Dawid Sip wrote:
> Hi,
> Im still doing my Polygon Path and I came across a following problem,
> I'm guessing this could be a bug. Inside of MyGraphicsView class i
> create a MyPolygon instance. MyPolygon class has an inner class
> definition of MyGraphicsEllipseItem which I use to visualize control
> points (vertices of the polygon). Now, when I press a mouse button over
> such ellipse the mousePressEvent is called and in this method I want to
> retrieve this ellipse item's position and print it in the console
> ->System.out.println("X "+this.scenePos().x()+" Y
> "+this.scenePos().y());. Unfortunately i allways get X 0 Y 0 although
> ellipses are placed in different positions then (0,0) in the scene. I
> think I tryed everything by now. I can easily overcome this problem by
> adding the position variable to the MyGraphicsEllipseItem where I keep
> the postion but I don't like to waste resources so if You could please
> explain why this doesn't work, I would be thankfull.
Hi Dawid,
This is not so much a bug, as it is a very common misconception ;-) The
graphics items have a geometry and an anchoring position. The geometry
is the shape of the item, in this case an ellipse located at:
> new
> MyGraphicsEllipseItem(pos.x()-8, pos.y()-8, 15, 15, this.parent,
The thing to realize is that this position is relative to the items
coordinate system. The shape and position of the items coordinate system
is set by its position, accessible via pos() or scenePos() depending on
relative to what you want, and its transformation matrix, accessible via
transform().
I often find it convenient to position the items geometry around the
origin and instead move the item to where I want it: e.g.:
Item i = new EllipseItem(-8, -8, 16, 16);
i.move(pos.x(), pos.y());
That will also give you the expected behaviour of pos() and scenePos()
aftwards.
best regards,
Gunnar
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