Bug 400837

Summary: Plasma Blur feature not ignoring shadows generated by other windows.
Product: [Plasma] kwin Reporter: Chris Raven <chris>
Component: effects-variousAssignee: KWin default assignee <kwin-bugs-null>
Status: RESOLVED INTENTIONAL    
Severity: normal CC: alex19930329, plasma-bugs-null
Priority: NOR    
Version First Reported In: unspecified   
Target Milestone: ---   
Platform: Other   
OS: Linux   
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:
Attachments: Screenshot of the issue
Video of non-expanded blur
Video of expanded blur (correct)

Description Chris Raven 2018-11-08 08:11:26 UTC 
SUMMARY:
If you put two windows next to each other, for instance 2x Konsole windows, the Shadow of the active window shows up as a White Blur type shadow on the inactive program. Basically, the Blur feature of Plasma / Kwin is also attempting to bluer the shadow generated by other windows, which should not occur. As a note this does not always occur.


STEPS TO REPRODUCE
1. Setup default blur on any window
2. Set that window (for instance Konsole) to any kind of blur / transparency amount
3. Place that window next to another window, for instance 2x Konsole windows side by side.

OBSERVED RESULT:
The shadow shows up behind the blurred part of the other app, as a white glow type vision.


EXPECTED RESULT:

Blur should ignore the Shadow of other windows.


SOFTWARE VERSIONS (Has been the same since Blur was introduced):
KDE Plasma Version:  5.14.3
KDE Frameworks Version: 5.51.0
Qt Version: 5.11.2
Comment 1 Chris Raven 2018-11-08 14:47:21 UTC 
Created attachment 116182 [details]
Screenshot of the issue

Based on the attachment, you can see the white colour showing through on the Transparent / Blurred Konsole session on the right. It seems that the blur is pulling the colour of the window next to it, although they aren't touching / overlapping
Comment 2 Alex Nemeth 2018-11-09 17:28:11 UTC 
Created attachment 116213 [details]
Video of non-expanded blur

The way the blur works is it takes a "screenshot" of the area bigger than the actual blur area. 
This is needed to avoid some glitches around the edges of the window.
See in the attached video what happens when we don't use "expanded blur". The window next to Konsole just pops in the the blur area out of nowhere, there is no smooth transition.
Comment 3 Alex Nemeth 2018-11-09 17:33:14 UTC 
Created attachment 116214 [details]
Video of expanded blur (correct)

This is how it works currently.
Look at the edges of Konsole where it touches the colorful image.
There is a smooth transition.
This is the correct behavior.
Comment 4 Alex Nemeth 2018-11-09 17:36:26 UTC 
Works as intended