Bug 196281

Summary:	kwin: overexpose effect for transparent images with compositing enabled - makes virtualbox unusable
Product:	[Plasma] kwin	Reporter:	Thomas Meixner <tom.meixner>
Component:	compatibility	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	RESOLVED NOT A BUG
Severity:	normal	CC:	tom.meixner
Priority:	NOR
Version:	unspecified
Target Milestone:	---
Platform:	Ubuntu
OS:	Linux
Latest Commit:		Version Fixed In:
Attachments:	KDE Screenshot showing an overexposed effect under tranparent images Transparent console window of virtualbox with effect Blur disabled

Description Thomas Meixner 2009-06-13 03:38:44 UTC

Version:            (using KDE 4.2.90)
OS:                Linux
Installed from:    Ubuntu Packages

I realized that transparency for some non native KDE Applications namely cairo-dock and virtual-box results in an overexpose effect for some areas. While in the first instance this is only cosmetical it renders a virtual box command line window virtually unusable unless compositing is disabled. Since 2 Applications are affected in a similar way I thought I would write a Kwin Bug report first. 

Please look at the attached screenshots for a better understanding of what this looks like: ( don't know how to upload screenshot. please check comments for a link)

I tried the same using Compiz and the transparent Background in cairo-dock is not visible and virtualbox has a black background as expected.

Especially in the case of Virtual box it would be nice to not be forced to shut down compositing to be able to use the application.

Comment 1 Thomas Meixner 2009-06-13 03:43:41 UTC

Created attachment 34491 [details]
KDE Screenshot showing an overexposed effect under tranparent images

Comment 2 lucas 2009-06-13 04:39:32 UTC

Does it still occur if you keep compositing enabled but disable all the actual effects? If it doesn't can you try to work out which effect is causing it?

Comment 3 Thomas Meixner 2009-06-13 05:17:21 UTC

Good call ! I disabled all effects and put them one by one back in.
Blur seems to add to the trouble but doesn't seem to be the only one involved. 
I'm using the folllowing version:
Blur
Version 0.2.0
Using KDE 4.2.90 (KDE 4.2.90 (KDE 4.3 Beta2))

For below please compare to screenshot:
Disabeling Blur fixes the overexposed issues in cairo-dock and makes text in virtualbox readable if the window is above a dark background. I still don't understaqnd why the console window in virtual box is transparent.

Comment 4 Thomas Meixner 2009-06-13 05:18:28 UTC

And btw thanks for you quick reply.

Comment 5 Thomas Meixner 2009-06-13 05:20:36 UTC

Created attachment 34493 [details]
Transparent console window of virtualbox with effect Blur disabled 

The problems with Blur enabled for the cairo-dock are sorted but the transparent background for the console window remains.

Comment 6 lucas 2009-06-13 09:00:38 UTC

The blur effect has been removed from KDE 4.3 thus making this problem invalid. Thank you for testing.

Comment 7 Thomas Meixner 2009-06-17 02:58:57 UTC

I have contacted the original Author about his opinion and advice. Unfortunately he doesn't have the time to work at the Blur anymore but for the sake of completeness and if someone wants to look into this in the future below his response:

I took a look at your screenshots and VirtualBox actually looks correct 
(assuming it's supposed to have transparent background) - 
<< TM: I don't think it shoule be transparent. In Compiz the background is black, only KWin seems to interpret it as transparent>>
if you have white text on a transparent background in front of a white window, then the blur isn't really going to change anything and white on white remains unreadable.

But the dock indeed seems to be broken by the blur effect. I can't say what 
the problem is though. I remember KWin being switched to using premultiplied 
alpha at some point, I wonder if that might have anything to do with it. 
Perhaps the dock uses an unexpected color format (one that KWin doesn't expect 
it to use) or messes up its alpha channel or something.


---

TM: I'm not a KDE Developer and I can only try to guess but from his description it looks as if the error is not with the Blur Effect itself but either with Kwin or the way the two non native KDE applications implement transparency.