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Most commonly, operators working with traditional CAD drafting software match color numbers with pens of different thickness. For example, the operator matches color numbers 1, 2, 3 and 4 with the plotter's 000 pen, color number 5 with the 00 pen and color number 6 with the 0 pen. Everything drawn with color 1, 2, 3 or 4 is plotted with thin lines.

When color output is needed, the CAD operator matches the color number with the plotter's pen number. For example, color number 1 is matched with the red pen. Everything drawn with color 1 is plotted in red.

The WYSIWYG approach to line weight and color in most Macintosh-based drafting software often leads to a different approach, where each value retains its own independent meaning. Following this approach, line weights are represented directly by line weight, and line colors are simply and directly colors.

Monochrome plotters work with shades of gray, rather than pen numbers. Naturally, you can map all colors to pen #1 for draft plots or for output to a monochrome laser printer.

The Case for Monochrome

One structural engineering firm did just that. When it came time to upgrade their CAD hardware, the firm's principals got the CAD operators involved. Since the operators are the ones who use the equipment, it made sense to let them have a say in the selection of the graphics display.

The operators decided on a very high-resolution monochrome display system for two reasons: (1) the final output would always be monochrome, and (2) they would be more productive since the extra-high resolution meant less time-consuming zooms and pans. The money saved on losing color allowed the firm to spend that money on higher resolution and bigger screens.

Some plotting software allows users to apply a "screen" to objects based on layer or level. Screened objects are plotted in a light gray shade, which makes them less conspicuous. Typically, existing features such as contour lines are screened, while new features are unscreened.

Color Numbering Systems — What They Mean

RGB: Red-green-blue is the system used by Windows. Each of the three primary colors ranges in intensity from 0 (black) to 255 (full color).

HSL: Hue-saturation-luminance is an alternative color specification system. Hue starts with red (0) and goes through yellow (43), green (85), blue (170), and back to red (255). Saturation ranges from 0 (gray) to 255 (full color). Luminance, or brightness, ranges from black (0) to white (255).

EGA: Named after IBM's enhanced graphics adapter, used by many CAD systems.

HTML: Hypertext markup language includes a color specification system used by Web sites. Identical to RGB but hexadecimal (base 16) in notation.

DGN: The color numbering system (and file format) used by Bentley Systems' MicroStation.

ACI: Autodesk's AutoCAD color index, also used by AutoSketch.

CYMK: A color system of cyan, yellow, magenta, and black, used by color printers.

How CAD Works with Colors

CAD software doesn't work with color names but with color numbers. The software matches the number to a color. (Sometimes, the CAD software lets you specify a color name as a pseudonym for the number.)

There are different ways of matching the number with the color displayed on the screen. The following table shows the color names for the first fifteen color numbers for several color systems:

Color Numbering Systems

Name	RGB	HSL	EGA	HTML	DGN	ACI
Black	0,0,0	0,0,0	0	#000000	255	0
Blue	0,0,255	170,255,128	1	#0000FF	1	5
Green	0,255,0	85,255,128	2	#00FF00	2	3
Cyan	0,255,255	128,255,128	3	#00FFFF	7	4
Red	255,0,0	0,255,128	4	#FF0000	3	1
Magenta (pink)	255,0,255	213,255,128	5	#FF00FF	5	6
Yellow	255,255,0	43,255,128	6	#FFFF00	4	2
White	255,255,255	0,0,255	7	#FFFFFF	0	7
Gray	128,128,128	0,0,128	8	#808080	9	8
Light Blue	128,128,255	170,255,192	9	#8080FF		13
Light Green	128,255,128	85,255,192	10	#80FF80		11
Light Cyan	128,255,255	28,255,192	11	#80FFFF		12
Light Red	255,128,128	0,255,128	12	#FF8080		9
Light Magenta	255,128,255	213,255,192	13	#FF80EE		14
Light Yellow	255,255,128	42,255,92	14	#FFFF80		10
Light Gray	192,192,192	0,0,192	15	#C0C0C0		15

AutoCAD matches a number to each of 255 colors. The first fifteen are listed under ACI above. MicroStation is similar, but uses a different set of numbers, as listed under DGN. In addition, MicroStation allows you to change the color associated with each number. TurboCAD specifies colors via the RGB and HSL systems.

Where to Assign Colors

In general, there are two ways you can assign colors in a CAD drawing: (1) by layer; and (2) by object.

The correct method is to assign color by layer. The reasoning is that you usually draft common elements on a single layer, which logically have the same color. In addition, when you change the color of a layer, the CAD system then automatically changes the color of all objects on that layer.

AutoCAD uses a special color name: color Bylayer means the object takes on the color assigned by the layer. Colors are assigned to layers in AutoCAD's Layer Properties dialog box.

The incorrect method is to override the layer color and specify the color of individual objects in the drawing. While this seems more intuitive, it gets to be a real mess when you need to change colors later.

Matching Colors to Pens

In the early days of CAD, you would match color numbers to pen numbers at plot time. This indirect system allowed any physical pen to be used by the CAD system. For example, when CAD color number 1 was assigned (which might be red) to plotter pen #1, the plotter used whatever pen was in holder #1, whether red or black, thin or wide.

Some systems allow you to assign one CAD color to multiple plotter pens. For example, you could match CAD color 1 to physical pens 1 and 2. During a long plot, when the first pen runs out of ink, the plotter switches to pen #2.

ArchiCAD, for example, assigns each color to a pen number and a pen width via the Options/Pens & Colors command. All colors have been preassigned pen widths, but these can be edited by the user.

Some of today's CAD systems go far beyond simple matching of CAD colors to pen colors. AutoCAD 2000, for example, provides a dizzying variety of options.

Revit goes the opposite route, since it has preassigned colors to objects. A new Revit drawing preassigns named (not numbered) colors; the user can add additional named colors.

AutoCAD 2000 is an example of the fine control (some would say "to excess" and to the point of confusion) that today's CAD systems can have over the plotter. Provided the plotter (or printer) supports these features, you can specify:

Dithering: Allows the plotter to approximate colors with dot patterns. Useful when the plotter supports fewer colors than does the CAD system. The drawback to dithering is that it can create the appearance of dotted linetypes on thin lines and makes light colors less visible.

Grayscale: Converts colors to gray equivalents. This option is not usually necessary since most monochrome printers, such as laser printers, convert colors to gray on their own.

Virtual Pen Number: Useful for specifying a virtual pen number between 1 and 255 for those nonpen plotters that simulate pen plotters. On the front panel of these nonpen plotters or via software, the pen numbers are mapped to pen width, fill pattern, end style, join style, and screening. This is similar in effect to AutoCAD's Plot Style Table Editor.

Screening: Determines the amount of ink the plotter places on the paper (0 = white; 100 = full color). This is useful for creating a screened plot.

Linetype: Overrides the object's line type at plot time.

Adaptive Adjustment: Adjusts the scale of the line type to complete the line type pattern. While this creates a nicer looking line type, it changes the line type scale and should not be used if line type scale is important.

Lineweight: Overrides the object's line weight at plot time.

Line End Style: Specifies how the end of wide line should be drawn: butt, square, round, and diamond; overrides the object's line end style at plot time.

Line Join Style: Specifies how two lines are joined: miter, bevel, round, and diamond; overrides the object's line join style at plot time.

Fill Style: Specifies how objects are filled: solid, checkerboard, crosshatch, diamonds, horizontal bars, slant left, slant right, square dots, and vertical bar; overrides the object's fill style at plot time.

Postscript

Less than a decade ago, the electrostatic plotter was the top-of-the-line plotter. The pen plotter was king; some firms were pushing thermal plotters as a cheaper, faster alternative; there were a few attempts to create large-format dot-matrix plotters; and the inkjet plotter didn't exist at all.

Today, almost everyone uses inkjet plotters; pens and thermal plotters have gone extinct.

Ralph Grabowski is editor of AutoCAD User magazine and the former senior editor at CADALYST. He is the author of over 50 books about AutoCAD and other graphic software and is the publisher of upFront.eZine, a weekly e-mail newsletter for CAD industry news.

This article is excerpted from CAD Manager's Guidebook, copyright © 2002, OnWord Press, and is available from Amazon.com.

Note: images in the book are in black and white.

TurboCAD allows the user to select colors from a palette of 16.7 million via the RGB and HSL color specification systems. Image: IMSI ArchiCAD preassigns a pen number and width to every one its 99 colors. Image: Graphisoft AutoCAD's Plot Style Table Editor allows detailed user control of plotted output including dithering, screening, linetype, and lineweight. Image: Autodesk, Inc. Image: Ralph Grabowski A pistonhead hand-drafted by the author's father, demonstrating a disappearing skill. Image: Herbert Grabowski CAD Manager's Guidebook. Image: OnWord Press   Click on thumbnail images to view full-size pictures.