Created attachment 6023 [details]
Image 2 of screen in KDE.

[... continued]

As can be seen from the images some parts are not drawn on screen (like the K menu) and that happens often with window content. The window shadow zone contains distorted images of what the window contains; typically a window would not be draw until the mouse passed over some element (a close button, for instance).

It's even hard to use, because there's no visual feedback to know where to click to e.g. start an application. Fortunately Ctrl-Alt-Del works as expected.

Leaving the session and logging again with Xfce, I noticed everything worked fine. All window apps rendered perfectly, even video could be watched.

For reference, I'm attaching some logs. The computer is not in use, but I can only answer when I get home, at night.

Thanks.

Created attachment 6024 [details]
Journalctl dump. I don't know how to extract the relevant info.

Created attachment 6025 [details]
Xorg.0.log

PS:

The videocard is an old Geforce 6200. There is still a proprietary Nvidia driver for it, but I chose to use the Nouveau driver which seems to be wiser in the long term.

The CPU is an old AMD Sempron 2300+, considered to be an i686, but lacking an important instruction (SSE2).

PS2: Forgot to mention that Kwin segfaults after I choose to leave/close the session to return to the kdm login screen.

There's a Baloo error shown which is described in bug 15457 https://bugs.mageia.org/show_bug.cgi?id=15457 , AFAIK unrelated to the present issue.

After a hunch, I decided to invert window managers.

In Xfce, using "kwin --replace" makes the screen garbled;

In KDE, using "xfwm4 --replace" makes everything ok, including KDE software like menus, systemsettings, Dolphin, Konqueror etc.

So, I'd say it has to do with kwin. But then I'm only a user...

Changed bug title to reflect my recent discovery.

Summary: Display corrupted by kwin. => Display corrupted (kwin OpenGL 2 default?)

Changed title again.

Apparently the problem was solved. Further testing required.

The reason for bug seems to be KDE/kwin defaulting to OpenGL 2.0; the Geforce 6200

Summary: Display corrupted (kwin OpenGL 2 default?) => Display corrupted (kwin/nouveau OpenGL 2 ?)

[... continuing]

... the Geforce 6200, though reported by Nvidia ( http://www.nvidia.com/object/geforce6_techspecs.html ) as supporting OpenGL 2.0, possibly does so with the use of proprietary drivers.

For this bug, toggling kwin composition (SystemSettings / Desktop Effects / Advanced Tab / Composition type) to OpenGL 1.2 makes the display correct (hint: change to Xrender, image becomes ok, change then to OpenGL 1.2).

Changing back to Open GL 2.0 makes the display garbled (after a time without confirmation, composition is reset automatically to OpenGL 1.2.

Therefore, I think it's a matter of somehow changing kwin defaults for old hardware. I haven't the slightest idea of how this can be done. Good luck! :-)

Closing as RESOLVED WORKSFORME.

Status: NEW => RESOLVED
Resolution: (none) => WORKSFORME

Additionally, I report slight problems (sudden onscreen artifacts, X still working) with the Accurate scaling method (Advanced tab in Desktop Effects). Using Crisp as recommended apparently helped (without further investigation there's really no certainty).

Even though I found a workaround which WORKSFORME, I'm reopening because someone might have to change some configuration based on the fact that Nouveau didn't like OpenGL 2.0.

Or, alternatively, there might be a way to make OpenGL 2.0 work.

Status: RESOLVED => REOPENED
Resolution: WORKSFORME => (none)

Just as a hint, if someone is facing this bug, soon after logging into KDE the screen becomes garbled.

Right-click on the empty desktop and move the mouse over the newly opened menu until Konsole becomes visible. Click on it and type "systemsettings"[Enter]. Strive then to click on Desktop effects and choose the advanced tab. Click then on the renderer dropbox: normally the options won't be correctly displayed, but one can use the up/down arrow keys to change selection (Xrender and OpenGL 1.2 should work with the Geforce 6200 card and Nouveau). Try to find then the "Apply" button (drag the window with Alt-left-mouse-button if needed).

It would be nice to add a note in Errata.

Whiteboard: (none) => MGA5TOO FOR_ERRATA

Sorry not to have found this before, but the KDE people already have a note about that in https://userbase.kde.org/Desktop_Effects_Performance .

I quote:

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

 OpenGL Version

KWin supports OpenGL 1.x and OpenGL 2.x. By default KWin uses features of OpenGL 2 if they are available. OpenGL 2 allows to use more effects, but requires stronger hardware. It is important to know that even if your driver only supports e.g. OpenGL 1.4 it is possible that KWin uses features of OpenGL 2 which are available through extensions in the driver.

You can find the OpenGL version provided by your driver in the "OpenGL/ES version" string in KInfoCenter as described above. The version is defined by the first two or three digits, e.g. 2.1.

It is not trivial to find out whether KWin uses OpenGL 1 or 2 as this is completely determined at runtime. The best available test is to use the Invert effect which can be enabled in Systemsettings -> Desktop Effects -> All Effects tab. After enabling the screen should be inverted when using the shortcut Meta + Ctrl + I. If it inverts OpenGL 2 is used, if it does nothing OpenGL 1 is used.

It is possible to force KWin to use OpenGL 1 through Systemsettings -> Desktop Effects -> Advanced tab and unchecking Use OpenGL 2 Shaders. This can improve the performance for older hardware. In general it is completely safe to use OpenGL 2 Shaders if the driver reports a version of 3.x (only available with NVIDIA at the time of this writing).
Ktip.png
 
Tip
For modern hardware the OpenGL 2 Shaders yield a better performance than OpenGL 1. Disabling this option is no receipt for best performance.

\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/



Since it is very hard to navigate in a garbled display, it would be useful to have an option to start KDE without OpenGL.

Alternatively, a better way might be to start e.g. LXDE and once inside it, run KDE's "systemsettings".

Along the same reasoning I wrote in comment #6, in bug 15594, I think the present bug could be closed as WORKSFORME (please see comment #10) and, since there is a workaround, I think we can lower the severity and priority fields.

Please correct if things should follow a different course.

Thanks.

Priority: Normal => Low
Severity: major => enhancement

Duh.

Just press Ctrl Shift F12 to deactivate desktop effects. Then the screen will be shown correctly. You will be able to set rendering (in the Advanced tab) to OpenGL 1.2 and reactivate (Ctrl Shift F12 again) the special effects again.

Please read comment #17 again, but the correct shortcut to disable Desktop Effects is Alt Shift F12 (not Ctrl, sorry for the confusion).

Note added to "Hardware Issues" in Errata.

Thanks, it only lacks a link to this bug report.

Whiteboard not changed to IN_ERRATA... is it done by another person?

(In reply to Renato Dali from comment #21)
> Whiteboard not changed to IN_ERRATA... is it done by another person?

can be done by anybody :)

Link added (I intend to add links in other notes I wrote, too).

Since there's a workaround and it's already noted in the Errata, and since this is only useful is (growingly) uncommon situations, I guess this bug could be considered as solved (I'm considering it as WORKSFORME).

Please feel free to undo that if there's another way to solve it.

Status: REOPENED => RESOLVED
Resolution: (none) => WORKSFORME