Pseudo Colour applications on True Colour desktops

Some applications, including several Starlink supported ones, were designed to be used on Pseudocolor displays. While this is not a problem on the Sun, DEC or NCD hardware, most of which supply a Pseudocolor display by default, the newer Linux machines supplied by Starlink are usually configured to take full advantage of their superior graphics hardware and provide Truecolor displays by default. Trying to use applications which were designed to run uder Pseudocolor displays on a Truecolor desktop may cause the program to display poorly, fail to run, or in extreme cases crash the X server.

There are several approaches to this problem. The least desirable option is to run your Linux X server in Pseudocolor (so called 8 bpp) mode by starting X Windows using the following command:

% startx -- -bpp 8

this does mean however that you are not utilising your graphics hardware at it best performance. This is somewhat irritating, hence there is a second (better) approach to the problem, you can run two different X servers on the same machine at the same time.

Linux offers the ability to use virtual consoles, these enable you to have several simultaneous sessions on the same machine. You can change between sessions by hitting Alt-F$\star$, e.g. Alt-F2 (hold down the Alt key and press the F2 key), or if you are in X Windows Ctrl-Alt-F$\star$.

So how does this help us run multiple X servers? Login to your Linux machine as normal and start X Windows using the startx command. Once X Windows has booted, hit Ctrl-Alt-F2 to switch to a second virtual console, and login again. Once you've logged in start a second X server, this one in 8 bpp mode, using the following command:

% startx -- :1 -bpp 8

You can now switch between the two X servers using Ctrl-Alt-F7 (the original Truecolor X server on :0) and Ctrl-Alt-F8 (the Pseudocolor X server on :1). Applications can even be started in an xterm on one X server and displayed in another by using the -display localhost:0 and -display localhost:1 command line options, which most X applications will accept.

You should note that your system administrator may have configured your Linux box to automatically start a Pseudocolor display. If so you can follow the instructions above towards the opposite goal, having a Truecolor display on :1 by starting the second X server using the following command:

% startx -- :1 -bpp 16

or if you have more graphics memory (and your X server has been configured to use it):

% startx -- :1 -bpp 24

There is a down side, since you are now running two X servers you are using twice as much memory, it is inadvisable to try this approach on machines with only a small amount of system memory (RAM). However, yet another additional solution to the problem of Pseudocolor applications and Truecolor desktops is presented in the section on the VNC application.