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Inside AutoCAD 14

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- 9 -

Creating Complex Objects

by David M. Pitzer

This chapter tells you about two complex AutoCAD objects: regions and multilines. Regions are rather esoteric in that they are 2D objects that behave in many ways like solids; you can place holes in regions, for example. Multilines, on the other hand, are very practical AutoCAD objects that consist of sets of parallel lines that behave as a single line. They are useful as drawing objects representing walls. Specifically, this chapter will cover the following topics:

Creating Regions

An AutoCAD region is a two-dimensional enclosed area formed from closed shapes called loops. The term loops is perhaps a little misleading because loops can be composed of straight lines. Specifically, a loop is a curve or a sequence of connected curves that forms an area on a plane that has a boundary that does not intersect itself. Loops can be combinations of lines, polylines, circles, arcs, ellipses, elliptical arcs, 3D faces, traces, and solids. Loops can be composed of several such objects, as long as the objects themselves are closed or form closed areas by sharing endpoints with other objects. Although you use regions primarily in three-dimensional models, they are also useful in two-dimensional drawing since they allow the construction of shapes that would be difficult to create using standard editing commands.

You can create regions from multiple loops and out of open curves whose endpoints are connected to form loops. Open curves that intersect in their interior, for example, arcs or self-intersecting curves, cannot form regions. You use the REGION and BOUNDARY commands to create regions.

NOTE: The term closed as used here does not mean that the Close option of the LINE or PLINE command was used to close a series of such lines, but rather, that the coordinates of the beginning line segment and the last line segment are coincident.

The REGION Command

The REGION command prompts you to select objects that you want to convert to regions. You can use any standard selection method. Valid selections are closed lines, polylines, splines, circles, or any group of objects whose endpoints connect to form a closed loop. Self-intersecting objects or objects whose endpoints do not connect do not qualify. Figure 9.1 shows some examples of valid and invalid selections for the REGION command. The REGION command converts valid selections into regions, reporting the number of loops extracted and the number of regions created on the command line.

TIP: By default, the REGION command "consumes" the selected object(s) in making the regions. That is, it replaces the selected objects with regions and deletes the original objects. You may sometimes use the loop objects as a template for several regions and may, therefore, want to keep the original objects. The system variable DELOBJ controls whether objects are retained. If you want to keep the original objects, set the DELOBJ system variable to 0 (off).

Figure 9.1 Examples of valid and invalid selections for the REGION command.

In the following exercise, you create various regions from pre-drawn closed loops.

CREATING REGIONS

1. Begin a new drawing called CHAP9.dwg using this book's IAC901.dwg as a template (it should resemble figure 9.2). The layer LOOPS should be current. This drawing is included on the accompanying CD.

Figure 9.2 Creating regions from closed loops.

2. The LOOPS layer contains 10 closed curves or loops. The curves represent polylines, circles, lines, and splines. You will convert these into regions.

3. Start the REGION command by clicking on the Region tool on the Draw toolbar, or by choosing Region from the Draw pull-down menu. After the Modeler DLL loads, the Select objects: prompt appears.

4. Select all the curves by typing all and pressing Enter. AutoCAD should find 25 objects. Press Enter. AutoCAD responds:

10 loops extracted. 10 regions created.

5. Note that although 25 objects were selected, AutoCAD found 10 closed loops and created 10 regions.

6. You will use this drawing in the next exercise. For now, press Ctrl+S to save your regions.

The 10 regions you created in the preceding exercise do not look any different from the original closed loops. Later, however, you see how differently they act.

Creating Regions with BOUNDARY

You can create regions another way. The BOUNDARY command can create either a polyline boundary or a region from an enclosed area. An enclosed area differs from a closed area (required by the REGION command) in that the curves that the BOUNDARY command considers need not have their endpoints coincident or connected. For example, referring to figure 9.1, the first set of overlapping arcs on the lower line are not closed but they do enclose an area. The BOUNDARY command can form a region from these arcs; the REGION command cannot. In such cases, the region that BOUNDARY creates takes the same shape as it would were the curves closed.

NOTE: Unlike the REGION command, BOUNDARY creates new regions from objects but leaves the original objects in place regardless of the setting of the DELOBJ system variable. You can, of course, delete these objects using the ERASE command.

In the following exercise, you create a region from curves that enclose an area but do not themselves form a closed loop. You use the Region object type option of the BOUNDARY command. Figure 9.3 shows the Boundary Creation dialog box with the Region option selected in the Object Type drop-down list.

Figure 9.3 The Boundary Creation dialog box.

CREATING REGIONS WITH BOUNDARY

1. Continue in the drawing from the previous exercise. Thaw the SHIELD layer and make it the current layer. Refer to figure 9.4 for this exercise.

Figure 9.4 Creating a boundary from an enclosed area.

The objects on the SHIELD layer consist of a line 1, a polyline 2, and two arcs 3 and 4. These objects enclose an area.

2. From the Draw pull-down menu, choose Boundary to start the BOUNDARY command. The Boundary Creation dialog box appears (refer to figure 9.3). Select Region from the Object Type drop-down list.

3. Still in the Boundary Creation dialog box, choose Pick Points and pick anywhere near 5 in figure 9.4. The following prompts appear. Press Enter in response to the final prompt.

Select internal point: Selecting everything... Selecting everything visible... Analyzing the selected data... Analyzing internal islands... Select internal point: ¯

4. AutoCAD creates the region and displays the following two prompts:

11 loops extracted. 11 regions created.

5. AutoCAD has created a new region in addition to the 10 regions created in the previous exercise.

6. Use the ERASE command (type E and press Enter) to erase the arcs at 3 and 4. Your drawing should now resemble figure 9.5. Save your work.

Figure 9.5 The completed regions.

Extracting Data from a Region Model

Because regions behave in many ways like solids, you can extract a great deal of physical information from them. The MASSPROP command allows you to display certain physical parameters, such as area.

The MASSPROP command prompts you to select a region or regions. AutoCAD then displays a list of physical properties on the text screen. The following is the data that AutoCAD displays for the shield model you constructed in the exercises for this chapter.

 ----------------   REGIONS   ----------------
Area:                    61.0053
Perimeter:               80.9890
Bounding box:         X: 1.5000  --  12.0000
                      Y: 1.2500  --  7.8750
Centroid:             X: 6.8334
                      Y: 4.5380
Moments of inertia:   X: 1490.2179
                      Y: 3432.9384
Product of inertia:  XY: 1887.8823
Radii of gyration:    X: 4.9424
                      Y: 7.5015
Principal moments and X-Y directions about centroid:
                      I: 233.8782 along [0.9999 -0.0110]
                      J: 584.3485 along [0.0110 0.9999]

After AutoCAD displays this information, a prompt appears that allows you to save the information to a file. Note that, because regions are actually 2D areas, the information includes such parameters as Area and Perimeter rather than Mass and Volume. In many ways, regions are mathematical "fictions" because any true object--or a computer model of any true object--would have both mass and volume. Nonetheless, regions are useful AutoCAD objects, or models, that may be manipulated in real-world ways.

Understanding Boolean Operations

Regions are interesting objects. Although they have no thickness, they behave in many ways like solids. In AutoCAD, you can act on regions as you would if they were actual 3D solids. For example, if you have a region, you may want to place holes in it or attach another region to it or trim it in some manner. In true solids, you can perform such operations by performing mathematical operations collectively known as Boolean operations. Three Boolean operations and commands exist in AutoCAD: UNION, SUBTRACT, and INTERSECT.

The UNION command enables you to mathematically join two or more regions into a single region of their combined areas. Interestingly, the regions do not have to touch (much less overlap) to be joined. Figure 9.6 shows the result of using the UNION command.

Figure 9.6 The results obtained with the UNION, SUBTRACT, and INTERSECT commands.

The SUBTRACT command allows you to cut holes into or areas out of regions. These holes and cutouts become a part of the region; if you move the region, the holes also move. Figure 9.6 illustrates the effect of using the SUBTRACT command. AutoCAD prompts you for the region to subtract from and then asks for the regions you want to subtract.

The INTERSECT command creates a new region from the area common to two or more overlapping regions. Figure 9.6 depicts the effect of using the INTERSECT command.

WARNING: Using the INTERSECT command on regions that do not intersect or overlap creates a region that has no area, called a null region. Essentially, AutoCAD deletes the regions you selected. If this occurs, you can use UNDO to restore the regions.

In the following exercise, you will create some Boolean holes in the shield region you created in the preceding exercise. You use the regions you created from closed loops earlier with the REGION command.

CUTTING HOLES IN REGIONS WITH SUBTRACT

1. Continue from the preceding exercise. The layer SHIELD should remain current.

2. Start the SUBTRACT command by choosing Modify, Boolean, Subtract. The following prompt appears:

Select solids and regions to subtract from...

3. Pick the shield region at a point such as 1 in figure 9.7 and then press Enter. The following prompt appears:

Select solids and regions to subtract...

4. Use a window to select the regions within the shield, as shown in figure 9.7, at 2 and 3, and then press Enter. AutoCAD subtracts the regions.

5. Save your drawing.

Figure 9.7 Select the interior regions to create holes.

Although your drawing now looks exactly as it did when you began the preceding exercise, you actually cut 10 holes into the larger shield region. Figure 9.8 shows a rendered, perspective view of your drawing in its present state. As you can see, there are holes in the shield.

Figure 9.8 A rendered view of the shield drawing.

TIP: Because regions act much like solids, you can change their shape only by using Boolean operations. If you want to change regions back into their constituent lines, circles, and arcs, you must use the EXPLODE command.

Regions are interesting objects. Although they are technically two-dimensional, they behave in many ways like real-world, three-dimensional objects. Boolean operations allow you to easily alter AutoCAD's region objects in ways not available using conventional editing methods, resulting in "shapes" that would be difficult to draw otherwise. In the next section, you will learn about another interesting AutoCAD object: multiple parallel lines, or mlines.

Creating Multiple Lines

Multilines (mlines) are objects that contain multiple parallel lines called elements. Multilines are convenient when drawing objects such as walls in floor plans, which are often represented by two or more parallel lines. Each element of a multiline is defined, in part, by its distance or offset from a center, where the center is considered zero offset. Figure 9.9 shows the parts of a typical mline object.

Figure 9.9 Multiline elements.

Drawing with the MLINE Command

Multilines are created using the MLINE command, which returns the Justification/Scale/STyle/<From point>: prompt.

By default, AutoCAD draws multilines with two elements, one having an offset of 0.5 and the other with an offset of -0.5, resulting in a default mline of two elements, 1 unit apart. By adjusting the mline scale, you can control the distance between elements. For example, setting the mline scale to 6 produces two parallel lines, 6 units apart.

In the following exercise, you begin to create a floor plan for a house using the default multiline style.

CREATING EXTERIOR WALLS WITH MULTILINES

1. Begin a new drawing called FLOORPLN.dwg, using this book's IAC902.dwg on the accompanying CD as a template. If necessary, set the PLAN layer current.

2. Start the MLINE command by clicking on the Multiline tool on the Draw toolbar, or by choosing Draw, Multiline. Respond to the MLINE prompts as shown:

Justification/Scale/STyle/<From point>: S¯ Set Mline scale <1.00>: 6¯ Justification/Scale/STyle/<From point>: 30',30'¯ <To point>: @22'<0¯

3. AutoCAD starts the mline and draws the first segment. Note that since you set the scale to 6, the distance between the two lines is 6 units (inches in your drawing) apart. Since the floor plan lines are orthogonal to each other, you can reduce your typing by using Direct Distance entry. Press F8 to turn ORTHO mode on. The Undo/<To point>: prompt is current.

4. Respond to this prompt by moving your cursor above the last point and typing 26' and pressing Enter. ORTHO mode forces the new segment to a point @26'<90 from the previous point. The Close/Undo/<To point>: prompt appears.

5. Move the cursor to the left of the last point and type 22' and press Enter. Direct Distance entry draws the next segment @22'<90 to the last point. The Close/Undo/<To point>: prompt returns. Respond by typing C and pressing Enter.

6. AutoCAD closes the mline by drawing a segment from the last point to the start point. Your drawing should now resemble figure 9.10.

7. Save your work.

Figure 9.10 Floor plan outline using mlines.

Defining Mline Styles with MLSTYLE

The default multiline style consists of two lines, each 0.5 units from the zero offset. It is called the Standard mline style. By adjusting the scale and justification, the Standard mline style can serve many purposes--but still is limited to only two lines. The MLSTYLE command allows you to create more complex multiline styles.

Issuing the MLSTYLE command opens the Multiline Styles dialog box, shown in figure 9.11. The Multiline Style area of the dialog box gives you information about the multiline styles available in the drawing and offers options for loading, saving, adding, and renaming multiline style definitions. Unless you save them to a file, multiline styles exist only in the current drawing. To share multiline styles among drawings, you must save the multiline style definition to a file. Multiline styles are stored in MLN files. The Standard multiline style is stored in ACAD.MLN, usually found in the \SUPPORT directory or folder of your Release 14 installation.

Figure 9.11 The Multiline Styles dialog box.

Clicking on the Element Properties button displays the Element Properties dialog box shown in figure 9.12. The Element Properties dialog box enables you to define the elements of a multiline style. Each element is defined as a distance from the zero offset. AutoCAD draws elements that have a positive offset above the zero offset and elements that have a negative offset below the zero offset. The elements that have the greatest absolute offsets become the top and bottom justification elements.

Figure 9.12 The Element Properties dialog box.

You can define as many as 16 elements in a single multiline style. In addition to an offset distance, you can assign each element a linetype and color, which enables you to create very complex multiline styles.

NOTE: Although you can assign a separate linetype and color to each element of a multiline, the multiline itself resides on a single layer. If you freeze a layer that contains a multiline, you freeze the entire multiline.

Clicking on the Multiline Properties button opens the Multiline Properties dialog box (see fig. 9.13). The Multiline Properties dialog box presents you with some options for the display and generation of multilines. The Display joints check box provides you with the means to control the display of corner joints, as shown in figure 9.14.

Figure 9.13 The Multiline Properties dialog box.

The Caps area of the dialog box enables you to control the creation of the start and end of multilines. The Angle option enables you to control the miter angles of the end cap. Figure 9.15 shows the various capping effects.

Figure 9.14 Multilines with and without joint display.

Figure 9.15 Multiline end cap options.

The options in the Fill area of the Multiline Properties dialog box enable you to fill in the multiline with a solid color. If you set the Fill option to on (mark the On check box), AutoCAD fills in the area between the outermost offset elements with the color you specify in the multiline style.

In the following exercise, you create two new multiline styles for the interior walls and closet shelves of your floor plan drawing.

CREATING MULTILINE STYLES

1. Continue from the preceding exercise. Layer PLAN should still be current.

2. Choose Format, Multiline Style. The Multiline Style dialog box appears. Click on the Element Properties button. The Element Properties dialog box appears.

3. In the Elements list, select the 0.5 element and type 2 in the Offset edit box. Now, select and highlight the -0.5 element and enter -2 in the Offset edit box. Click on OK to dismiss the dialog box.

4. Back in the Multiline Styles dialog box, click on the Multiline Properties button and in the Multiline Properties dialog box, place an X in the Start and End Line check boxes. Click on OK to close the dialog box.

5. In the Multiline Styles dialog box, type WALL_4 in the Name box and 4"WALL in the Description box. Click on the Add button. Next, you create another multiline style for the closet shelving.

6. While still in the Multiline Styles dialog box, click on Element Properties. In the Element Properties dialog box, select the 2.0 element in the elements list and enter a new value of 20 in the Offset edit box. Then select the -2.0 element and enter 16 in the Offset edit box.

7. Now click on Linetype, and then in the Select Linetype dialog box, select Dashed from the list of available linetypes. Click on OK to close the Select Linetype dialog box and OK again to close the Element Properties dialog box.

8. In the Multiline Styles dialog box, click on Multiline Properties and clear the Xs from the Start and End Line check boxes, and then click on OK.

9. In the Multiline Styles dialog box, type Shelf in the Name box and 20"Closet-shelf in the Description box, and then click on the Add button. Click on OK to close the dialog box.

10. Save your drawing.

Now, in addition to the Standard multiline style, you have two additional styles. In the following exercise, you use these multiline styles to create the interior walls of the floor plan.

CREATING ADDITIONAL MULTILINE STYLES

1. Continue from the preceding exercise. Make sure ORTHO mode is on. First, perform a ZOOM with Center option.

Command: Z¯ All/Center/Dynamic/Extents/Previous/Scale(X/XP)/Window/ <Realtime>: C¯ Center point: 41',43'¯ Magnification or Height <1.00> : 1.5x¯

2. Start the MLINE command by typing MLINE and pressing Enter at the Command: prompt. Respond to the prompts as follows:

Justification/Scale/STyle/<From point>: S¯ Set Mline scale <6.00>: 1¯ Justification/Scale/STyle/<From point>: ST¯ Mstyle name (or ?): WALL_4¯ Justification/Scale/STyle/<From point>:

Figure 9.16 Floor plan with interior walls and shelf.

3. To respond to the current <From point>: prompt, refer to figure 9.16. Using the FROM and the ENDPoint object snap, pick at 1.

<Offset>: @13'<90¯ <To point>: @9'6<0¯

4. At the Undo/<To point>: prompt, pick a point near 2. This segment is too short right now, but you will clean it up later. For now, just press Enter to end the command.

5. Restart the MLINE command by pressing Enter again.

6. Use the FROM and ENDPoint combination again and pick at 3.

<Offset>: @13'<90¯ <To point>: @7'6<180¯ <To point>: ¯ (Note the end cap on the wall.) Command: ¯ Justification/Scale/STyle/<From point>: Use the FROM and ENDPoint combination and pick at 4. <Offset>: @30<90¯ <To point>: @6'<0¯ Undo/<To point>: Using the PERPendicular osnap, pick 5. Close/Undo/<To point>: ¯ Command: ¯ to repeat the MLINE command. Justification/Scale/STyle/<From point>: ST¯

7. You will now pick a new multiline style and justification.

Mstyle name (or ?): SHELF¯ Justification/Scale/STyle/<From point>: J¯ Top/Zero/Bottom <top>: Z¯

8. At the Justification/Scale/STyle/<From point>: prompt, pick at 4 using the ENDPoint osnap.

9. At the <To point>: prompt, pick at 6 using the PERPendicular osnap, and then press Enter to end the command.

10. Save your drawing.

NOTE: You currently cannot redefine or reload a multiline style after you create it. If you need to make changes to a multiline style, you must EXPLODE or ERASE all multilines that have that style, PURGE the multiline style from the drawing, re-create the multiline, and re-create any multilines that use the new style.

If you have saved the multiline style to a MLN file, you can redefine the style by changing the DXF codes in the style definition file. The changes affect only new multilines drawn with that style, however; existing multilines do not change. See The AutoCAD Customization Guide for information about DXF codes.

Editing Multilines with MLEDIT

Because multilines are rather unique, they have a unique set of editing tools. The MLEDIT command gives you a set of tools for cleaning up and editing multilines.

MLEDIT does not actually "cut" or break the multilines, but simply suppresses the display of the various multiline cut segments. This is done so that you can later redisplay (or "heal") the cut segments.

TIP: To tell whether a multiline has cut segments, simply pick on any portion of the multiline. The entire multiline segment highlights, and grips (if enabled) appear at each multiline vertex.

When you issue the MLEDIT command, the Multiline Edit Tools dialog box opens (see fig. 9.17). This dialog box provides 12 tools for editing multilines. You can divide the MLEDIT tools into four categories: crosses, tees, corners, and cuts.

Figure 9.17 The Multiline Edit Tools dialog box.


Crosses

The MLEDIT command's three cross tools (Closed Cross, Open Cross, and Merged Cross) enable you to clean up various crossing intersections. Figure 9.18 shows the various effects of these tools.

Figure 9.18 Closed, open, and merged crosses.

When you choose one of the cross tools, AutoCAD prompts you to select two mlines. AutoCAD always cuts the first mline you choose. AutoCAD cuts the second mline according to the tool you use.

For merged crosses, AutoCAD creates a corner out of matching pairs of mline elements. In other words, the outermost elements of the first mline create a corner with the outermost elements of the second mline, the second pair of elements in the first mline creates a corner with the second pair of elements in the second mline, and so on. If you leave an unmatched element in one of the mlines, AutoCAD leaves it uncut.

Tees

You use the next three multiline editing tools to clean up intersections. The tools behave in much the same way as the crossing tools. Figure 9.19 shows the effects of using the various tee tools.

Figure 9.19 Closed, open, and merged tees.

As with the cross tools, AutoCAD prompts you to select two mlines. AutoCAD cuts the first mline to make the base of the tee and cuts the second mline according to the tool you use.

Corners

The MLEDIT command's corner tools enable you to clean up corner intersections between two multilines as well as add and remove vertices from multilines. The corner joint tool creates a clean corner joint from two multilines. Figure 9.20 shows the effect of the corner joint.

Figure 9.20 A corner joint.


TIP: When you make a corner joint, make sure that each mline has the same number of elements; that's a good way to ensure that the corners miter properly. Doing otherwise can lead to unpredictable results.

When you create a multiline, AutoCAD places a vertex at each point you pick. The Add Vertex and Delete Vertex tools enable you to create and remove vertices from a multiline. Figure 9.21 shows the effect of adding a vertex. The top multiline is the original mline with a vertex at each endpoint. The middle multiline shows the effect of adding a new vertex and the bottom shows the mline after you use grips to stretch the mline.

Figure 9.21 Adding a vertex to a multiline.

You can use the Delete Vertex tool to remove a vertex from multilines that have three or more vertices. If you select an mline that has only two vertices, AutoCAD simply ignores the pick and allows you to try again.

Cuts

The cut tools enable you to break sections out of the individual elements of a multiline. You can use the Cut Single tool to break a section out of a single element of a multiline. You simply pick two points on the element and AutoCAD cuts the portion of the element between the two points. The Cut All tool works similarly but cuts all the elements. Figure 9.22 shows the effects of using the Cut Element and Cut All tools.

Figure 9.22 The multiline cut tools.

AutoCAD doesn't actually cut the multiline itself--it simply suppresses the display of those sections. The multilines are still a single object. The Weld All tool prompts you to pick two points on a multiline. AutoCAD redisplays any cut sections between the two points you pick. The fact that AutoCAD doesn't break the multiline enables you to reverse the effects of the cut tools, using the Weld All tool. Figure 9.23 shows the effect of using the Weld All tool.

Figure 9.23 Welding a cut mline.

In the following exercise, you use some of the multiline editing tools to create door openings in the walls of your floor plan.

EDITING MULTILINES

1. Continue from the preceding exercise. The PLAN layer should still be current. Refer to figure 9.24.

2. Choose Modify, Object, Multiline to start the MLEDIT command. The Multiline Edit Tools dialog box appears.

3. Double-click on the Closed Tee icon (top icon, second column).

4. At the Select first mline: prompt, pick the mline at 1. At the Select second mline: prompt, pick the mline at 2. Press Enter twice to end the command.

Figure 9.24 Editing with MLEDIT.

5. Restart the MLEDIT command and double-click on the Merged Tee icon (middle icon). At the Select first mline: prompt, pick the mline at 3. At the Select second mline: prompt, pick the mline at 4. At the next Select first mline (or Undo): prompt, press Enter.

6. Next you use the Cut All tool to create some door openings. At the Command: prompt, press Enter.

7. Double-click on the Cut All icon (middle icon, last column).

8. Using the FROM and ENDPoint object snap, pick at 5.

Offset: @6<0¯ Select second point: @4'8<0¯

9. Using the FROM and ENDPoint object snaps, pick 6.

Offset: @6<90¯ Select second point: @30<90¯

10. Using the FROM and MIDpoint object snaps, pick at 7.

Offset: @3'<180¯ Select second point: @6'<0¯ Select mline(or Undo): ¯

11. You have completed making the opening in your mlines. If you want, you can cap the open lines using a running ENDPoint osnap and the LINE command. When you finish, your drawing should resemble figure 9.25.

This completes your work in this drawing.

Figure 9.25 Completed floor plan with capped openings.

Mlines make it very easy to draw sets of two or more parallel lines, a frequent requirement when making drawings such as floor plans. The ability to set the distances between Mline elements and to assign various linetypes to the individual elements make Mlines very flexible. Mlines can be edited in a number of ways and their styles can be saved and recalled as needed. They are one of the most flexible elements in AutoCAD.

Summary

The complex objects that this chapter has discussed extend your AutoCAD drawing capabilities. Regions enable you to model real objects in 2D space and Boolean operations enable you to modify regions in ways you cannot edit other enclosed two-dimensional geometry. The resulting geometry would be difficult to achieve using standard, non-Boolean, editing commands. Multilines enable you to draw and edit sets of customizable parallel lines and make it easy to present parallel sets of lines in visually effective ways. They enable you to save significant amounts of time in the many drawing situations that require drawing and editing parallel lines.

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