How QPainter repaint viewport() and how to optimize it?

qwe3

Hi,

I have QScrollArea and big widget on it, like this:

MainWindow:

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    myBigWidgetObject = new myBigWidget(this);

    scrollArea = new QScrollArea(this);

    scrollArea->resize(500,500);
    scrollArea->setWidget(myBigWidgetObject);
}

big widget:

myBigWidget::myBigWidget(QWidget *parent): QWidget(parent)
{
    setFixedSize(5000,5000);
}

void myBigWidget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);

    for(int i=0; i<100; i++)
    {
        painter.drawRect(i*100,i*100,50,50);
    }

    for(int i=0; i<100; i++)
    {
        painter.drawEllipse(5000-i*100,i*100,50,50);
    }
    ...
}

In my real app I have bigger widget and more expensive operations in paintEvent() than in this example above.

I have QTimer, which very often scrolling myBigWidgetObject and do repaint() on this object ( I have many if's statements in paintEvent() ).

And my questions are:

What does paint do, when I call repaint()? Is it draw the whole widget or only what I see on the screen ( viewport )?
What does paint do, when I call change value on scrollBar in QScrollArea? Is it draw the whole widget or only what I see on the screen ( viewport )?
What is more expensive: call something like painter->drawRect(.....) or really draw this rect on the screen?

I would like to optimize this drawing because I draw this widget many times.

I see something like QRegion. Maybe this is a good idea?

Christian Ehrlicher

Draw the stuff outside your paintEvent() and only blit the QImage/QPixmap in the paintEvent()