[Okular-devel] [okular] [Bug 362856] New: [Frameworks] Wrong render resolution, possibly caused by KDE 5 "Scale Display"

[Yi Yang via KDE Bugzilla]

https://bugs.kde.org/show_bug.cgi?id=362856

            Bug ID: 362856
           Summary: [Frameworks] Wrong render resolution, possibly caused
                    by KDE 5 "Scale Display"
           Product: okular
           Version: unspecified
          Platform: Other
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: ahyangyi at gmail.com

KDE 5 includes a new feature, "scale display", for better support for high
resolution screens. To enable it, go to settings > Display and Monitor >
Display Configuration > Scale Display.

I use a 4K screen and I enabled 1.5x scale display.

I noticed that all PDFs are rendered in a way that looks extremely bad. A
closer look reveals that the PDF is somehow rendered in a way, so that the
rendered bitmap consists of 2x2 regions of the same color. In other words,
Okular rendered PDFs with 1/2 of actual resolution.

I tried to dive into the source code. Apparently, the render resolution is
decided  then sent to the PDF module via a PixmapRequest. Therefore, I guess
this is not only a PDF problem, but a global one, hence the general component
instead of PDF backend in this bug report.

The geometry sent in PixmapRequest comes from uncroppedWidth() &
uncroppedHeight(), which in turn come from m_uncroppedGeometry. The calculation
of m_uncroppedGeometry looks somewhat magical, which is where I stop looking at
the code and decide to write a bug report instead...

The KDE5 Scale display feature is the most possible cause of this problem.
However, I'll happily give you more information if that's needed for diagnosing
this bug. 

Reproducible: Always

Steps to Reproduce:
1. Enable KDE 5 scale display.
2. Open Okular, open a PDF.
3. Look at how text is rendered (other things are also affected, but text is
more important and ubiquitous)

Actual Results:  
Okular renders at half of actual resolution, then scale the result, producing
poor results.

Expected Results:  
Okular renders at actual resolution.

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