demo-image-viewer-qt

demo-image-viewer

It’s a demonstration only image viewer made with Qt6. The binaries are available.

Screenshot

Features

Opens an image in a time independent of the count of images in a directory where it is located.
Handles the images of the directory in a separated thread with QtConcurrent::run.
The directory parsing is fast. It does not use QFileInfo in std::sort, but a custom struct.
Handles the directory only once, until it is changed. (For example, drag’n’dropping a file from the same directory does not trigger the directory parsing, since it’s not needed to do, just find the file by name in the list structure.)
Decodes 8.3 file names to long file names with Win API (GetLongPathNameW). An 8.3 can be faced on a user input of a file with long path (260+ chars) from disc C from Windows Explorer: on Drag’n’Drop and when a user opens an image with a double click (the image path is passed as a command line argument to the program.)
Opens images with a long path starts with \\?\ (\\?\UNC\).
Preloades the adjacent images in a separate thread.
Sorts by mtime, btime, size.
Updates the image position (in the title) on the sorting change.
Lists hidden files (QDir::Hidden).
All long time taking operations log the execution time in the console with qDebug().

Here is the example of the console log, after the program is started (~70 ms):

The speed results for 350 KB image with a directory with 30000+ files (Run it with Qt Creator to look at the qDebug() logs):

[timer][displayImage]: 84 ms — time to display the image.

At this moment the image is visible.

That is performed in a background thread:

[timer][entryInfoList]: 143 ms — time to get the list with QFileInfo of all files (31973) in the directory
[timer][filterBySupportedExts]: 6 ms — time filter that list to exclude the unsupported files
[filterBySupportedExts] fileInfoList.size: 31973
[filterBySupportedExts] fileInfoListFiltered.size: 30006
[timer][initFileEntryList]: 200 ms — time to create a list of data structure with all necessary information for all filtered files (30006)
[timer][initFileList]: 352 ms — the total time of the work with directory files ~(143 + 6 + 200)
[timer][sortByMtime]: 6 ms — to sort all files by mtime

Note: Do NOT use code that calls methods of QFileInfo in std::sort. It’s incredibly slow.

That is up to 600 times slower

        std::sort(fileEntryList.begin(), fileEntryList.end(),
                  [](const FileEntry& a, const FileEntry& b) {
                      return a.fileInfo.lastModified() < b.fileInfo.lastModified();
                  }
        );

than that

        std::sort(fileEntryList.begin(), fileEntryList.end(),
                  [](const FileEntry& a, const FileEntry& b) {
                      return a.mtime < b.mtime;
                  }
        );

3200-3600 ms vs 6 ms.

So, store all required fields of QFileInfo in your own data struct.

Try by yourself (uncomment the line 307 and comment lines 298-304): https://github.com/AlttiRi/demo-image-viewer/blob/ad9ac9b1c9d78244462064983e676542700e96d9/core.h#L296-L311

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