Z-order of items

Have you wondered what happens when multiple items are painted in the same area of the scene? Let's try to do this:

QGraphicsEllipseItem *item1 = scene.addEllipse(0, 0, 100, 50);
item1->setBrush(Qt::red);
QGraphicsEllipseItem *item2 = scene.addEllipse(50, 0, 100, 50);
item2->setBrush(Qt::green);
QGraphicsEllipseItem *item3 = scene.addEllipse(0, 25, 100, 50);
item3->setBrush(Qt::blue);
QGraphicsEllipseItem *item4 = scene.addEllipse(50, 25, 100, 50);
item4->setBrush(Qt::gray);

By default, items are painted in the order they were added, so the last item will be displayed in front of the others:

However, you can change the z-order by calling the setZValue() function:

item2->setZValue(1);

The second item is now displayed in front of the others:

Items with a higher z value are displayed on top of the items with lower z values. The default z value is 0. Negative values are also possible. If items have the same z value, the order of insertion decides the placement, and items added later overlap those added earlier.

Ability to change the z-order of items is very important when developing 2D games. Any scene typically consists of a number of layers that must be painted in a specific order. You can set a z value for each item based on the layer this item belongs to.