[Development] QGraphicScene with thousands of QGraphicItems freezes the APP

[Samuel Rødal]

On 01/23/2013 08:02 PM, Jose wrote:
> Hello,
>
> I am developing a Qt-based application. The main part of the application
> if a QGraphicView widget
> whose scene is populated with thousands (sometimes hundreds of
> thousands) of QGraphicItems, more
> precisely a subclass because to overload the "paint" function in order
> to draw a circle for each item.

Each call to paint() involves a bit of set up / tear down of the 
QPainter state as well as the overhead of a virtual function call. You 
can get rid of some of the set up / tear down cost by setting the 
DontSavePainterState optimization flag on the QGraphicsView, but then 
you have to make sure that your items don't rely on a clean painter 
state (and that they don't change the painter state).

Also, make sure you don't have QGraphicsItem::ItemClipsToShape set on 
any item in the parent chain.

Still, it's always going to be faster to draw all the circles as a 
single QGraphicsItem.

> The problem is that the application basically freezes when I load a lot
> of items.
>
> Is this normal? Is there a limit on the number of  QGraphicItems that
> you can visualize?
>
> Also, if you want QGraphicItems to respond to a certain signal, do you
> need to connect
> the signal to an slot for each of the items?

Doesn't sound like you want to do this for a hundred thousand items. Why 
not have a controller item?

> As an alternative, I can draw all the dots as a one single QGraphicItems
> which will solve
> the problem, I guees, but I would lose the interaction with the dots
> individually.

Unless you allow interaction with the dots via that controller item.

--
Samuel