How to rescale GraphicItems without disturbing the shape? - c++

Question

How to rescale GraphicItems without disturbing the shape?
I am trying to rescale QGraphicsItems from a part of a scene. When I rescale the QGraphicsItem it transforms the shape.
QList<QGraphicsItem*> items;
for (auto it : items)
{
qreal scale = 1.75;
QPointF c = it->mapToScene(it->boundingRect().center());
it->setScale(scale);
QPointF cNew = it->mapToScene((it->boundingRect()).center());
QPointF offset = c - cNew;
it->moveBy(offset.x(), offset.y());
}

Answer 1

Create QGraphicsItemGroup which is containing all your items and apply the transform on it.
You just have to recenter the group according to the previous position.
QList<QGraphicsItem*> makeItems()
{
return QList<QGraphicsItem*>() << new QGraphicsLineItem(0, 0, 50, 0)
<< new QGraphicsLineItem(50, 0, 50, 50)
<< new QGraphicsLineItem(50, 50, 0, 50)
<< new QGraphicsLineItem(0, 50, 0, 0)
<< new QGraphicsRectItem(24, 24, 2, 2)
<< new QGraphicsEllipseItem(0, 0, 50, 50);
}
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QApplication app(argc, argv);
QGraphicsScene* scene = new QGraphicsScene(-500, -500, 1000, 1000);
auto* view = new QGraphicsView;
view->setScene(scene);
QList<QGraphicsItem*> items, originalItems;
originalItems = makeItems();
items = makeItems();
QGraphicsItemGroup* itemGroup = new QGraphicsItemGroup;
for (auto* item: originalItems)
{
scene->addItem(item);
}
for (auto* item: items)
{
itemGroup->addToGroup(item);
}
scene->addItem(itemGroup);
qreal scale = 1.75;
QPointF const beforeScale = itemGroup->mapToScene(itemGroup->boundingRect().center());
itemGroup->setScale(scale);
QPointF const afterScale = itemGroup->mapToScene(itemGroup->boundingRect().center());
QPointF const offset = beforeScale - afterScale;
itemGroup->moveBy(offset.x(), offset.y()); // The center will be the same than before the scaling
view->show();
return app.exec();
}
Use QPen::setCosmetic to keep the look-and-feel before and after the transformation