Free Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design by Stephen J. Schoonmaker Page A

of geometric entities cre- ated by CAD software may seem simple. However, this task can be complicated,

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    such as when the CAD program is relying on the graphics adapter to create shaded 3-D models. In this case, the CAD program may be completely relieved from interacting with the user for the manipulation of the 3-D model. Once the CAD program defines the desired 3-D surfaces to the graphics adapter, the user may manipulate and “slice through” the 3-D model by interacting only with the graphics adapter. In this case, the coprocessor may be doing millions of compli- cated computations per second.
    Beyond the presence and capability of the coprocessor, probably the next most important characteristic of a graphics adapter is the amount and configura- tion of the graphics memory. As mentioned earlier, the graphics memory may be simply a section or region of the computer system’s main memory. In this case, there will be a somewhat fixed amount of graphics memory to consider since it is an integral part of the computer design. In other cases, the computer may allow for variation or expansion of the graphics memory. In general, more graphics memory (or Graphics RAM, Video RAM, etc.) indicates a better graphics adapter system.
    The graphics memory has enough addresses or locations for storing data within it so that each pixel on the monitor gets a certain predetermined amount of information. If a graphics adapter operates at a resolution of 1280 by 1024, then there are 1,310,720 pixels (1280 times 1024). So, at least these many addresses are needed (or about 1.2 megabytes) in the graphics memory. Of course, each byte can contain any pattern of 8 ones and zeros. These 8 bits can then be used to define specific characteristics of a given pixel. The most obvious characteristic is color.
    Looking at an 8-bit per pixel system, then, the emission of colored light is based on 3 primary colors (red, green, and blue), so 2 of the 8 bits can represent the amount of red, 2 of the 8 bits can represent the amount of green, and 2 of the 8 bits can represent the amount of blue (ignoring the other 2 bits). Any of those 2 bit combinations can have 4 different values (00, 01, 10, 11), so there are 4 levels or shades of each individual color possible for this fictitious graphics adapter. Therefore, the total number of possible shades (combinations of the red, green, and blue) is 4 times 4 times 4 or a total of 64.
    Note that all the pixels have some sort of definition in their corresponding location in graphics memory, even if it looks like the screen is blank. If the graph- ics adapter is functioning and blank areas of the screen are black, then it just means that those parts of the Graphics Memory is set to all 0s (no red, no green, no blue/black) by whatever program is running. As mentioned previously, it is the job of the CAD software to get this graphics memory loaded with the right data (on its own or by calling on the coprocessor), and then it is the graphics adapter that looks at the pattern of bits and then sends the appropriate signal to the moni- tor so that it displays the image desired. This process of turning bits (or digital data) into a “real” electrical signal (or analog) is called a digital-to-analog con-

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    version, and the reverse is called analog-to-digital. This process is performed by “D to A” converters (DACs). This is an important function of the graphics adapter, but it is performed behind the scenes.
    Although the resolution in the example of graphics memory is pretty good (about 1 megapixel), the number of colors is not very good. Of course, at one time 64 colors was acceptable for 2-D CAD, but more likely at least 64,000 col- ors would be needed to show realistic 3-D models for a CAD system (curved sur- faces need to show a very gradual change in the color to give the illusion of depth). The way to improve the available colors (or the palette) is to increase the amount