Bug 493593

Created attachment 174032 [details]
Krita file with slow thumbnail generation

SUMMARY

When working with large images (7680 x 4320) with large transparent areas, the undo image thumbnail generation freezes redrawing of the Krita UI after every operation for a long time.

The reason for this is that even if the undo history docker is hidden, thumbnails is generated.
The slow code path is `m_paintDevice->exactBounds()` that was introduced in:
https://invent.kde.org/graphics/krita/-/commit/b67e2556bb2944f56c0a4ca9bd77a1f29f8e05df

Because it loops over potentially all pixels in the image to find any opaque pixels, probably best to not do this in a locking manner for the undo system. (Or perhaps generate the undo thumbnails on demand if the undo history docker is visible?

Note that I'm specifically pointing to the undo thumbnail creation here as I ran krita with a profiler (hotspot) and it showed that after each stroke it seems like all activity was in a single thread doing `exactBounds` calculations.

If I shorted out the exact bounds function to return early without doing any calculations, krita was really fast and responsive again.

STEPS TO REPRODUCE
1. Open the krita file I have attached in the .7z archive
2. Draw a stroke in the middle of the image
3. Notice that the krita UI freezes after finishing the stroke
4. Add a opaque pixel at every corner of the image
5. Notice that now drawing in the middle is really fast and responsive

OBSERVED RESULT
The krita UI freezes after every stroke when an image has large transparent areas

EXPECTED RESULT
Thumbnail creations shouldn't lockup the UI and degrade the overall performance of Krita.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Gentoo Linux
KDE Plasma Version: 6.1.5
KDE Frameworks Version: 6.6.0
Qt Version: 6.7.2