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I'm currently setting up a new Xen paravirt domain running KDE (4.2.2, from Kubuntu 9.04). As I have been unable to get the virtual framebuffer working in it, I've decided to set up VNC (from the vnc4server package), and run KDE over Xvnc.

This is all fine and good, and KDE starts up okay. However, all the colours look dithered, especially on the task bar and title bar, making them impossible to see. From my web searches, it appears to be because these items are drawn using Porter-Duff. This is especially the case when using the Oxygen style, and Oxygen and Ozone window titlebars (selecting these styles generates messages about Porter-Duff being unavailable); not using those styles at least makes most of the UI widgets and window titles usable again.

But this doesn't solve the problem for the task bar, nor for the desktop, where the only theme available to me is Oxygen (this is under the "Desktop Settings - Plasma Workspace" window, just for reference). So, unless I have a way to use a non-Porter-Duff theme for those, it seems that KDE would still be unusable under VNC.

So if someone experienced with KDE can advise on how to work around, or even fix, these issues, I'd appreciate it very much. :-)

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4 Answers

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Why don't you use remote xdmcp?

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Well, that also can have its rendering issues, like if fonts are missing? Of course, one can set up the font server to get fonts over over the wire as well (Google xfs and xset +fp tcp/your-kde-machine:7100). And when controlled using a different keyboard (like from a Mac) then one might need to solve keyboard issues as well, especially when using Xnest or Xephyr (Google xmodmap, or use xev for debugging). I am not sure about speed: it can be very fast, but others say it needs a lot of bandwidth. – Arjan Oct 2 '09 at 8:15
Yes, that's a good question. I might try that and see if it helps the situation, although VNC has other benefits, like not having to lose your session when your thin-client machine is being rebooted, much like using screen for console stuff. – Chris Jester-Young Oct 27 '09 at 18:34
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Dithering generally occurs when a color palette of higher depth must be displayed using a color palette of lower depth. For example, rendering a 24-bit color palette (2^24=16M colors) in a 16-bit display (2^16=64K colors) or a 16-bit color palette in a 8-bit display (2^8=256 colors)

Just a shot in the dark, but vncserver has a -depth flag you can specify to change the color depth of the server to match your desired client display settings. I assume vnc4server has a similar flag? It defaults to 16-bit, so I'm wondering if you run your VNC server under 24-bit depth if that would prevent the dithering.

I'm not sure of the performance implications or whether that even matters to you.

There may be other ways to solve your problem, but this is the first thing that comes to mind.

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you can check the vnc server's logfile to see what depth it was running at. the location depends on how it was run; check in /var/log or in ~/.vnc first. – quack quixote Oct 9 '09 at 2:38
I always run Xvnc at 24 bits, so this isn't the problem. There is no alpha-blending capability with Xvnc though (that I'm aware of), so that's where the dithering comes in (if I understand correctly as to the function of Porter-Duff). – Chris Jester-Young Oct 27 '09 at 18:30
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I was playing with this today (on Kubuntu 10.04), but I eventually gave up and went with XFCE4 for VNC connections, and KDE for a local display.

Install the package xfce4 and edit ~/.vnc/xstartup/ to look like this:

#!/bin/sh

unset SESSION_MANAGER
. /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &

startxfce4 &
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I had success switching my VNC server from the default vncserver to http://xf4vnc.sourceforge.net/. This appears to be more stable and prevents the ugly dithering effect on the window decorations. I'm on Ubuntu 10.04

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