How to time QPixmap::scaled?

Eligijus

Hello,

Is there an easy way to check/print how long QPixmap::scaled scaling with painting took?

raven-worx

Eligijus

@raven-worx QElapsedTimer on it's own does not give any meaningful results because of Qts Event system. I can clearly see that scaling takes longer than 1 ms but printing QElapsedTimer shows 0.

raven-worx

@Eligijus
can you please show how you used it.

Eligijus

@raven-worx

label->setPixmap(myPixmap);
label->setScaledContents(true);
QElapsedTimer timer;
timer.start();
label->resize(size);
qDebug() << timer.elapsed();

J.Hilk

@Eligijus
I would do that slightly differen.

//QElapsed timer as member and reimplementing a virtual function
//In QLabel subclass
public:
void startTimer(){eTimer.start();}

protected:
QElapsedTimer eTimer;

virtual void resizeEvent(QResizeEvent * event){
   Q_Unused(event);
   qDebug() << eTimer.elapsed();
}

//in your cpp
label->setPixmap(myPixmap);
label->setScaledContents(true);
label->startTimer();
label->resize(size);

resizeEvent is called after the Widget has the new size e.g it should be painted than.

raven-worx

@Eligijus said in How to time QPixmap::scaled?:

@raven-worx

label->setPixmap(myPixmap);
label->setScaledContents(true);
QElapsedTimer timer;
timer.start();
label->resize(size);
qDebug() << timer.elapsed();

exactly this won't work, because of the resize and paint event being executed in the next event loop iteration (after your timer.elapsed() call).

Try the following to force a paint:

label->setPixmap(myPixmap);
label->setScaledContents(true);
QElapsedTimer timer;
timer.start();
label->resize(size);
label->repaint();
qDebug() << timer.elapsed();

Alternatively sub-class QLabel and simply override the paintEvent() like this:

void MyLabel::paintEvent(QPaintEvent* event)
{
   QElapsedTimer timer;
   timer.start();
   QLabel::paintEvent(event);
   qDebug() << timer.elapsed();
}

Eligijus

Thanks sub-classing and overrinding paintevent method worked.