Part I: CAD and AutoCAD for the Theater

Chapter Eight: 2d Primitives


    1. From A to B
      1. line
      2. ray
      3. xline
      4. AutoCAD Reference: Default Command Line Shortcuts
      5. point
    2. Going Around
      1. arc
      2. circle
      3. elipse
    3. Complex Lines
      1. mline
      2. polyline
      3. polygon
      4. spline
      5. sketch
    4. Down in Writing
      1. text
      2. mtext
      3. ddstyle
      4. special symbols
      5. AutoCAD Reference: Special Text Codes
      6. attributes

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From A to B:

Connect two points and you have a line. In AutoCAD simple point to point lines come in three varieties: LINE, RAY and CONSTRUCTION LINE (XLine). Line or [L] draws a single line from point to point. [Ray] draws a line from a point through another point, and continues on forever. No matter how big the drawing gets, the Ray continues to radiate outward. The construction line, drawn with the [XL] command, is similar except that it continues through both points, radiating out both directions.

When you type [L] and enter, AutoCAD replies, "Specify first point". Enter coordinates at the command prompt or pick a point with a screen pick. Use Snap, Osnap or other tools to pick more precisely. AutoCAD then says, "Specify next point or [Undo]". Enter more coordinates, make another pick or 'u' and enter to Undo the last pick. You can continue adding lines to the end of each subsequent line. To finish out of the Line command, press enter again or right mouse click, or to close a series of lines type [C] and press enter. A final line will close the series by finding the first pick point. This does not work with the XLine or Ray commands. Each line in the series is a seperate object.

The following list includes one or two letter shortcuts typed at the command line prompt to issue commands used in this chapter.

AutoCAD Reference: Default Command Line Shortcuts

A       ARC                  PL      PLINE
C       CIRCLE               PO      POINT
DO      DONUT                POL     POLYGON
L       LINE                 T       MTEXT
M       MOVE                 XL      XLINE
P       PAN                  Z       ZOOM

AutoCAD also has a single POINT object. A Point object, like any point, has no geometry other than location. Points are used to anchor other geometry. Commands such as Measure and Divide create Points by default. The drafter then Snaps to those points to attach other drawing entities to them. To confuse the issue, AutoCAD uses the term "Node" to refer to a Point object in the Snap menu. For our purposes a NODE is a Point object.

The [Point] command defaults to a single dot display, but how the Point is displayed can be changed with the DDPTYPE command. [DDPType] brings up a window showing various display styles and the current size in relative or absolute measure. The current settings are highlighted. Click on new settings and click on "OK". All Points change the new style display.

chapter index.

Going Around:

There are lots of ways to create an arc or circle. The CIRCLE command is [Circle] or just [C] which brings up the Circle command line dialog: "Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: ". The default is to pick a center point and a radius length. If you remember your geometry, a circle can be defined by three point on the circumference, which is what [3P] (three points) does. Given three pick points, [3P] will create a perfect circle which passes through all three. The two points [2P] entry defines a diameter, i.e. the center point is placed midway between the two pick points and the circle passes through the two points.

More complex is TTR or tangent-tangent-radius, which creates a circle tangent to two lines, that are not parallel. When you respond to the circle command line dialog with [TTR], you will be asked to select two lines and a radius. AutoCAD will locate a circle of the given radius so that it touches both lines (or their logical extensions) at only one point on the circumference and so that the lines are at right angles to a radius touching that point (tangent).

ARC has even more possibilities. The [Arc] or [A] command line dialog is deceptively simple: "Specify start point of arc or [CEnter]: ". There are only two choices: pick a start point or type [CE], hit Enter and pick a center point. Try picking a start point. The command prompt now says, "Specify second point of arc or [CEnter/ENd]: ". If you pick another point, the Arc command will pass an arc through that point to an end point selected next. But try using [CE]. When a center point for the arcs radius is picked, Arc will then use the next pick to select the angle that the arc covers in a counter-clockwise direction. Or you can specify the angle [+n] is counter-clockwise and [-n] is clockwise) or a chord length (the strait line distance between end points) where a negative number will take the chord in a clockwise direction but not the arc.

Now try picking a start point and use [EN] to pick an end point. The command dialog now gives four choices: "Specify center point of arc or [Angle/Direction/Radius]: ". As you can see the possibilities are expanding. The Draw pull-down menu, shown at the right, lists ten pick methods. While three point picking allows you to draw an arc in clockwise or counter clock-wise direction, many pick methods only draw counter-clockwise arcs.

It is also possible to draw an elliptical arc, but first let's discuss the ELLIPSE command. Typing [Ellipse] brings up the Ellipse command line dialog, "Specify axis endpoint of ellipse or [Arc/Center]: ". To draw a full ellipse pick an end point or type [C], hit enter and pick a center point. Pick a point to establish the first axis and another to establish the length of the second axis.

The [Arc] choice, from the Ellipse command line dialog, draws an ellipse in the same manner, but then allows you to choose two angles to cut away a portion of the ellipse leaving an elliptical arc. The remaining elliptical arc will be the portion of the ellipse included in the counter-clockwise inclusion of the two selected angles.

chapter index.

Complex Lines:

AutoCAD has several kinds of complex lines. MULTILINE draws multiple lines in parallel and gussies up the corners automatically. POLYLINE is the workhorse of lines. A polyline is a line made up of multiple lines and arcs which form a single line object. POLYGON creates a polygon which is actually a polygon shaped polyline. A SPLINE is a smooth curving line bent through NURBS. A Polyline can be turned into a Spline, but a Spline cannot be turned into a Polyline, unless it is a splined Polyline. A DONUT is a wide line circular polyline. And finally SKETCH leaves a trail of line segments, either as individual lines or as a single polyline, that sketches a line drawn with the mouse.

I'll skip Multiline for now, so refer to the help file if you want to know more, and move directly to polylines. Get very familiar with the POLYLINE command. Type in [Polyline] or [PL] to start a polyline and pick a point. The command dialog tells you that the "Current line-width is 0.0000 " and asks you to "Specify next point or [Arc/ Close/ Halfwidth/ Length/ Undo/ Width]: ". Change from line segment to arc segment with Arc or [A] and enter. Change back to straight line segments with [L]. Each arc segment begins tangent to the end of last segment. Polylines can have a width, and can change width at each corner (Vertex) where the segments meet. [Halfwidth] and [Width] set the width (or halfwidth) of the line from the start of the current segment to the end of the current segment. The width of subsequent segments will match the end width unless it is the Width or Half-width is reset. Close or [C] closes the polyline by adding a segment connecting back to the original start point of the polyline. Undo or [U] undoes the last segment and [Length] establishes a new segment of specific length in the direction of the current crosshair location.

SKETCH draws a series of lines and arcs or a polyline to follow the path of the movement of the cursor. It closely resembles sketching in paint programs. Use this command to follow irregular edges such as the outline of a tree or stone wall. Turn Ortho and probably Snap off before using sketch or you will get a stairstep. The [Sketch] command line dialog informs you of the "Record increment <0.1000>: " If the segment length ("Record Increment") is too small, the drawing becomes loaded down with many little lines and arcs. The more unnecessary entities in the drawing the larger the file size becomes and the slower that Save, Regen and Open function. Set the Record Increment so that the line in the final drawing scale looks faintly choppy but not annoyingly so.

Anecdote: I received a drawing from another company which took five or ten minutes to load. At first I believed the file to have become corrupt. It turned out that the drawing which contained a drawing of a large (40' in diameter) turntable covered in foliage which had been scanned or sketched with line segments of approximately 1/8" each. The result was a bloated drawing of enourmous size in memory. Most of it had to be thrown out and re-drawn.

CADvise:
Use the largest acceptable increment size when using the Sketch command.

With the Record Increment set, Sketch is active. The command line dialog now reads, "Sketch. Pen eXit Quit Record Erase Connect." Pen, [P] or a left mouse click is the pen down command and makes sketch actively draw all movements of the mouse as a series of line segments. Left mouse click again and the pen is up or not drawing. EXit, or [X], quits out of Sketch and records the drawing as polyline(s) or line segments. Quit, or [Q], quits without recording the segments. They disappear. Record, [R], records the current sketching by turning it into Line or Polyline objects but keeps Sketch active. Erase, [E], lets you back up over unrecorded sketching and erase as you go. With the pen up, Connect to the last line by typing [C], hit enter and pass the cursor over the last unrecorded segment. The pen automatically goes down and starts sketching from that point.

The command SKPOLY toggles the Sketch command between drawing a polyline and drawing lines and arcs. Set [Skpoly] to <1> (on) so that Sketch draws a polyline rather than individual lines and arcs.

chapter index.

Down in Writing:

Text or MText are used to add text to the drawing. The TEXT command adds text one line at a time in only one type face or font. It is useful for one line notes or labels. For larger blocks of text or text with font changes, use Multiline Text or MTEXT.

Text and MText can be Justifed Left, Right or Center, and Top, Bottom or Middle. Text can also be Justified to Align on an angle or to Fit a specified width. MText can be rotated on an angle, and the text box resized to a specified width. The text in this case is not stretched or squeezed to Fit.

Font Styles are set with the DDSTYLE (Dynamic Dialog for Style) command, which opens the Text Style dialog box. AutoCAD sets a "Standard" style with the "Txt.shx" font by default. Because Txt.shx is a bit blocky, I usually change the "Standard" style font to "Romans.shx", which is cleaner, but a slightly more memory intensive style. AutoCAD has its own compiled Shape fonts (.shx), but any True Type font can be used.

Press the New… button to create new styles and call them something descriptive: "Titles", "HandLetter", "Notes". With the style name visible in the Style Name window, set the Font Name: and Font Style: in the middle. The Height set at "0.00" allows text height to be specified individually. Set any Effects, and then press Apply to set the new style. when you close the new styles will be available for Text and MText entries.

When insert Multiline Text with the [MText] command, the command line dialog will tell you the current text style and height and ask you pick two corners to establish a box for the text. Between picks there are several factors that can be set at the command line such as justification, style, height, rotation and line spacing. When you make the second pick a Multiline Text Editor (or a text editor of your choice) opens for you to type in text, import text from another text file, and change many of the same attributes mentioned above. The Multiline Text Editor allows you to change font, style, color, size and other attributes of individual letters are words, as well as large blocks of text.

It is often necessary to add special symbols such as the degree sign, phase or diameter sign, or the plus/minus symbol. There are special symbols or controls that can be added in the text by adding two percent signs [%%] followed by the control code or an ASCII character decimal code [%%nnn]. The %%nnn will be replaced by the proper ASCII character. To underscore or underline add [%%U] in front and behind the text to be underlined. To overscore add [%%O]. There are special characters: the Degree symbol [%%D], the Plus/Minus symbol [%%P] and the Circle Diameter symbol [%%C]. As of AutoCAD 2000, typing the three or four letter Unicode, ASCII, or ISO lettering code with Alt pressed [Alt+nnn] will work as well, providing a larger number of available symbols. Degree () is [Alt+0176]; plus/minus() is [Alt+0177]; and the diameter or phase symbol () is [Alt+0216].

AutoCAD Reference: Special Text Codes

%%c	circle diameter/phase symbol
%%d	degree symbol
%%o	overscore
%%p	plus/minus symbol
%%u	underscore
%%%	percent symbol
%%nnn	ASCII (or ISO) decimal code nnn

The three letter ISO (International Standards Organization) codes are listed in Appendix C. Not all font definitions have all the basic symbols. If font doesn't have a definition for a symbol, it will likely substitute something odd like a box or question mark. In earlier releases, if you wanted to add a % sign in your text you would have to add it with two percent signs in front [%%%].

Later we will get into block definitions. Blocks are drawings within drawings. A single block can be inserted into a drawing several times. Blocks have special text attachments called Attributes. Standard Text and MText entities in a block will be the same in all instances of the block, but Attribute entities can be altered for each seperate instance of the block. Other than that they act much like normal text entities. I'll save the full discussion until we cover Blocks.

Every drawing object, be it primitive or complex, has AutoCAD defined characteristics known as Properities. Properties govern the appearance, placement and other features of drawing objects. Before we try to edit or modify drawing objects we must look at properities.

chapter index.

last chapter. AutoCAD Backstage home. AutoCAD Backstage Index. next chapter.

Copyright © 1999 Wm W Wells. All rights reserved.