Rendering rectangles speed issue

[Boron]

Some years ago, during my studies, I took over a student project, written in C++/Qt-3.
It was a simple user interface, whose main part was a simplified soccer field (a bitmap of about 500x300 pixels). Players were also drawn pixel-wise. Well, not pixel by pixel but in bitmap style, somehow.
A robot soccer game was shown with 3 vs. 3 players.
And the whole picture was flickering and awful slow. CPU utilisation was about 95% on an 1.5Ghz machine. Just für six players (shown as coloured circles) and on white filled circle as ball.

I decided to rewrite some code.
QCanvas of Qt3 was the "chosen one" for presenting/drawing the game.
After I did this the CPU utilisation was ridiculous low. Everything went smooth. And it even looked better .

Long story short:
Use QGraphicsScene, which replaces the QCanvas from Qt3 in Qt4.

[macbeth]

Thanks for your answer!

I'm just thinking about rewriting some code to see how would it behave with QGraphicsScene, it looks like it has many good features and supports printing and rendering to QPaintDevice, which is crucial for me...

[macbeth]

Hm, another strange thing that showed up: when I add 250 000 small (5x5pix) rectangles on the GraphicsScene and then want to show it using GraphicsView, the cpu 'user' load rises to ~90%. Not just during plotting, but for all the time after the rectangles are drawn. That's looks quite strange to me... (If I draw ~50 000 rectangles, the cpu usage is about 50%)

Does anybody know, what is GraphicsView doing in the background? Still updating the view?
I'm quite disappointed, because they say in Qt docs:

Graphics View uses a BSP (Binary Space Partitioning) tree to provide very fast item discovery, and as a result of this, it can visualize large scenes in real-time, even with millions of items.

But how can one use it, if the cpu load is 90% :/

Well, maybe I've made a mistake somewhere, but I just tried to edit one of the examples (portedcanvas in examples/graphicsview) and instead of adding one rectangle I added many of them, so I don't know what's wrong...

Any advice appreciated, thanks...