Weird execution times QImage by reference

[Tony123]

Hi guys

Take a look on following code the Image is a class member of type QImage.

void ImageViewer::setImage(const QImage &newImage)
{
Image = newImage; // takes 108 milliseconds
imageLabel->setPixmap(QPixmap::fromImage(Image)); / takes 58 milliseconds
}

because I don't need to set newImage to Image class member anymore ,I simply use reference of newImage and save time. BUT I WAS SUPRISED!!

void ImageViewer::setImage(const QImage &newImage)
{
imageLabel->setPixmap(QPixmap::fromImage(newImage)); / takes 158 milliseconds
}

ITS TAKE SAME TIME ..WHAT I'M MISSING HERE ??!!

[J.Hilk]

@Tony123

the = operator does not do a deep copy of QImage, you have QImage image_copy = image.copy(); for that,

Image = newImage; should take up next to no time at all as its only asome pointer shuffeling.

[raven-worx]

@Tony123
see Implicit sharing

[Tony123]

Thank for response @J.Hilk . So your said if I make a Image = newImage; thats not a deep copy so when I creating pixmap from Image that's why it takes less time than creating a pixmap from newImage which is therefore more time consume? Am I right ?

[Tony123]

@J.Hilk So I try what you said and it's doesn't matter if you are creating QImage by = or copy , the setPixmap is taking same time.

[J.Hilk]

@Tony123 said in Weird execution times QImage by reference:

Thank for response @J.Hilk . So your said if I make a Image = newImage; thats not a deep copy so when I creating pixmap from Image that's why it takes less time than creating a pixmap from newImage which is therefore more time consume? Am I right ?

What I'm saying is that after the compiler is done, the difference between the 2 functions is only that, the refcount of the image-data is increased by 1 and the pointer of Image was changed.

The time intensive part, as far as I know, is coming from the QPixmap::fromImage function

That said ~100ms to create a Pixmap is a lot, is it a big image and do you do you tests in Debug or release mode, those make a huge difference. and how exactly did you meassure the time it takes ?

I did some tests myself:

void MainWindow::setImage(const QImage &img)
{
    quint64 time1, time2, time3;

    elap.start();
    refImage = img;
    time1 = elap.nsecsElapsed();

    showImg->setPixmap(QPixmap::fromImage(refImage));
    time2 = elap.nsecsElapsed();

    QImage deepCopy = img.copy(0,0,img.width(),img.height());
    time3 = elap.nsecsElapsed();

    qDebug() << "total time:" << time3 << "[ns]" << endl
             << "create RefImage" << time1 << "[ns]" << endl
             << "Time to create Pixmap and set it" << time2-time1 << "[ns]" << endl
             << "Time for deepcopy" << time3-time2<< "[ns]";
}

//Multiple runs, averaged out

Result for release build
total time: 111225 [ns]
create RefImage 364 [ns]
Time to create Pixmap and set it 62724 [ns]
Time for deepcopy 48137 [ns]

Result for debug build
total time: 274963 [ns]
create RefImage 364 [ns]
Time to create Pixmap and set it 234849 [ns]
Time for deepcopy 39750 [ns]