[okular] [Bug 508719] New: Blended colors in CMYK color blending space rendered incorrectly

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https://bugs.kde.org/show_bug.cgi?id=508719

            Bug ID: 508719
           Summary: Blended colors in CMYK color blending space rendered
                    incorrectly
    Classification: Applications
           Product: okular
      Version First 23.08.1
       Reported In:
          Platform: Microsoft Windows
                OS: Microsoft Windows
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: heinrich.bohne at gmx.at
  Target Milestone: ---

Created attachment 184436
  --> https://bugs.kde.org/attachment.cgi?id=184436&action=edit
This zip-archive contains a sample PDF file and two screenshots of the file
rendered in Okular

I have a PDF file containing yellow text over a dark blue background, and the
yellow text has the "exclusion" blend mode applied to it. The color blend space
is in CMYK mode, but Okular appears to render the text color as if the blending
space were an RGB space (I compared the resulting colors with the color Krita
shows when I blend the two colors in an RGB and a CMYK space, and also with how
Acrobat Reader displays this file and an almost identical file where the
blending color space set to an RGB profile).

Strangely enough, this only happens when "overprint preview" is disabled in the
PDF backend settings. Upon enabling "overprint preview", the color is rendered
correctly (or at least like in Acrobat Reader, which, seeing as the color
displayed there appears to match the color I get in Krita when blending in
CMYK, I presume is correct), even though the document does not contain any
overprint and should therefore look the same with "overprint preview" enabled
and disabled.

I have attached a sample PDF file to demonstrate this behavior, along with two
screenshots of how Okular renders the file with and without overprint preview.


STEPS TO REPRODUCE
1. Open a PDF file that contains an element with a blend mode applied (in my
case, the blend mode is exclusion, where the difference between blending in
CMYK and RGB space is said to be particularly noticeable, but I presume the
problem is the same with other blend modes), a CMYK color profile as a blending
color space and overprint set to false. The element should be directly before a
non-empty background.
2. Turn overprint preview off.

OBSERVED RESULT
The element in the foreground is shown as if the blending color space were in
RGB mode. With the attached sample PDF file, the text is displayed with the
color shown in screenshot 1. Whether this final blend color has been converted
to the document's destination color space, I cannot tell.

EXPECTED RESULT
The element in the foreground is shown with a color resulting from blending the
two source colors in the color space defined as the foreground element's
blending color space, and only this resulting color should, if needed, be
converted to the document's destination color space (in the case of the
attached sample PDF file, the color should look something like in screenshot 2,
which is what is displayed when overprint preview is turned on).


SOFTWARE/OS VERSIONS
Windows 10 Home edition, 64-bit (x64)
Okular version: 23.08.1. This seems to be an old version, but it is the version
I get from installing Okular via the Microsoft Store, which is the recommended
way of installing Okular on Windows, according to the Okular website. However,
I also tried opening the PDF file with the nightly 25.11.70 version, and the
problem persisted, so it does not seem to have been fixed in the meantime.

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