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16.7 million colors = 24 bits

Most technicians won’t say, for example, “I set my video card to show over 16 million colors.” Instead, they’ll say, “I set my color depth to 24 bits.” Talk in terms of bits, not colors. It is assumed that you know the number of colors for any color depth.

You can set the color depth for a Windows 2000 or Windows XP computer in the Display Properties applet on the Settings tab (Figure 19-36). If you set up a typical Windows XP computer, you’ll notice that Windows offers you 32-bit color quality, which might make you assume you’re about to crank out more than 4 billion colors, but that’s simply not the case. The 32-bit color setting offers 24-bit color plus an 8-bit alpha channel. An alpha channel controls the opacity of a particular color. By using an alpha channel, Windows can more effectively blend colors to create the effect of semi-transparent images. In Windows XP, you see this in the drop shadow under a menu; in Windows Vista, almost every screen element can be semi-transparent (Figure 19-37).

Your video card and monitor are capable of showing Windows in a fixed number of different resolutions and color depths. The choices depend on the resolutions and color depths the video card can push to the monitor and the amount of bandwidth your monitor can support. Any single combination of resolution and color depth you set for your system is called a mode. For standardization, VESA defines a certain number of resolutions, all derived from the granddaddy of video modes: VGA.

Figure 19-36 Adjusting color settings in Windows XP

Figure 19-37 Semi-transparency in Windows Vista

VGA

With the introduction of the PS/2, IBM introduced the video graphics array (VGA) standard. This standard offered 16 colors at a resolution of 640 × 480 pixels. VGA supported such an amazing variety of colors by using an analog video signal instead of a digital one, as was the case prior to the VGA standard. A digital signal is either all on or all off. By using an analog signal, the VGA standard can provide 64 distinct levels for the three colors (RGB)—that is, 643 or 262,144 possible colors—although only 16 or 256 can be seen at a time. For most purposes, 640 × 480 and 16 colors defines VGA mode. This is typically the display resolution and color depth referred to on many software packages as a minimum display requirement. Every video card made in the past 15 years can output as VGA, but VGA-only cards are now obsolete.

Beyond VGA

The 1980s were a strange time for video. Until the very late 1980s, VGA was the highest mode defined by VESA, but demand grew for modes that went beyond VGA. This motivated VESA to introduce (over time) a number of new modes with names such as SVGA, XGA, and many others. Even today, new modes are being released! Table 19-1 shows the more common modes.

Table 19-1 Typical Display Modes

The video card must have sufficient RAM to support each combination of color depth and resolution. Many years ago this mattered, when video cards had scant megabytes of memory. A video card with only 2 MB of RAM, for example, could handle a high color (16-bit) display at 1024 × 768, but not the same resolution with true color (24-bit). Table 19-2 shows common modes and the minimum video memory needed. All modern video cards can handle true color at any resolution.

Table 19-2 Common Modes and the Minimum Video Memory Required

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EXAM TIP To accommodate rotated LCD monitors in portrait view, the video resolution numbers might be reversed. Rather than 1280 × 1024, for example, you might see 1024 × 1280. The amount of RAM needed remains the same regardless.

Motherboard Connection

Using more color depth slows down video functions. Data moving from the video card to the display has to go through the video card’s memory chips and the expansion bus, and this can happen only so quickly. The standard PCI slots used in almost all systems are limited to 32-bit transfers at roughly 33 MHz, yielding a maximum bandwidth of 132 MBps. This sounds like a lot until you start using