Question

I have a QGraphicsScene where I initially drew the background in the drawBackground() function. However, this required quite a few calculations and turned out to be pretty slow so I created a bunch of items instead. This had the expected speedup.

My question: is there a way to treat these items as the background? Would it even matter if I treated them as background items?

Thanks

Answer 1

There are three layers in terms of QGraphicsScene (see Qt docs):

• QGraphicsScene::ItemLayer

The item layer. QGraphicsScene renders
all items are in this layer by calling
the virtual function drawItems(). The
item layer is drawn after the
background layer, but before the
foreground layer.

• QGraphicsScene::BackgroundLayer

The background layer. QGraphicsScene
renders the scene’s background in this
layer by calling the virtual function
drawBackground(). The background layer
is drawn first of all layers.

• QGraphicsScene::ForegroundLayer

The foreground layer. QGraphicsScene
renders the scene’s foreground in this
layer by calling the virtual function
drawForeground(). The foreground layer
is drawn last of all layers.

Threrefore, there is no legal way to put an item into the background layer.
However, you could use QGraphicsItem Sorting to place some items behind others, making them to appear as a background.