[Interest] How to change parent of scaled QGraphicsItem while retaining scene bounding rect?

[Patrick Stinson]

Thanks for that. Though I am having some trouble. If you have PyQt, I wonder what problem you see with this example:

def tests_QGI_parent_transform():
    app = QApplication(sys.argv)
    view = QGraphicsView()

    scene = QGraphicsScene()
    rect = QRectF(-50.0, -50.0, 100.0, 100.0)

    parentA = QGraphicsRectItem(rect) # red                                                                                                                                                                                                  
    parentA.setPos(-100, -100)
    parentA.setScale(.5)
    parentA.setPen(Qt.red)
    scene.addItem(parentA)

    parentB = QGraphicsRectItem(rect) # blue                                                                                                                                                                                                 
    parentB.setPos(100, 100)
    parentB.setScale(1.0)
    parentB.setPen(Qt.blue)
    scene.addItem(parentB)

    child = QGraphicsRectItem(rect) # green                                                                                                                                                                                                  
    child.setPos(-100, 100)
    child.setPen(Qt.green)
    scene.addItem(child)
    view.setScene(scene)

    buttonParent = QWidget(view)
    parentAButton = QPushButton('Set Parent A', buttonParent)
    parentBButton = QPushButton('Set Parent B', buttonParent)
    noParentButton = QPushButton('Set No Parent', buttonParent)
    ButtonLayout = QVBoxLayout(buttonParent)
    ButtonLayout.addWidget(parentAButton)
    ButtonLayout.addWidget(parentBButton)
    ButtonLayout.addWidget(noParentButton)
    buttonParent.move(10, 10)
    def setParent(parent):
        if parent == child.parentItem():
            return
        sceneTransform = child.sceneTransform() # I assume even if the child has a parent.                                                                                                                                                   
        if parent:
            inverted, ok = parent.sceneTransform().inverted()
            localTransform = inverted * sceneTransform
            print(parent.pen().color().name())
        else:
            localTransform = sceneTransform
        child.setTransform(localTransform)
        child.setScale(1.0)
        child.setRotation(0.0)
        child.setParentItem(parent)
    parentAButton.clicked.connect(lambda: setParent(parentA))
    parentBButton.clicked.connect(lambda: setParent(parentB))
    noParentButton.clicked.connect(lambda: setParent(None))

    view.resize(800, 600)
    r = scene.itemsBoundingRect()
    sceneRect = r.marginsAdded(QMarginsF(r.x() * 2,
                                         r.y() * 2,
                                         r.width() * 2,
                                         r.height() * 2))
    view.fitInView(sceneRect)
    view.centerOn(0, 0)
    view.show()
    app.exec()


tests_QGI_parent_transform()




> On Aug 7, 2018, at 10:05 PM, Christian Gagneraud <chgans at gmail.com> wrote:
> 
> On 8 August 2018 at 15:49, Patrick Stinson <patrickkidd at gmail.com> wrote:
>> Thanks for the reply. I am just particularly dense in this sort of cognitive operation (something about flipping symbolic representations like in fractional arithmetic) and might do well with just a bit of code or pseudo code. Do you think you might provide some?
> 
> I haven't done graphics stuff in a while (unfortunately), but this
> should be as simple as:
> scene_xform = item->sceneTransform();
> local_xform = next_parent->sceneTransform().inverted() * scene_xform;
> item.setTransform(local_xform);
> item.setScale(1.0);
> item.setRotation(0.0);
> item.setParentItem(next_parent);
> 
> AFAIR, QGI's rotation and scale need to be reset b/c they are combined
> with QGI's transform()
> 
> Hope this help!
> Chris
> 
>> 
>> Thanks!
>> 
>>> On Aug 7, 2018, at 6:24 PM, Christian Gagneraud <chgans at gmail.com> wrote:
>>> 
>>> On 8 August 2018 at 11:08, Patrick Stinson <patrickkidd at gmail.com> wrote:
>>>> Hello!
>>>> 
>>>> I am trying to change the parent of a QGraphicsItem without it appearing to move or change size on the scene. Both the item, old parent, and new have arbitrary scale values set. How can this be done?
>>> 
>>> If you grab the item's sceneTransform() before re-parenting, you
>>> should be able to deduce it's new local transform from it's new parent
>>> sceneTransform().
>>> 
>>> 
>>>> 
>>>> Thanks!
>>>> _______________________________________________
>>>> Interest mailing list
>>>> Interest at qt-project.org
>>>> http://lists.qt-project.org/mailman/listinfo/interest
>> 

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