https://bugs.kde.org/show_bug.cgi?id=493593
Bug ID: 493593
Summary: Undo preview generation visibly freezes krita for
serveral seconds when working with large transparent
images
Classification: Applications
Product: krita
Version: 5.2.5
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: * Unknown
Assignee: krita-bugs-n...@kde.org
Reporter: darkdefe...@gmail.com
Target Milestone: ---
Created attachment 174032
--> https://bugs.kde.org/attachment.cgi?id=174032&action=edit
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
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