[okular] [Bug 526590] New: Incorrect colors in PDF when "Overprint Preview" is set to "Automatic"

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https://bugs.kde.org/show_bug.cgi?id=526590

            Bug ID: 526590
           Summary: Incorrect colors in PDF when "Overprint Preview" is
                    set to "Automatic"
    Classification: Applications
           Product: okular
      Version First 26.08.1
       Reported In:
          Platform: Fedora RPMs
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: jonas-miles-kwecl at luxusmail.gq
  Target Milestone: ---

DESCRIPTION
A PDF file display doesn't work if if overprint preview option is not set to
'never' in okular. This bug occurs regularly with pdf plans provided by Ile de
France Mobilité on page https://www.iledefrance-mobilites.fr/le-reseau/plans

STEPS TO REPRODUCE
1. Download and open the following PDF in Okular:
   https://eu.ftp.opendatasoft.com/stif/PlansDeSecteur/Plans/plan23.pdf

2. Use Okular 26.08.1 on KDE Plasma.

3. Set Overprint Preview to Automatic.

4. Look at the colored bus lines on the map.


OBSERVED RESULT
Some colors of the bus lines are displayed incorrectly. They appear
significantly different from their expected colors, in some cases approximately
as complementary colors.

The problem affects only some colors; this is not a global color inversion.

The same PDF is displayed with the expected colors in a web browser.

EXPECTED RESULT
The colors displayed by Okular should match the colors of the PDF as rendered
by other PDF viewers, including a web browser.

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 44
KDE Plasma Version: 6.7.5
KDE Frameworks Version: 6.30.0
Qt Version: 6.11.2

WORKAROUND
If Overprint Preview is changed from Automatic to Never, the affected colors
are displayed correctly.

ADDITIONAL INFORMATION
Printing/exporting the PDF from Okular also produces a PDF whose colors appear
correct when subsequently viewed, although the resulting rendering has somewhat
degraded contrast.

The PDF appears to contain vector graphics rather than being only a raster
image.

I have not determined whether the problem is related to CMYK colors,
overprinting, transparency, blending modes, or another PDF rendering feature.

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