https://bugs.kde.org/show_bug.cgi?id=476186
Bug ID: 476186
Summary: Screen recording quality is terrible
Classification: Frameworks and Libraries
Product: KPipeWire
Version: unspecified
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: plasma-b...@kde.org
Reporter: mar...@marcan.st
CC: aleix...@kde.org
Target Milestone: ---
Screen recording quality in Spectacle is bad to the point of being unusable for
anything more than casual use.
Selecting quality is a codec, specific issue, but for x264 in particular, the
current code is very questionable:
https://invent.kde.org/plasma/kpipewire/-/blob/master/src/libx264encoder.cpp
This sets `m_avCodecContext->global_quality` from some weird formula, but it's
unclear how that maps to encoder settings in the end within ffmpeg. What I see
in the resulting files is that Average Bitrate mode is being used (rc=abr),
with bitrate somehow varying based on dimensions. This is a very bad choice for
x264.
The correct "just give me a given quality please" mode in x264 is CRF (constant
rate factor) mode, which does not force any given bitrate but rather targets a
specific visual quality. If the quality settings are not configurable (or not
configurable beyond a simple quality slider), then that mode should be the
default to give decent output without more fuss. CRF mode is
resolution-independent, and will automatically scale bitrate depending on the
requirements (video size, motion complexity, etc.). It's the best option to
default to for most users.
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