Working with Regions

Regions are special AutoCAD objects. In many ways, regions act like 2D planar objects; in other ways, regions exhibit many of the properties of 3D objects. Like 3D surfaces, for example, regions can hide objects “behind” them and can have materials applied to them for rendering purposes. Like 2D objects, regions have no third dimension, or Z-axis information. In many ways, regions can be considered infinitely thin solids.

Technically, regions are enclosed 2D areas. You create regions from closed shapes. Closed shapes consist of a curve or a sequence of curves that define an area on a single plane with a non-self-intersecting boundary. Closed curves can be combinations of lines, polylines, circles, ellipses, elliptical arcs, splines, 3D faces, or solids. The objects that compose a closed curve must either be closed themselves or form a closed area by sharing endpoints with other objects.

As you can see, a large number of AutoCAD objects can join together to form a closed curve from which a region can be created. The only restrictions are that the object must be co-planar, or exist on the same plane, and sequential elements must share endpoints. Assuming that the arcs and lines shown in Figure 28.27 are co-planar, the set of curves on the left could not be converted into a region, whereas the curves on the right could.

To convert a set of qualifying curves into a region, you use the REGION command. In the following exercise, you create a region from a set of straight polyline “curves” using the RECTANG command.

Exercise 28.10 Creating Regions with the REGION Command

1.	Open the file REGION27.DWG from the accompanying CD-ROM. This drawing is composed of only a circle and a polyline drawn at an elevation of -1.0 units.
2.	From the Draw menu, choose Rectangle. When prompted for the first corner of the rectangle, type 4,2 . At the Specify other corner: prompt, type @3.5,5.5 . Your drawing should now resemble Figure 28.28. Figure 28.28. A closed polyline curve drawn over a circle and line.
3.	From the Draw menu, choose Region. At the Select objects: prompt, select the rectangle you just created and press Enter. AutoCAD reports 1 Region created at the command line. The polyline used to create the rectangle has now been converted to a region.
4.	Type HIDE and press Enter to invoke the HIDE command. Note that the newly created region acts as a planar surface hiding the circle and line objects behind it as shown in Figure 28.29. Figure 28.29. A region created with the REGION command. Regions hide objects behind them. In the following step, you rotate the region in 3D space.
5.	From the Modify menu, choose 3D Operation, then Rotate 3D. When prompted to select objects, select the region and press Enter.
6.	At the next prompt, type Y to indicate you want to rotate the region about its Y-axis.
7.	When prompted to specify a point on the Y-axis, use an Endpoint osnap and pick at in Figure 28.29. To specify an angle of rotation, type 90 and press Enter.
8.	From the View menu, choose 3D Views, then SE Isometric. Your drawing should resemble Figure 28.30. Figure 28.30. Regions can be moved and positioned in 3D space.
9.	Again type HIDE to invoke the HIDE command. Note that the region is rotated in 3D space and hides objects behind it as shown in Figure 28.31. Figure 28.31. Regions behave like solids and hide objects behind them.
10.	You can close this drawing without saving changes.

As you can see from the preceding exercise, regions have many of the properties of 3D surfaces; they hide objects behind them and they can be easily moved in 3D space. One of the most interesting and useful properties of regions is their ability to undergo Boolean operations—a trait they share with AutoCAD solids. Boolean operations will be discussed and demonstrated later in Chapter 30, “Solid Modeling.”