XLIB Programming Manual, Rel. 5, Third Edition

7.3 Differences in Display Hardware

The description of color mapping given in the previous section was actually somewhat over-simplified. There are significant differences in how the colormap is used on mid-range color screens, monochrome and gray-scale screens, and high performance color screens. Color handling in X was designed to work with any of these hardware types.

7.3.1 Mid-range Color Displays

The most common type of color screen has between four and eight planes and uses the colormap indexing technique described above. This type of screen is so widespread because it provides a flexible color system while being moderately priced. The mapping of pixel values to colorcells, with arbitrary RGB values stored in each colorcell, allows a very large range of possible colors, even though a more limited number can be shown on the screen at any one time.

Mid-range color screens usually have only one hardware colormap. In other words, the pixel values in all the windows on the screen are mapped to colors using the same colormap. On most of these systems, however, the color in any colorcell in the hardware colormap can be individually changed, and therefore, the entire colormap can be replaced with a new set of values. X provides the concept of the virtual colormap, so that more than one set of colorcells can be maintained, even though only one of them can be in use at a time. Virtual colormaps are swapped in and out of the hardware colormap by the window manager. This makes it possible ...