How can I reduce color depth from 24 down to 16 in KDE Plasma (for remote desktop)?

Help
1

Hi. I’m on Kubuntu (though I’ve experienced the same with Neon). KDE is 5.24.7 , X version is 7.7
As I’m working with desktop of my host remotely (it’s a cloud VM on vultr), I’m trying to minimise everything that affects bandwidth consumtion. I’ve already performed some basic changes, so now I’m into some lower level settings.

And I have hard times figuring out how to reduce color depth for my desktop. Currently it’s 24 as per “xwininfo -root | grep Depth”

What I’ve done so far and what I experience:
I’ve generated xorg.conf for my system with “xorg -configure” and have placed it under /etc/X11 (it was not present originally).
The system boots just fine if I keep originally generated xorg.conf

However, if I add DefaultDepth 16 under Section “Screen” , the system stops to boot into KDE:
I do see “kubuntu” logo at startup, then it’s showing terminal boot line “[OK] Finished Terminate Plymouth Boot Screen” and then - nothing, it doesn’t respond to any key kombinations.

I have tried to search for some cues in logs - couldn’t spot anything above warning.
I have tried to delete .Xauthority as it was suggested in some posts - it didn’t help.

As long as I comment out the line DefaultDepth in xorg.conf (via rescue boot), KDE is back to normal.

Please advise either how to properly reduce color depth from 24 to 16 using some higher-level approach, or at least - how to diagnose what’s the issue so I can try to fix it.

Thank you.

2

AFAIK you set the color depth in the remote display application, e.g. for vncviewer (man vncviewer):

       -FullColor, -FullColour
              Tells the VNC server to send full-color pixels in the best format for this display.  This is default.

       -LowColorLevel, -LowColourLevel level
              Selects the reduced color level to use on slow links.  level can range from 0 to 2, 0 meaning 8  colors,  1  meaning  64
              colors  (the  default), 2 meaning 256 colors. Note that decision if reduced color level is used is made by vncviewer. If
              you would like to force vncviewer to use reduced color level use -AutoSelect=0 parameter.
3

Thank you for the suggestion, it may be a kind of compromise as a last resort.

I’m not using VNC. And overall - instead of creating a need for any kind of post-processing (that will require resources) on the side of remote access server (VNC, etc), I thought it may be better just to config Plasma in a way so it shows lower color depth.
Are you aware on how to do it in KDE/X?

4

The VNC was hust an example I’m familiar with, I’m sure other protocols have something similar but yes, if you’re only connecting remotely, a server side solution would be best.

Unfortunately I don’t know how to do that, but according to man Xorg:

    -depth n
        Sets the default color depth.  Legal values are 1, 4, 8, 15, 16, and 24.  Not all drivers support all values.

and:

    -fbbpp n
        Sets the number of framebuffer bits per pixel.  You should only set this if you're sure it's necessary; normally the server can deduce the correct
        value  from  -depth above.  Useful if you want to run a depth 24 configuration with a 24 bpp framebuffer rather than the (possibly default) 32 bpp
        framebuffer (or vice versa).  Legal values are 1, 8, 16, 24, 32.  Not all drivers support all values.

So maybe you can force it “by hand”? e.g. from SDDM (man sddm)

    [X11] section:

    ServerArguments=
        Arguments to the X server.  Default value is "-nolisten tcp".

Make a file, say /etc/sddm.conf.d/depth.conf with content like this:

[X11]
 ServerArguments="-nolisten tcp -depth 16"
1 Like
5

thank you, I’m right now in the process of extending my cloud-init with minimal x install, then will move to sddm/KDE, so will check if I can do it

6

Again, I thought I can manage it via xorg config, but it didn’t work out…

7

From that, maybe you’re just unlucky :frowning: