The ability to construct accurate, easily modified models is helping make manufacturers more competitive.
To construct three dimensional displays from a model in a CAD database, software must first be given a viewpoint of the observer. With information about the viewpoint, the computer can calculate which surfaces of the CAD model would be visible from that particular perspective. The typical approach to sorting visible surfaces starts with those closest to the viewer and works back. The closest objects block the view of some objects farther away, and so forth.
There are also two clipping operations associated with the view on the screen. The first operation uses the distance between the viewer and the model to calculate which part of the model would be visible on the screen. Everything else outside the viewing box is ignored. The second operation decides which objects would not be visible because they are completely hidden by objects closer to the viewer.
Once the computer has calculated which surfaces are visible, it determines the color of each visible surface, which depends both on lighting and on the original surface color. Advanced CAD programs can often assign a variety of simulated light sources such as spot lights, colored lights, directional light sources as like light coming through a window, and so forth.
Once the CAD program has determined basic coloration, it calculates a specific shade for each pixel in the display. Advanced programs optimized for realism may employ sophisticated methods for generating this sort of shading. The calculations involved, however, are lengthy and work best on high-end graphic workstations.
High-end software may also add refinements such as antialiasing to reduce the jaggedness of diagonal lines, transparency for producing effects such as fog or tinted glass, and texture mapping instead of calculating a color value. This provides the appearance of surfaces such as wood grains.
