Specifications

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Chapter 15 Video Hardware
applications use another process, called MIP mapping, which uses different versions of the same
texture that contain varying amounts of detail, depending on how close the object is to the
viewer in the three-dimensional space. Another technique, called depth cueing, reduces the color
and intensity of an object’s fill as the object moves farther away from the viewer.
Using these elements, the abstract image descriptions must then be rendered, meaning they are con-
verted to visible form. Rendering depends on two standardized functions that convert the abstractions
into the completed image that is displayed onscreen. The standard functions performed in rendering are
■ Geometry. The sizing, orienting, and moving of primitives in space and the calculation of the
effects produced by the virtual light sources that illuminate the image
■ Rasterization. The converting of primitives into pixels on the video display by filling the shapes
with properly illuminated shading, textures, or a combination of the two
A modern video adapter that includes a chipset capable of 3D video acceleration has special built-in
hardware that can perform the rasterization process much more quickly than if it were done by soft-
ware (using the system processor) alone. Most chipsets with 3D acceleration perform the following ras-
terization functions right on the adapter:
■ Scan conversion. The determination of which onscreen pixels fall into the space delineated by
each primitive
■ Shading. The process of filling pixels with smoothly flowing color using the flat or Gouraud
shading technique
■ Texture mapping. The process of filling pixels with images derived from a 2D sample picture or
surface image
■ Visible surface determination. The identification of which pixels in a scene are obscured by other
objects closer to the viewer in three-dimensional space
■ Animation. The process of switching rapidly and cleanly to successive frames of motion sequences
■ Antialiasing. The process of adjusting color boundaries to smooth edges on rendered objects
Common 3D Techniques
Virtually all 3D cards use the following techniques:
■ Fogging. Fogging simulates haze or fog in the background of a game screen and helps conceal the
sudden appearance of newly rendered objects (buildings, enemies, and so on).
■ Gouraud shading. Interpolates colors to make circles and spheres look more rounded and smooth.
■ Alpha blending. One of the first 3D techniques, alpha blending creates translucent objects
onscreen, making it a perfect choice for rendering explosions, smoke, water, and glass. Alpha
blending also can be used to simulate textures, but it is less realistic than environment-based
bump mapping (see the section “Environment-Based Bump Mapping and Displacement
Mapping,” later in this chapter). Because they are so common, data sheets for advanced cards
frequently don’t mention them, although these features are present.
Advanced 3D Techniques
The following are some of the latest techniques that leading 3D accelerator cards use. Not every card
uses every technique.
Stencil Buffering
Stencil buffering is a technique useful for games such as flight simulators, in which a static graphic
element—such as a cockpit windshield frame, which is known as a HUD (heads up display) and used
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