Bug 501305

Summary:	Tablet calibration doesn't work properly with some graphics tablets not supported by Linux Wacom driver and can mess up the mapping
Product:	[Applications] systemsettings	Reporter:	alt
Component:	kcm_tablet	Assignee:	Plasma Bugs List <plasma-bugs-null>
Status:	RESOLVED FIXED
Severity:	normal	CC:	aleixpol, alt, josh
Priority:	NOR
Version First Reported In:	6.3.2
Target Milestone:	---
Platform:	Manjaro
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:	6.4.0
Sentry Crash Report:

Description alt 2025-03-10 16:49:41 UTC

SUMMARY
Tablet calibration doesn't work properly with some graphics tablets that are using proprietary user-space drivers and are not supported by Linux Wacom (Veikk A30 tablet in my case).

First of all I can't see the pointer when calibrating, and it seems that the screen mapping in the calibration window is incorrect to begin with because I'm not able to tap the second target and the following ones (this happens in this window only, I don't really need calibration because it's already good by default, I just wanted to try this feature).
The first time I tried it I touched the upper left corner of the tablet screen but couldn't proceed with the calibration, so I hit "Cancel". And yet the calibration got misconfigured and I couldn't use my tablet on Wayland.

Why isn't there a "reset to defaults" option? I tried clicking on "Defaults" but nothing changed, still misconfigured.
I had to delete the tablet configuration in ~/.config/kcminputrc to reset it correctly. The calibration matrix was full of "Nan".

STEPS TO REPRODUCE
1. Connect a graphics tablet like Veikk's (not supported by Linux Wacom) and make sure that the proprietary user-space driver is installed and no other kernel-space drivers compatible with the tablet are installed or active (like OpenTabletDriver).
2. In Drawing Tablet settings click on the pen tab and then "Calibrate".
3. Try to follow the instructions for calibration and press "Cancel" if you're not able to proceed.

OBSERVED RESULT
Can't calibrate correctly, the mapping gets messed up making the tablet unusable. Can't reset the calibration from the settings.

EXPECTED RESULT
If I press on "Cancel" it shouldn't mess up with the default mapping. If the tablet is not supported, the calibrate option shouldn't affect its default mapping in any way instead of messing it up.

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux
KDE Plasma Version: 6.3.2
KDE Frameworks Version: 6.11.0
Qt Version: 6.8.2
Kernel Version: 6.12.17-1-MANJARO (64-bit)
Graphics Platform: Wayland

Comment 1 Joshua Goins 2025-07-30 21:07:48 UTC

Note that it's unsupported to use the calibration matrix feature (or really, any Drawing Tablet KCM settings) with a non-kernel, user-space driver. There is too many unknowns, as KCM and KWin assume that libinput is the one doing any transformations on the input stream. Of course this *could* work, but I have to say it probably won't!

> First of all I can't see the pointer when calibrating

This is intentional, because you are supposed to ignore the current mapping which is what the cursor shows. Otherwise people would jend up calibrating nothing.

> Why isn't there a "reset to defaults" option? I tried clicking on "Defaults" but nothing changed, still misconfigured.

This is fixed in Plasma 6.5, the whole UX in this exact misconfigured situation is much better now and it's easier to get unstuck. Sorry about that!

> 3. Try to follow the instructions for calibration and press "Cancel" if you're not able to proceed.

This is fixed in Plasma 6.4, where if you misconfigured it so much that you can't even press any buttons the KCM will auto-reset in the confirmation step. (Or if you close the window, whichever happens first.)

It seems the ultimate issue is being stuck with a misconfigured tablet, so I'll mark it as fixed in 6.4 but it will be truly good in 6.5 :-) Let me know if you have any other questions!