Qt: Scale polygons but not children - c++

Question

I am new to Qt. I am representing events from my application as polygons in my scene, using a custom class that inherits QGraphicsPolygomItem. The polygon dimensions are (event duration, fixed height), using 1s : 1px. Event duration can be as low as 1E-6, so I simply scale my view so that the smaller polygon is scaled up to MIN_POLY_WIDTH (10px):
view->scale(qreal(MIN_POLY_WIDTH/min_event_duration), qreal(1.0));
So far so good. However, I have a QGraphicsTextItem child for each polygon, which get stretched by the scale operation to a point they get way outside the polygon boundaries:
The text item is created as follows:
void EventPolygon::setId(QString id) {
if (!this->id) {
this->id = new QGraphicsTextItem(id, this);
} else {
this->id->setPlainText(id);
}
this->id->setPos(0, this->polygon().boundingRect().height() / 2 - this->id->boundingRect().height() / 2);
}
That function is usually called by EventPolygon constructor. I though that was the issue, since scale is done after all items are added to the scene, so it would affect the text items. So I tried calling setId after the scale operation, by iterating over all items in the scene. That way I though only the polygons would be stretched. That was not the case, and the text remained stretched.
I also tried using the following instead of scale:
QTransform t = QTransform();
t.scale(qreal(MIN_POLY_WIDTH/min_event_duration), qreal(1.0));
view->setTransform(t, false);
I thought "false" would avoid the transformation being applied to the polygons children, however it seems that is not the case. Is there any way I can scale the polygons to 10px width min and have a readable text inside them?

Answer 1

The QGraphicsScene forms a scenegraph hierarchy based upon the parent-child relationships, transformations are inherited down this hierarchy - there is no way round this (see the Transformations section here).
So to fix your issue, you will need to make the child QGraphicsTextItem invert the scaling transformation of the parent multiplied by the scaling of your view.
In fact I really recommend that you never set view transformations that are not for simulating a camera operation (pan, zoom, etc.) for this reason. I would simply allow for the seconds per pixel ratio to vary and allow the child items to be able to query this from the view - in other words have the progress items take care of their own size on screen.
That was not the case, and the text remained stretched.
Changing the transformation stack will cause a redraw, that's why it doesn't matter when you set the scale.
I thought "false" would avoid the transformation being applied to the
polygons children
No, the combine argument when false just overrides the existing transformation matrix with the one you are providing.