[okular] [Bug 424817] New: Ugly rendering of images containing text in PDF files

[Jan Rathmann]

https://bugs.kde.org/show_bug.cgi?id=424817

            Bug ID: 424817
           Summary: Ugly rendering of images containing text in PDF files
           Product: okular
           Version: 1.9.3
          Platform: Kubuntu Packages
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: jan.rathmann at gmx.de
  Target Milestone: ---

Created attachment 130504
  --> https://bugs.kde.org/attachment.cgi?id=130504&action=edit
Example pdf illustrating the bug (generated with 'img2pdf -o test.pdf
test.png')

SUMMARY

Hello,

I have observed that unfortunately in Okular PDF files that contain images with
text (e.g. scanned pages) are rendered in a relatively "ugly" way at certain
zoom levels when compared with other open source PDF readers.

STEPS TO REPRODUCE
1. Open attached file "test.pdf" with Okular.
2. Set zoom level to 75%.

OBSERVED RESULT

Text looks "rather ugly".
(See also attached screenshot.)

EXPECTED RESULT

Text looks "clearly readable".
(Compare screenshot from same document opened in Evince.)

I have compared the rendering of other open source PDF viewers with Okular (at
same zoom level):
Evince, MuPDF, Firefox embedded PDF viewer, Chromium embedded PDF viewer
The rendering looks fine on all of them with Okular being the only exception.

When I check/uncheck "Enable Graphics Antialias" in Okular, this makes no
difference for me.

I'm using Kubuntu 20.04 as my main system, but I also verified that I see the
same result in the current KDE Neon live image
(neon-unstable-20200726-1102.iso).

SOFTWARE/OS VERSIONS
Linux: 5.4.0-42-generic
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

Kind regards,
Jan

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