https://bugs.kde.org/show_bug.cgi?id=505272
Bug ID: 505272
Summary: Imprecise/Wrong sRGB output from LCMS2
Classification: Applications
Product: krita
Version First unspecified
Reported In:
Platform: Ubuntu
OS: All
Status: REPORTED
Severity: normal
Priority: NOR
Component: Color models
Assignee: krita-bugs-n...@kde.org
Reporter: xp8...@outlook.com
Target Milestone: ---
SUMMARY
LCMS2 will do sRGB curve-fitting to pow 2.2 if `cmsFLAGS_NOOPTIMIZE` is not
set for `cmsCreateTransform()` calls under specific conditions, this leads to
noticeably incorrect result especially around dark regions when outputting
image from LCMS. From a quick search in Krita code base, I don't see this flag
present in places where it's actually needed for correctness.
This can be a non-bug for krita ONLY IF `cmsFLAGS_NOOPTIMIZE` is set for
`cmsCreateTransform()` so that LCMS2 will convert the pixels properly via sRGB
transfer curves for output.
I am the developer of [OurPaint](https://www.WellObserve.com/OurPaint) and just
today I found out why the result is different than doing the color space
conversion algorithm manually. I have reported the same thing on
[LCMS2](https://github.com/mm2/Little-CMS/issues/496) as well. Krita seems to
have the same thing going on, but I doubt if anybody is serious enough to
pixel-peep the output.
STEPS TO REPRODUCE
By "the look":
If you are a seasoned artist, and if you observe darker lines/regions in your
krita canvas exported "just a little bit darker/richer", then that's the exact
problem I'm describing. (Effect more pronounced if use Clay/AdobeRGB compatible
color spaces on your canvas)
It's quite tedious to reproduce completely via krita _and_ to get an observable
result from the start, but you could try doing it like this:
1. Set a new image to 16bit, Rec2020-ellie-v4-g10.icc (anything that has a
different primaries than sRGB and a linear transfer curve.
2. Draw a linear gradient from black to white across the image.
3. Export as PNG, make sure that "convert to sRGB" is selected. This is image
`A`
4. Export as PNG, disable "Embed sRGB tag" and convert to sRGB". This is image
`B`
5. Open both images in GIMP and convert `B` to something like a Rec2020 but
with sRGB transfer function, , then convert the result to sRGB, now you got
`B1` (This is the guaranteed correct transform I found for now)
6. In GIMP convert `B` straight to sRGB, you get `B2` (This is broken the same
way as krita since both uses lcms2)
OBSERVED RESULT
- `A` matches `B2`
- B1 and B2 is slightly different especially at dark parts.
EXPECTED RESULT
- Both `B1`, `B2` matches `A` (3 identical images, with maybe off-by-1
conversion errors)
SOFTWARE/OS VERSIONS
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: Ubuntu 22.05
KDE Plasma Version: -
KDE Frameworks Version: -
Qt Version: 5?
ADDITIONAL INFORMATION
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