https://bugs.kde.org/show_bug.cgi?id=397391
Bug ID: 397391
Summary: Unusual Output from Levels Filter in Krita
Product: krita
Version: 4.1.0
Platform: MS Windows
OS: MS Windows
Status: UNCONFIRMED
Severity: minor
Priority: NOR
Component: Filters
Assignee: krita-bugs-n...@kde.org
Reporter: jem.hollo...@gmail.com
Target Milestone: ---
The Levels filter in Krita doesn't conform with the expected behaviour of such
a filter. Whereas other applications apply the filter in HSV space, Krita uses
LAB space, which produces a poor result.
To reproduce:
1. Start with an ordinary 8-bit RGB image, containing colour information, like
this:
https://nofile.io/f/wtOOsvPgU09/moonlit+blue+sky_sm.png
2. Apply a Levels filter, and set the minimum input slider to 127, like this:
https://nofile.io/f/OhGtyS7QvY0/levels_dialog_adjusted.png
3. One would expect all pixels with a Value of 127 or less to be clipped to
black. And pixels with a Value from 128-255 should be re-mapped to the
specified output range (in this case, 0-255).
So the result should look like this:
https://nofile.io/f/O7Yee3qgMD9/levels_test_correctResult.png
And that's the result I get from the Levels filter in every other image
manipulation software I've tried, including Gimp, Inkscape, Photoshop, Affinity
Photo, and even compositors such as Fusion.
But Krita's output is the odd one out. None of the dark areas have been
clipped, and all the details in the bottom half of the image are still visible:
https://nofile.io/f/s2MWUd6fFwV/levels_test_incorrectResult.png
In the relevant forum thread
(https://forum.kde.org/viewtopic.php?f=137&t=153522&sid=f98ff5a4bfd173434daa7574efba1eb0&p=402380#p402377),
Krita developer boudewijn comments:
"I do see that gimp 2.8 and Krita 4.1.1 do something different, and I suspect
that's because Krita converts the image to LAB and modifies the L channel, and
Gimp to HSV and modifies the Value channel."
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