XLIB Programming Manual, Rel. 5, Third Edition

2.3 Introduction to X Graphics

This section provides a brief introduction to the terms and concepts used in graphics under the X Window System. You will see these terms used in Chapter 3 and Chapter 4 before we get to a serious treatment of graphics in Chapter 5, Chapter 6, and Chapter 7.

2.3.1 Pixels and Colors

The X Window System is designed to control bitmapped graphics displays. In the simplest black-and-white display, there is a single bit per pixel: the state of that bit determines whether the pixel will be black or white. In color systems or on monochrome systems allowing gray-scale displays, there are multiple bits per pixel.

The state of the multiple bits assigned to each pixel does not directly control the color or gray-scale intensity of that pixel. Instead they are used as an index to a lookup table called a colormap, as shown in Figure 2-8. On a color display, a pixel consists of separate red, green, and blue phosphors, each sensitive to a separate electron beam; the relative intensity of these three colors fools the eye into thinking it sees a single color. Accordingly, the colormap contains an array of red, green, and blue (RGB) triples. In other words, if the value of the bits for a given pixel (a pixel value) is 14, the RGB values of the fourteenth member of the colormap will be displayed at that location on the screen.

Figure 2-8. Mapping of pixel value into color through ...