Page 282 OpenGL Super Bible! If the PC (Affordable web design)
Page 282 OpenGL Super Bible! If the PC graphics card is in 24-bit color mode, then each pixel is displayed precisely in the color specified by the 24-bit value (three 8-bit intensities). In the 15- and 16-bit color modes, Windows passes the 24-bit color value to the display driver, which converts the color to a 15- or 16-bit color value before displaying it. In 24-bit color mode, the RGB color cube measured 255 (or 8 bits) per side. In 15- or 16-bit color mode, the color cube measures 32 (5 bits) or 64 (6 bits) on a side. The device driver then matches the 24-bit color value to the nearest color match in the 15 or 16-bit color cube. Figure 8-13 shows how an 8-bit red value might be mapped to a 5-bit red value. Figure 8-13 A medium-intensity red being mapped from an 8-bit value to a 5-bit value At the low end of the scale, 4-bit color mode can only display 16 colors. These colors are fixed and cannot be modified. Internally, Windows still represents each color with a 24-bit RGB value. When you specify a color to use for drawing operations using the RGB macro or glColor3ub, Windows uses the nearest color of the 16 available to fulfill the request. If the color is being used for fill operations, the color is approximated by dithering the available colors. Dithering Having only 16 colors to work with makes the 4-bit color modes poorly suited for graphics. One thing the Windows GDI will do to help is to perform dithering on solid shapes and objects in this mode. Dithering is a means of placing different colors close together to produce the illusion of another composite color. For example, if you place yellow and blue squares together in a checkerboard pattern, the pattern will take on a greenish appearance. Without actually mixing the colors, the green would have a grainy appearance. By changing the proportion of yellow to green squares, you are effectively changing the intensities of yellow and green.
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