XLIB Programming Manual, Rel. 5, Third Edition

7.8 Standard Colormaps

A standard colormap is one in which the mapping between pixel values and colors is predictable. The purpose of standard colormaps is to encourage sharing of entire colormaps (not just individual cells) between applications that have too demanding color needs to be able to allocate read-only colors out of the default colormaps.

X defines a set of properties that contain information describing commonly used colormaps. An application reads these properties by calling XGetRGBColormaps(). This call returns an XStandardColormap structure that contains enough information so that the application can calculate the colors in every colormap cell (or a certain range within the colormap). This structure may also include the ID of a colormap matching this description that was created by the window manager or another client. The X distribution from MIT includes a standard client, xstdcmap, that creates the standard colormaps. The user can arrange for xstdcmap to be invoked when the X server starts up, or in the user’s .xsession or .xinitrc file. If this program has not been run, the application can create a new colormap and use the information in the standard colormap properties to allocate and set the colors according to the information in the property. There are functions in the Xmu (miscellaneous utilities) library for allocating standard colormaps.

But how does the sharing work? After creating this colormap, the application (or xstdcmap) sets the ID of the created colormap ...