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Rocketing Data to the Monitor

At this point, the pixels aren't ready to paint the screen. After each chunk of image data is converted to pixel data, it gets sent back to memory for storage. Most CRT monitors comprehend only analog signals. That's where the graphics board's random access memory digital-to-analog converter, or RAMDAC, comes into play. As its name implies, the RAMDAC changes the digital data for each pixel into the red, green, and blue analog signals that your monitor can use to display the image. The faster the RAMDAC is able to convert data (measured in megahertz), the higher the resolution the graphics board can produce, and the faster it can refresh. (A few CRT--and many LCD displays can accept the digital signal; those displays require a special card with no RAMDAC.)

The monitor displays analog pixels one line at a time, and to reproduce the illusion of movement, the graphics board must produce many screenfuls of pixels every second. Graphics board vendors call the speed at which the board paints pixels its fill rate; current boards provide fill rates in the range of a gigapixel (roughly 1 billion pixels) per second. Faster fill rates lead directly to faster frame (or screen refresh) rates--how many times the entire screen can be redrawn each second. Rates of at least 30 frames per second (the frame rate for television and video) trick the eye into seeing fluid motion.

When you raise the resolution (a lot of cards support resolutions of up to 2048 by 1536 pixels), you add to the graphics board's workload by forcing it to produce a larger number of pixels. Likewise, when you increase the color depth from 16-bit "high color" to 32-bit "true color," it adds to the complexity of the work the card must perform. Because a larger frame requires more pixels to fill it, the frame rate can slow down if the workload reaches the maximum capacity of the GPU and RAMDAC to produce pixel data.

Meeting the 3D Challenge

Like Superman bending steel, newer graphics boards easily manage the processing of two-dimensional images (such as those created when you browse the Web or work in office applications). These so-called 2D images get rendered only in the vertical and horizontal, like drawing on a piece of paper.

But to create realistic 3D images on a two-dimensional surface like your monitor requires more out of the card: the perception of depth. If colored pixels or dots are the building blocks of 2D graphics, polygons--shaded in with textures on the sides to add the illusion of depth--make up 3D images. Most people encounter 3D images when they play games on their PC, but a handful of applications, from CAD to 3D modeling programs, also use the 3D processing power in the card. Handling polygons takes far more processing power than assigning colors to pixels, but today's GPUs--some of which have as much processing power as the CPUs in budget PCs--are up to the task.

Tricks of the 3D Trade

Graphics boards use the large bag of tricks built into their firmware and drivers to render 3D perspective in two dimensions, tricking the eye into believing that polygons displayed on a flat monitor tube have depth. Depending on your board, some of these features are handled by the GPU, while others are handled by your computer's CPU through the graphics driver. The tricks include the following:

• Hardware transform and lighting handles the placement of polygons and lighting effects.
  
  
• Bump mapping applies the look of smoothness or roughness to polygon textures, which helps add to the illusion of depth.
  
  
• Antialiasing removes the jagged edges of diagonal lines in drawn images.
  
  
• MIP mapping prevents the card from trying to draw detailed polygon textures on "distant" objects (like the ground in flight simulator games, when you're flying at cruising altitude) until you get closer, which saves processing power.
  
  

In addition to the graphics driver, software called the Application Programming Interface controls all the other aspects of this 3D rendering process in a common language that the graphics board (or its driver) can understand. There are several competing APIs, including Microsoft's Direct3D (part of DirectX), OpenGL, and the proprietary GLide API made by 3dfx for its own boards. Most graphics boards are designed to work with more than one API for their 3D rendering.

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