understanding wrong?

Thanks ^^

[marcel]

Now another simple question since you seem to know pretty much about the QGraphics framework:

Not really that much...

Use void QGraphicsView::centerOn ( const QPointF & pos ).
As pos pass the center of your QGraphicsPixmapItem. Pos is scene coords.

As an alternative, use void QGraphicsView::ensureVisible ( const QRectF & rect, int xmargin = 50, int ymargin = 50 ).
rect is scene cords.

Regards.

[hickscorp]

Hello again

Well i'm sorry, but i'm not sure i understand how to do what you recomend
When i call centerOn(), i have to give some coordinates to the method. Do i have to give the coordinates of the center of my scalled Item as you say: let's take as example my item is scalled down to 0.5 two times, do i have to take my QGraphicsPixmapItem width / height, divide it by 4 (Get item's size in view local coords system after scale), divide the result by 2 (For the center), and then give to centerOn the result added to the position of my item in the scene?

I tried several methods, for you i have made some screenshots of what i get.
On the pictures, you can clearly see that after the item is resized, it is "stuck" to top left corned, and is still draggable (But not to the center).

Anyway thanks, maybe if i try various stuff i'll find why i'm wrong
Pierre.

[hickscorp]

I just have tried this (Note that iZoomFactor is an int, which is 100 for a scale of 1, 50 for a scale of .5 etc):

pPicture	= (QGraphicsPixmapItem*)grphVwOriginalPicture->items().first();
	pPicture->scale(.5, .5);
	QPoint	cCenterOn;
	// [EDIT:] Here i get the size of the item in the view coords system, divide it by 2 (For the center) and add it's position.
	cCenterOn.setX( (100/pPicture->pixmap().width()*iZoomFactor) / 2 + pPicture->pos().x() );
	cCenterOn.setY( (100/pPicture->pixmap().height()*iZoomFactor) / 2 + pPicture->pos().y() );
	grphVwOriginalPicture->centerOn(cCenterOn);

pPicture	= (QGraphicsPixmapItem*)grphVwOriginalPicture->items().first();
	pPicture->scale(.5, .5);
	QPoint	cCenterOn;
	// [EDIT:] Here i get the size of the item in the view coords system, divide it by 2 (For the center) and add it's position.
	cCenterOn.setX( (100/pPicture->pixmap().width()*iZoomFactor) / 2 + pPicture->pos().x() );
	cCenterOn.setY( (100/pPicture->pixmap().height()*iZoomFactor) / 2 + pPicture->pos().y() );
	grphVwOriginalPicture->centerOn(cCenterOn);

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But the same problem is here... I'm not sure of what i can try next, but for now i'm stuck.

[EDIT:]Attached is what i want, it's achieved by scaling the whole scene instead of the item. But i cant keep scaling the whole scene, since i'll have to put another item in it which has to not be scaled.

Pierre.