QImage too slow to load ...



  • Hi guys ;
    I would like to show a small thumbnail image in my app, when user selects an image file. Here is my code :

            QPixmap MyPixmap;
            MyPixmap = QPixmap (":/Images/Holder.png");
            if (!MyPixmap.isNull()) { ui->imageLabel->setPixmap ( MyPixmap.scaled ( 98,  55,  Qt::KeepAspectRatio));}
    

    Everything works fine for small images (700 KByte size), but it take long time to show when I load an image with 3MegaByte file size...!
    Is there a way to improve the performance ?

    Thanks.
    MSD.


  • Qt Champions 2016

    Hi
    Have you tried the 3 MB file from a
    normal file and not from a resource ?
    Just to see time difference.



  • I guess the time is not taken for loading the image but for scaling the image. If the image is loaded from resource (means: static and will not change after compilation) you should scale the image once and include the scaled version into your binary...



  • You could try out QImageReader which, for some formats, can speed up reading by reading in a scaled image.

    On top of that you could read in the image in a separate thread (in case the loading is blocking some UI you might have).

    And on top of that you could use a quicker method of scaling the image. Should you want to reuse the image, don't forget to cache the scaled image.

    PS: Also, there is no need to guess: Make use of QElapsedTimer if you want to know how long certain actions take.


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