https://bugs.kde.org/show_bug.cgi?id=457817
Bug ID: 457817
Summary: window preview quality is very low due to cheap / fast
scaling algorithm
Product: kwin
Version: 5.24.4
Platform: Archlinux Packages
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: kwin-bugs-n...@kde.org
Reporter: adam.m.fontenot+...@gmail.com
Target Milestone: ---
Created attachment 151292
--> https://bugs.kde.org/attachment.cgi?id=151292&action=edit
screenshot of the problem and proposed resolution
Everywhere a window preview is shown (e.g. in present-windows, in the window
previews of Plasma's icons-only task manager, etc), it is low quality - meaning
"pixelated" looking, with strong color fringing around text (when sub-pixel
anti-aliasing is enabled for font rendering on the system).
I am assuming that different tools utilizing window previews under KWin rely on
the same scaling code, since the issue is the same in each case, so I'm
reporting this as a general bug.
STEPS TO REPRODUCE
1. Make sure the compositor's scale method is set to "accurate" instead of
smooth - just in case it might have an effect.
2. Open a bunch of windows and then activate the "present windows" effect.
(Opening a bunch of windows will make the resulting preview size smaller, which
will make the problem easier to see on high resolution screens.)
OBSERVED RESULT
Quality is poor (see attached screenshot). Strong color fringing around text is
probably caused by using a nearest neighbor / point algorithm to rescale the
window preview.
EXPECTED RESULT
Quality is acceptable (see attached screenshot). For the sample shown in the
screenshot, I captured the window at full resolution and used a (still very
cheap and fast) bilinear filter to create a preview of the same size as the one
created by KWin.
Window rescaling is probably GPU accelerated anyway (?) so I don't think using
a slightly more expensive algorithm like bilinear should be a problem. If it
might be on some systems, perhaps a setting to enable or disable it could be
added.
SOFTWARE/OS VERSIONS
Linux: Arch Linux x86_64 (kernel 5.18.16-arch1-1)
KDE Plasma Version: 5.25.4 (x11)
KDE Frameworks Version: 5.96.0
Qt Version: 5.15.5
ADDITIONAL INFORMATION
I've always thought it would be nice if window previews also showed the window
decorations.
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