How QScrollArea repaints QWidget on it?

qwe3

Hi,
I have QScrollArea and one big widget ( inherits QWidget ) on it, like this:

    area = new QScrollArea(this);
    bigWidget = new bigWidgetClass;
    bigWidget->setFixedSize(5000,5000);
    area->setWidget(bigWidget);

In bigWidget I draw numbers, like this:

void bigWidgetClass::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    auto reg = visibleRegion();

    for(int i=0; i<400; i++)
    {
        for(int j=0;j<400;j++)
        {
            if(reg.contains(QPoint(j*30,i*30)))
            {
                painter.drawText(j*30,i*30,QString::number((i)%10000));
            }
        }
    }
}

I check if point is visible - drawText. If not - don't drawText: if(reg.contains(QPoint(j*30,i*30)))
I would like not to draw text, which I will don't see on screen, because I would like to have faster application.

When I change QScrollArea's ScrollBar value using mouse I don't see all text pixels:

When I minimize application, and next show it - everything is ok ( I see all text pixels ).

So how QScrollArea repaint screen? Using repaint(QRegion) ?

mpergand

I think you have to test that the visibleRegion and the rect occupied by your circle, text or whatever don't intercept:
see: intersects(const QRect &rect) const

mpergand

I think you should test the text rectangle occupied by the number instead. see QFontMetrics.

qwe3

@mpergand The texts is only example. In my real application I have circles, rects etc. And I don't understand when the text is in the center of visible region - I don't see pixels too...

mpergand

I think you have to test that the visibleRegion and the rect occupied by your circle, text or whatever don't intercept:
see: intersects(const QRect &rect) const

mpergand

Quick dirty example:

class BigWidget : public QWidget
{

    void paintEvent(QPaintEvent *event)
    {
       QPainter painter(this);
       auto reg = visibleRegion();
       int w=80;
       QSize s{w,w};
       QPoint pv=reg.boundingRect().topLeft();
       QRect rv(pv,s);
       
       while(reg.intersects(rv))
           {
           painter.drawRect(rv);
           painter.drawText(rv, Qt::AlignCenter,QString("%1,%2").arg(rv.x()).arg(rv.y()));
         
           QPoint ph=pv+QPoint(w,0);
           QRect rh=QRect(ph,s);
           
           while(reg.intersects(rh))
            {
            qDebug()<<rh;
            painter.drawRect(rh);
            painter.drawText(rh, Qt::AlignCenter,QString("%1,%2").arg(rh.x()).arg(rh.y()));
            ph+=QPoint(w,0);
             rh=QRect(ph,s);  
            }
           
           pv+=QPoint(0,w);
           rv=QRect(pv,s);
           }      
    }
};