Rotate QGraphicsPixmapItem

[durbrak]

Okay, I've been sitting on this issue for at least 4 hours now and just am exhausted.
I'm trying to simply rotate a QGraphicsPixmapItem by 45° around its own axis.


MyPixmap::MyPixmap() : QGraphicsPixmapItem()
{
this->setPixmap(QPixmap("wallpaper.jpg"));

QTransform t = this->transform();

t.translate(this->pixmap().width()/2, this->pixmap().height()/2);
t.rotate(45, Qt::XAxis);
t.translate(-this->pixmap().width()/2, -this->pixmap().height()/2);

this->setTransform(t);
}I tried alot of other options but the image is always moving around the whole screen and this is what comes out:

http://img292.imageshack.us/img292/5521/capturenm9.th.jpg (http://img292.imageshack.us/my.php?image=capturenm9.jpg)

Shouldn't the lower edge be large and the upper edge small, like a trapezoid? Anyway, that's what I want it to be but it seems like I'm missing something here :(

[marcel]

Why not use QGraphicsItem::rotate()?
You can also do translations with the QGraphicsPixmapItem::translate();

Be careful:
this->pixamp->width() and height() are in local item coordinates.
You should use boundingRect() and translate to scene coords, and make the rotation from the scene, not from the item...

[durbrak]

QGraphicsItem::rotate() only accepts one parameter (the degrees), so I can't find a way to tell it on which axis I want to rotate it on.

And from translate() doc:

Translates the current item transformation by (dx, dy).
So I think the local item coods are enough.

[jpn]

Maybe this article (http://labs.trolltech.com/blogs/2006/11/30/zoom-and-rotate-on-mouseover-effect-in-graphics-view/) will help?

[durbrak]

Thank you jpn, but I've already read that article and tried lots of examples from TT Labs but they all only work for the usual Z-Axis (which is not a problem and not really hard).

[marcel]

The QGraphicsScene class provides a surface for managing a large number of 2D graphical items.

Keyword: 2D.

I believe you'll have to do more computations than that to get a rotation around X. Like obtaining a projection on XY plane of the rotated image rect and then map the image on the resulting shape. Then you'll get a trapezoid.

[durbrak]

Well, I know. But I assumed since it also just uses QPainter with QTransform that wouldn't be a problem.
TT Labs also created examples to achieve that:

http://labs.trolltech.com/images/e/e1/Qtransform.png
http://labs.trolltech.com/images/4/43/Qtransform2.png
http://labs.trolltech.com/images/d/db/Browser.png
I looked into it but they all don't use QGraphicsView, they just use QWidget and paint the items on it with QPainter and QTransform (with similar code to mine above).

[marcel]

Are you sure they don't do any extra things to the pixmaps once they have the transforms?