Best way to preview a lot of big size images

  • Hello !
    I want to do an image overview for several hundred big size images (minimum 40mb, JPG).
    What i have done:
    I load 30 images for every page with QImageReader. To speed up the loading process i use QtConcurrent. Unfortunately this is still not fast enough. Setting QImageReader::setScaleSize() and QImageReader::setQuality() doesn`t increase the speed.
    How can read big images in very low resoltion just for previewing them in a Size of maybe 200x200 ?
    Thanks in advance !

  • Lifetime Qt Champion


    AFAIK, what's usually done for that kind of case is to create thumbnails for every images and load the original only when the user actually chooses an image.

  • Thank you for answering!
    I also thought of that, but in my case the number of pictures can go into the thousands. When i create thumbnails i have to additionally maintain thousands of them. If i can find a way to organize that well, i can do that. Nevertheless i would prefer a solution with the original files.

  • On Windows, the OS takes care of creating and organising those thumbnails, you can read the thumbs.db into your program directly, see

  • Thank you VRonin !
    Good idea! But when the user has deactivated Thumbs.db files i have a problem.
    I decided to write my own stuff for that. Thank you very much!

  • Lifetime Qt Champion

    One thing to take into account: depending on the format, some include a thumbnail in their meta data.

    However and in any case, you can't expect your target audience to load thousands of high quality images in memory for your application. It will kill their system. You should at least use a cache to keep a reasonable amount of data for the current use.

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