Converting old 8 Bit Colormap to 32 bit Colormap

Peter Harris peter.harris at
Fri Nov 2 09:55:53 PDT 2007

Edward Slezak wrote:
> I have not programmed X-Windows in some time. I have
> some old code that uses an 8 bit RGB colortable. My
> books do not show how to create a 32 bit colortable.

It generally isn't necessary, since TrueColor colours are not mutable,
nor are they a scarce resource.

> Any help would be appreciated. I just need a snippet
> of code that shows what to do. My code snippet is
> listed below.

Translated into pseudocode, you are doing roughly this:

XCreateColormap(alloc all)
end loop

The first step is to translate that into:
XCreateColormap(alloc none)
end loop

Note that you don't get to pick the pixel values, you have to use the
pixel values that the X server returns to you. The resulting code should
work with any kind of visual.

Then, if you detect you are on a TrueColor visual, you can skip the
XCreateColormap step, and just use the default colormap.

Peter Harris

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