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);
}

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);
}

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I tried alot of other options but the image is always moving around the whole screen and this is what comes out:



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 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:

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?