[Qt-interest] High CPU usage with QGraphicsPathItem

[Samuel Rødal]

Deepak Chandran wrote:
> I have noticed that the CPU usage goes high (rises from 0-3% to 30-50%) 
> when I have a few (~ 2-3) QGraphicsPathItem objects drawn on the scene. 
> The paths all contain beziers and their paths have been set using 
> QPathStroker. This is not normal, is it? 
> 
> When debugging, I noticed that the path graphics items are constantly 
> being redrawn, which would explain the issue. This is only happening for 
> the path graphics items and not for other graphics items that I have 
> drawn on the scene. The other graphics items are abstract shape items 
> with beziers and other complex shapes as well, but they do not redraw 
> themselves all the time. I tried overriding the boundingRect and 
> sceneBoundingRect methods of QGraphicsPathItem so that they just return 
> QRectF()  just to check whether the bounding rect is the problem. I also 
> did painter->setClipRect( option->exposedRect ) in the paint method. The 
> problem did not go away. 
> 
> Any ideas? 
> 
> Thanks.

Try enabling caching for the items with 
setCacheMode(QGraphicsItem::ItemCoordinateCache). If a pen is set on the 
path items rendering them will be more expensive as a stroke needs to be 
generated each time. You could also get around this by pre-generating 
the stroke with QPainterPathStroker and instead setting the brush of the 
items.

Regards,
Samuel