Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (12 page)

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.

Computer Hardware Basics 37

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 drawing are standardized by organizations such as ASME (American So-
ciety of Mechanical Engineers) and SAE (Society of Automotive Engineers). In-
deed, throughout this book, the term drawing means 2-D data and the term model
means 3-D data was used. Figure 4.1 is an example of 2-D data for a drawing.
Hardcopy for 3-D models is quite different (refer to Figure 2.3). The 2-D draw-
ings are really intended to convey specific technical information in a standardized
format. The 3-D hardcopy, however, is really intended to convey a sense of real-
ism for a design. Necessarily then, the type of device needed to create these two
types of hardcopy are quite different.

The hardcopy device for the 2-D drawings is going to need to draw simple
objects (such as lines and circles) in a fast and accurate manner. These hardcopies
are likely to be well used, copied repeatedly, and be read by a wide array of indi-
viduals from a variety of disciplines. Clarity and the simplicity of the image is
paramount. Particularly since it must generally produce excellent copies (or re-
prographics), this output is almost always just black ink on white paper. Since the
device only needs to get information on these black lines, arcs, etc., the data
transfer from the computer to the device is not that great (perhaps 500 kilobytes
for a typical drawing). This type of device could be connected with simple
cables.

38 Chapter 2

The hardcopy device for the 3-D model is going to have to be more sophis-
ticated. This device will almost certainly have to support “full color” (since the
illusion of shading mentioned earlier in the section on graphics adapters will
need to be preserved). This device will also have to print “all over” the paper
instead of just putting down the black ink lines on the white paper (similar to the
pixels on the monitor), and even the “background” may need to be printed. Thus,
the amount of data to transfer from the computer to the device is generally going
to be larger than for 2-D drawings. Assuming the device needs to get as much
data as resides in the graphics memory, there could easily be many megabytes of
data to transfer to the device. With this demand for data transfer, more robust
connections would be used, such as making the device a node on the network and
using the relatively fast network data transfer speeds.

The next basic distinction between hardcopy devices would be the type of
data actually being generated by the CAD software and being sent to the device.
This is a very important issue for making optimal use of CAD system resources.
It turns out that there are basically 2 types of data that will “encapsulate” the in-
formation for an image that is going to be printed. This collection of data for a
particular drawing or model is usually contained in a computer file (“saved” or
“written” to the Storage system or disk drive).

These 2 basic types of data are vector and bitmap. A vector is a mathemati-
cal concept that indicates position of something with respect to an origin and a
direction with respect to that origin. Vectors are discussed in some more detail in
later chapters, but for now it is only necessary to realize that a vector can be de-
scribed by an starting point and an ending point, and that there are mathematical
coordinates for these points (i.e. X and Y values). It is essential to realize that for
the vector data, it is possible to create the hardcopy of the image regardless of the
device chosen. The X and Y values are “analytical” in the sense that any device
can handle X and Y data. For example, the vector file can print a line from a point
at X = 10 mm and Y = 10 mm to a point where X = 250 mm and Y = 10 mm
(which may be the exact locations desired by the user). Of course, if the device is
not large enough to handle a 250 mm coordinate, then the data can easily be
“scaled” by multiplying all the values by a constant factor, and the drawing or
model will look “correct” in the hardcopy format (although as a smaller image).

Another important realization is that the vector data can be considered
“perfect”; there is no approximation of the drawing data within the computer file.
The coordinates of the points should be exactly what the user of the CAD soft-
ware intended (such as 250 mm and 10 mm). This means that the use of better
hardcopy devices could yield a noticeable improvement in the quality of the im-
age, since each device goes to the source for the drawing data and can then at-
tempt to plot the data. A typical example of a vector data file for a hardcopy is
called an HPGL file. This is a type of file created by Hewlett-Packard for support
of their plotters.

Computer Hardware Basics 39

The bitmap is quite different. The bitmap is similar to the graphics memory
discussed in the section on graphics adapters. In this case, there is no pure mathe-
matical definition of what is to be seen in the hardcopy. Instead, the device that is
creating an image generates or dumps the individual bits of data into a file. One
can see that this a type of computer file that can be generated directly from the
graphics memory. The computer file would simply contain the needed number of
bytes of data (i.e. the color) for each pixel. In other words, the bitmap data is
simply that—a file that has a map of what each bit should look like. The genera-
tion of bitmap files directly from the data in the graphics memory (being dis-
played already on the monitor) is often called a screen dump. Dumping (in
computer system’s terms anyway) generally refers to capturing data from mem-
ory or RAM (i.e., memory chips) to storage (i.e., a file on a disk drive).

There are a number of ramifications of the bitmap approach, particularly if
used in conjunction with a CAD drawing or model. First, since the file can not
contain the actual coordinate data that the user indicated, there is a level of ap-
proximation “built into” the file. For example, when a circle is approximated by a
set of specific X, Y bits (just like pixels), there is going to be some visible jag-
gedness to the image (as with a graphics adapter and monitor). Of course, the
higher the resolution of the data the more smooth the circle will appear, but the
hardcopy generally can be no better than what was available when the file was
first created (unlike the vector data that can allow different devices to improve the
hardcopy results). Resolution for these kind of files and hardcopy devices is gen-
erally given as a pitch that is independent of size of the printing area (unlike mon-
itors where resolution is a number of pixels regardless of physical size). In this
case, there are a certain number of dots per distance. For inch distances, the reso-
lution becomes so many dpi or dots per inch. 600 dpi (about 24 dots per mm)
would be fairly standard, with higher quality hardcopy devices using 1200, or
higher.

Another ramification of the bitmap file is size. Notice that the data in a vec-
tor type of computer file can be very compact. To define a single straight line, all
that is needed is the 2 X,Y values for the starting and ending point coordinates
(just 4 numbers or perhaps 32 bytes). For the case of a 240-mm long line (going
from X = 10 mm to X = 250 mm as discussed earlier) and a bitmap file being
used with a resolution of 12 dots per mm, the amount of data in the file expands
to 2880 bytes (using just 8 bits to define a very simple color). Of course, the num-
ber of X,Y values in a vector file with many geometric entities can get just as big,
but a generalization can be made that file size is a bigger concern for the bitmap
files. A typical example of a bitmap data file for hardcopy is a TIFF™ file. To
compensate for the size issue, the bitmap file types are often accompanied by
compression technology that shrinks the file size.

Considering the 2 types of data files, it has been somewhat customary to
consider hardcopy for vector data (2-D drawings) to be called plotting, while

40 Chapter 2

making hardcopy from bitmap data (including just printing text) would be called
printing. Thus, the devices for the vector data hardcopy would be called plotters,
and the devices for other hardcopy would be called printers. For a long time, only
the plotters would give the necessary level of detail and accuracy for the geomet-
ric entities of the CAD system. Also, generally only the plotters could create the
very large output (such as A0- or E-size drawings which are 1200 mm (44 in.) in
the horizontal direction). However, the distinction has become blurred since the
printers can now create the large output, and the “resolution” of printers of 600 or
more dots per inch can be as accurate as the plotters.