Important and Unimportant Video Card Features

Walk into a computer superstore, or even an office supply store, and you're likely to see shelves stacked with video cards costing $100 or more. What would such an expenditure buy, though? Is it really necessary? Features that are common on high-end video cards include:

Fast Video Bus Most video cards sold today use the Accelerated Graphics Port (AGP) bus. This bus was designed for use by high-speed video cards, and modern motherboards include an AGP bus for video cards. Older video cards use the Peripheral Component Interconnect (PCI) bus, which is still used by most nonvideo plug-in cards; or even slower busses, such as the Industry Standard Architecture (ISA) bus.

Multiple Outputs Some high-end video cards support multiple outputs—for instance, to handle both a monitor and a television. This feature is useful in certain specialized situations, but most Linux workstations don't need it.

2D Acceleration Video cards use on-board memory to store a bitmap of the entire screen. X can modify this RAM more-or-less directly to create windows, display text, and so on; but this process consumes a lot of CPU time and bandwidth on the video bus. A quicker approach is to include enough smarts on the video board itself to draw specific shapes in response to simple commands from the X server. This feature is known as 2D acceleration, and it's present in all modern video cards. Some do better 2D acceleration than others, though.

3D Acceleration The area of greatest improvement in video card designs over the past few years has been in 3D acceleration. This feature enables a video card to create a 2D representation of a 3D object. As with 2D acceleration, the computer's main CPU can perform this task, but it's more efficient to pass this task off to a dedicated video processor. This feature is most commonly used by video games, many of which present first-person views. In Linux, 3D acceleration requires support by a 3D graphics library such as Mesa (http://mesa3d.sourceforge.net), which in turn is an open-source implementation of the OpenGL (http://www.opengl.org) specification.

Lots of RAM The larger your video display and the greater the color depth, the more RAM a video card requires to support the display. It's almost impossible to buy a new video card today that comes with less than 32MB of RAM. As Table 1.2 shows, this amount of RAM is more than enough for even very high-resolution 2D displays. Video cards with more RAM use the extra memory mainly to support their 3D acceleration features.

Table 1.2: RAM Requirements for Various Video Resolutions, in Kilobytes

Resolution

8-bit

16-bit

24-bit

32-bit

640 x 480

300

600

900

1,200

800 x 600

469

938

1,406

1,875

1024 x 768

768

1,536

2,304

3,072

1280 x 1024

1,280

2,560

3,840

5,120

1600 x 1200

1,875

3,750

5,625

7,500

1920 x 1440

2,700

5,400

8,100

10,800

Note Color depth is the number of bits of data devoted to describing the color of each pixel. An 8-bit display provides 28, or 256, possible colors per pixel; and a 24-bit display provides 224, or 16,777,216, colors per pixel. As a general rule, X works best with 16-bit (65,536-color) or greater color depth.

Of these features, none is truly required for X operation. Even many ancient unaccelerated ISA video cards will work with X—although most modern motherboards lack the ISA slots needed by such outmoded cards! In practice, for basic desktop use in Linux, most modern video cards will work. RAM beyond 32MB (or even 10MB) and flashy 3D acceleration features aren't needed even for very large and color-heavy display modes. If you play first-person perspective video games or run some of the rare business or scientific applications that take advantage of OpenGL/Mesa 3D acceleration, you may want to examine video cards for which Linux Mesa support is available and pay attention to benchmark numbers for these video cards. Unfortunately, benchmarks in mainstream computer magazines are likely to use Windows applications, and Linux performance may not track Windows performance very closely. Fortunately, various people have posted Linux Mesa benchmarks online, including the Jongl benchmark page (http://www.jongl.de/benchmarks.html) and various reviews and articles at Linux Games (http://www.linuxgames.com).

Unfortunately, few reviews focus upon 2D performance, so if you're shopping for a card to use in a workstation, finding relevant performance information can be difficult. The good news is that 2D acceleration is common and very good on most video cards, so the lack of performance reviews isn't as serious a problem as it might be. You should, however, check on the status of X drivers for any card you're considering, as described in the next section, "Supported and Unsupported Video Card Features." Watch for words like "unaccelerated support" in X driver descriptions; this means that the card's 2D acceleration features are unused.

If a Linux computer is to be used as a server, even an ancient text-only video card should be fine. With the possible exception of initial setup, you don't need to run X on server computers. Therefore, you should buy the least expensive video card you can

Most laptop computers and even some desktop systems come with video circuitry integrated on the motherboard. Such systems can be convenient to buy, but they're risky because adding a video card to such a system can be tricky or impossible if the on-board video doesn't work. When buying such systems, you should be very careful to make sure your video hardware will be adequate, both in terms of features and in terms of X driver support. Also, some such systems use part of the computer's main memory as video RAM. For instance, if a computer has 128MB of RAM, you may have to set aside 4MB of that RAM to run a 1024 x 768 display at 32-bit color depth (see Table 1.2; the amount of RAM set aside is typically rounded up to the nearest power-of-two value in megabytes). Worse than this loss of RAM is the fact that the RAM on the motherboard is typically of a different type than the RAM on a video card. Standard motherboard RAM produces less-than-optimal performance with video card circuitry.

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