Performance benefit of update() vs update(QRect) on QPixmap

[CHG1730]

Not superfamiliar with how QT works under the hood, is there an actual performance benefit to be gained from calling update with a specific region to update or is it meant as a nice to have to not overwrite the entire picture at once?

[IgKh]

Calling update with a region makes the subsequent paintEvent be limited to at least that region; any paints outside the event's region won't have any effect (though the widget can still make them). Whether that has performance benefits depends on the widget's implementation really; the QPainter calls themselves are relatively cheap, it is mostly about if it can avoid making any expensive calculations (text shaping, or iterating over large internal structures, etc) by being aware of the event bounding rectangle.

If the widget is just painting a pixmap over its' entire content area, I personally wouldn't care much. I mean, sure, if it is easy for you to determine a smaller relevant region to update when you request an update then it certainly won't hurt, but it will also probably not help too much.

[IgKh]

QPixmap has no update method, did you mean perhaps QWidget?

[CHG1730]

@IgKh Whoops, yeah I meant the QWidget, in my case the entire QWidget is a pixmap hence the confusion.

[IgKh]

Calling update with a region makes the subsequent paintEvent be limited to at least that region; any paints outside the event's region won't have any effect (though the widget can still make them). Whether that has performance benefits depends on the widget's implementation really; the QPainter calls themselves are relatively cheap, it is mostly about if it can avoid making any expensive calculations (text shaping, or iterating over large internal structures, etc) by being aware of the event bounding rectangle.

If the widget is just painting a pixmap over its' entire content area, I personally wouldn't care much. I mean, sure, if it is easy for you to determine a smaller relevant region to update when you request an update then it certainly won't hurt, but it will also probably not help too much.

[CHG1730]

@IgKh Thanks for the detailed information! I'm gonna just use the normal update() for now and maybe if I feel like hyper optimizing I'll look into setting up regions for the most intensive parts.

Again, thank you so much!