Web design conference - Page 292 OpenGL Super Bible! devices that support
Page 292 OpenGL Super Bible! devices that support palettes (8-bit display cards). This doesn t mean you can t use color index mode on these devices; it only means there is no corresponding hardware palette with which you can perform animation. Palette animation occurs when you change the entries in the palette, which causes a corresponding change in all screen pixels having that palette index. This can produce color cycling for some special effects. Another reason to use color index mode is for applications that use color to indicate a third dimension to indicate the pressure at certain spatialregions, for instance. You can also use this mode for false color images that do not require an organized palette. Finally, color index mode can be somewhat faster in 8-bit color modes because only one color channel (as opposed to three, one each for red, green, and blue) needs to be manipulated instead of three. In addition to limiting the color selection, color index mode does not support some of OpenGL s other special effects including many lighting effects and shading, fog, anti- aliasing, and alpha blending. Generally, it is better to use RGBA mode. As mentioned, the most significant advantage of using color index mode is for more palette control on 8-bit display devices. The 3-3-2 palette limits your color choices, and if you want 200 shades of red to do really smooth shading on an 8-bit display, you are out of luck. In color index mode, however, the palette entries range from darkest to lightest colors. You can separate the palette into as many or as few bands as you like. The INDEX sample program displays a triangle shaded from black to bright red (see Figure 8-15). This shading is not possible in 8-bit color mode using at 3-3-2 palette. Figure 8-15 Output from INDEX showing over 200 shades of red for smooth shading
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