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

of graphics memory so that each pixel has a greater depth of information.
    For example, each pixel could go from 8-bit deep data (the example al- ready presented) to 16-bit deep. However, since there are 3 colors, and 24 is di- visible by 3, 24 bits is an even better example. Now with the 24-bit color graphics adapter, there can be 3 bytes of data for each pixel, and there can be 8 bits for each color at the pixel. There are 256 different levels of each color possi- ble (there are 256 combinations of 8 ones and zeros), and now the total number of combinations of red, green, and blue becomes over 16 million (256 times 256 times 256). This is basically sufficient to give the needed illusion of shading of 3-D models mentioned, and it is basically sufficient to make a digitized photo- graph appear real.
    So, for the 24-bit graphics adapter with 1280 by 1024 resolution, the total amount of graphics memory used is about 4 megabytes (there are about 1.3 mil- lion pixels and there are 3 bytes for each pixel resulting in 3.9 million bytes). 4 megabytes of graphics memory should be considered the “bare” minimum for 3- D CAD systems, though. The quality of the system will be noticeably improved by going beyond the 1280 by 1024 resolution, and there is more than just color data that can be associated with each pixel. If 4 bytes are allocated for each pixel (instead of just 3), then information such as translucency of surfaces or how far away the pixel is from the observer (Z buffer) may be stored with each pixel. This will further enhance the performance and capability of the graphics adapter. Therefore, 16 megabytes or more of graphics memory may be justified.
    The last characteristic of graphics adapters that needs to be mentioned is frames (or perhaps called pages). Frames are just a static state of the graphics data being sent to the monitor. Frames are relevant to animations or other opera- tions that need to quickly change the image shown on the monitor. Obviously, if a new frame is sent to the monitor more quickly, the animation is improved. Al- though animations may generally be associated with multi-media presentations or entertainment, they are of some value to CAD users that are working on me- chanical component and/or assembly design. As animations of how to assemble products become more common than assembly drawings, this will become a more important issue.

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    Since graphics adapters have processing capabilities of their own (such as the coprocessor), it turns out that the graphics adapter can actually be loading new data to one area of the graphics memory while the monitor is still displaying another set of data. Then, a signal is sent to have the monitor suddenly switch to displaying the area of graphics memory just completed being updated. This oper- ation is based on having 2 frames. One frame is loading while the other is dis- playing, and then the process is reversed for the 2 frames. As usual, this explanation is an oversimplification, but it can be seen that this sort of behavior is going to require double the amount of graphics memory (for a given resolution and number of colors to display).
    2.7.3 Hardcopy
    The next type of peripheral to be discussed is hardcopy devices. As the name im- plies, these are devices that create paper copies from the data in the computer system. This could be printed material (such as a document) or a graphical im- age. Printing text is not a very significant issue for CAD systems, so this discus- sion will be generally limited to printing of graphical images.
    The most basic distinction between hardcopy of CAD information is 2-D versus 3-D. 2-D hardcopy is going to be centered on the plotting of drawings. Drawings with respect to a CAD system has a very specific meaning; it implies standardized engineering drawings (see Chapter 4). These drawings are made up of 2 dimensional geometric entities such as lines, arcs, characters, etc. The format of these