Bug 497571

Summary: Window recording gives incorrect resolution with display scaling, in both spectacle and obs-studio
Product: [Plasma] kwin Reporter: Oxalica <oxalicc>
Component: screencastingAssignee: KWin default assignee <kwin-bugs-null>
Status: RESOLVED FIXED    
Severity: normal CC: nate, oxalicc, xaver.hugl
Priority: NOR    
Version: 6.2.4   
Target Milestone: ---   
Platform: NixOS   
OS: Linux   
Latest Commit: https://invent.kde.org/plasma/kwin/-/commit/4048d208f72b74f2393e6f129a1802ae447f228b Version Fixed In: 6.2.5
Sentry Crash Report:

Description Oxalica 2024-12-16 19:33:46 UTC 
SUMMARY

Window recording produces a video with resolution of actual resolution divided by scaling factor (logical geometry?), rather than actual resolution, resulting a blurry and small content. While full screen recording produces correct actual physical resolution.

It is also reproducible in obs-studio (https://github.com/obsproject/obs-studio) via "Screen Capture (Pipewire)": with the same full screen game, capturing the full screen gives a 2560x1440 clear source, while capturing the game window itself gives a 2048x1152 blurry source. (My display is 2560x1440 with 125% scale)

STEPS TO REPRODUCE

1. Have a 2560x1440 display, and set scale to 125%
2. Open "dolphin" and maximize it to occupy (almost) the full screen.
3. Open "spectacle", click "Recording" - "Window" - select the dolphin window. Wait for one second and stop recording.
4. Repeat 3 but record with "Full Screen".

OBSERVED RESULT

The video from step 3 (window recording) is 2048x1104 for me, which is approximately (2560/1.25)x(1440/1.25), minus the plasma panel on the top. The text in video is also blurry and hard to read.
The video from step 4 (full screen recording) is exactly 2560x1440, the text inside is clear and easy to read.

EXPECTED RESULT

The resolution of video from step 3 should be 2560x(1440 - top panel height) on maximized dolphin window. And the text should be as clear as in full screen recorded video.

SOFTWARE/OS VERSIONS

Operating System: NixOS 25.05
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.0
Kernel Version: 6.6.63 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 5700G with Radeon Graphics
Memory: 31.0 GiB of RAM
Graphics Processor: Mesa Intel® Arc

ADDITIONAL INFORMATION

Also reproduced in obs-studio 30.2.3
Comment 1 Oxalica 2024-12-16 21:11:46 UTC 
After some debugging, I made it work with the following patch on top of kwin v6.2.4. With it, both spectacle and obs-studio record the non-scaled correct resolution and the content looks clear. I hope it helps, but I'm not sure if it's ideal and it's not applicable on kwin master due to some non-trivial code change.

```
diff --git a/src/plugins/screencast/windowscreencastsource.cpp b/src/plugins/screencast/windowscreencastsource.cpp
index f2a1432e4d..2b2c6ed36d 100644
--- a/src/plugins/screencast/windowscreencastsource.cpp
+++ b/src/plugins/screencast/windowscreencastsource.cpp
@@ -49,12 +49,12 @@ quint32 WindowScreenCastSource::drmFormat() const
 
 QSize WindowScreenCastSource::textureSize() const
 {
-    return m_window->clientGeometry().size().toSize();
+    return m_window->clientGeometry().size().toSize() * devicePixelRatio();
 }
 
 qreal WindowScreenCastSource::devicePixelRatio() const
 {
-    return 1.0;
+    return m_window->preferredBufferScale();
 }
 
 void WindowScreenCastSource::render(QImage *target)
-- 
```
Comment 2 Zamundaaa 2024-12-16 22:07:43 UTC 
You can make that work on git master by replacing preferredBufferScale() with targetScale(). Would you like to make a merge request with that change?
Comment 3 Oxalica 2024-12-16 23:51:01 UTC 
(In reply to Zamundaaa from comment #2)
> You can make that work on git master by replacing preferredBufferScale()
> with targetScale(). Would you like to make a merge request with that change?

Thanks for the advice, but I don't have a kwin dev&test environment. I'm currently hot-patching kwin's Nix package in my production system. The master branch contains many dependency changes (kdecoration3 and all its dependents) which are incompatible with build scripts from released version. I'm unfamiliar with fixing it.

If it's more convenient for you to test the 2-line-change, feel free to adopt it in upstream. Thanks!
Comment 4 Bug Janitor Service 2024-12-17 00:14:05 UTC 
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/6906
Comment 5 Zamundaaa 2024-12-17 14:11:03 UTC 
Git commit b6e6000d9729f740f9a17cf4d4517703a8da09fa by Xaver Hugl.
Committed on 17/12/2024 at 13:29.
Pushed by zamundaaa into branch 'master'.

plugins/screencast: take scaling into account for window sources

Otherwise the textures will be too small, and the result blurry

M  +2    -2    src/plugins/screencast/windowscreencastsource.cpp

https://invent.kde.org/plasma/kwin/-/commit/b6e6000d9729f740f9a17cf4d4517703a8da09fa
Comment 6 Zamundaaa 2024-12-17 15:18:56 UTC 
Git commit 4048d208f72b74f2393e6f129a1802ae447f228b by Xaver Hugl.
Committed on 17/12/2024 at 15:18.
Pushed by zamundaaa into branch 'Plasma/6.2'.

plugins/screencast: take scaling into account for window sources

Otherwise the textures will be too small, and the result blurry


(cherry picked from commit b6e6000d9729f740f9a17cf4d4517703a8da09fa)

Co-authored-by: Xaver Hugl <xaver.hugl@gmail.com>

M  +2    -2    src/plugins/screencast/windowscreencastsource.cpp

https://invent.kde.org/plasma/kwin/-/commit/4048d208f72b74f2393e6f129a1802ae447f228b