Basic Surfacing Techniques

In the previous chapter the techniques used to create 3D lines and polylines were discussed. Objects modeled with such lines are called wireframe models. To create a true 3D object, however, surfaces must be applied across or around the edges of the wire frame. Such surfaces can then be used to hide lines and other surfaces that lie “behind” other surfaces in any particular view. Surfaces can also have hatch patterns and even materials applied to them to yield more realistic-looking objects.

In some cases, you need to create the wireframe “skeleton” of an object before you create the surfaces, using the wireframe as a starting point from which to apply the surfaces. This method employs two relatively simple 3D objects: 3D faces and polyfaces.

Using 3D Faces and Polyface Meshes

A 3D face is a surface defined by either three or four sides, forming either a rectangular or a triangular surface. In most instances, a triangular, three-sided face is preferable because a 3D face is a planar element. A rectangular 3D face can be either planar or non-planar. Figure 29.1 show a triangular and a rectangular 3D face with the upper-right corner of the rectangular face lying at a different elevation than the other three corners. Four-sided faces can be more difficult to visualize.

You construct 3D faces with the 3DFACE command, accessed by selecting Draw, Surfaces, 3D Face. Specifying the corner points creates the face. The 3DFACE command enables you to continue creating 3D faces by selecting more points.

When creating your 3D face, always work your way around the perimeter of the face. This is especially important if you decide to create a four-sided face. If you select points in an X-wise fashion, a bow-tie effect will result, as shown in Figure 29.2.

Although you can construct a surfaced box with the AI_BOX command, there are instances in which you may need to place 3D faces on a wireframe as shown in the following exercise.

Exercise 29.1 Using the 3DFACE Command to Surface a Box

1.	Load the drawing file 29EX01.DWG from the accompanying CD-ROM. The file contains a wireframe box, as shown in Figure 29.3. This “box” consists of 12 individual polylines arranged in such a way as to represent a three-dimensional box. Figure 29.3. A wireframe “box” that you surface with the 3DFACE command.
2.	At the Command: prompt, type HIDE and press Enter. Because there are no surfaces in this collection of polylines, the HIDE command has no visual effect.
3.	From the Tools menu, select Drafting Settings and in the Drafting Settings dialog box’s Object Snap tab, ensure that the Object Snap On and Endpoint Object Snap options are checked. Click OK to close the dialog box.
4.	From the Draw menu, choose Surfaces, 3D Face.
5.	Select the seven corners of the box shown in through in Figure 29.4. Be sure to select the corners in numerical sequence. Figure 29.4. Choose the corners in sequence to apply 3D faces.
6.	When you have selected the corner at press Enter twice to end the 3DFACE command.
7.	Type HIDE and press Enter. Figure 29.5 shows you the resulting box. Figure 29.5. The box with faces applied to two sides.
8.	Leave this drawing open for use in the next exercise.

In the previous exercise, you constructed only two of the six required 3D faces to completely cover the frame of the box. The other four sides could be applied in a similar manner. The second face actually consists of two three-sided faces with the diagonal line appearing between corners and representing the shared edge. Because of the manner in which the 3DFACE command works, four-sided faces are usually drawn this way because AutoCAD attempts to construct three-sided faces where possible.

AutoCAD provides two means to hide face edges. You can hide an edge during the construction of a face by typing an i just prior to specifying the first point that will create the edge. You can also make a 3D face edge invisible after it is formed by using the EDGE command.

The following exercise shows how to use the EDGE command to hide a face edge after it has been drawn.

Exercise 29.2 Using the EDGE Command

1.	Continue in 29EX01.DWG from the preceding exercise.
2.	From the Draw menu, select Surfaces, Edge.
3.	When prompted, select the diagonal edge appearing between corners and in Figure 29.4. Notice that AutoCAD switches you to a Midpoint Object Snap mode automatically.
4.	Press Return and the edge is hidden. After an edge is hidden, you can no longer select it. To unhide an edge, you can use the Display option of the EDGE command.
5.	Press Enter to restart the EDGE command.
6.	Select an edge of the face containing the hidden edge. Select between and , for example.
7.	Type D (for display) and press Enter. You are then prompted with the Display options.
8.	When prompted for the Display options, type A (for all) and press Enter. All the edges of the face become visible and highlighted.
9.	Select the edge you want to make visible and press Enter. The visibility of the edge is restored.
10.	Quit this drawing without saving changes.

3D faces are limited as surfacing tools, principally because they are limited to three or four sides and do not serve well for the creation of curved objects. Applying 3D faces to define a large surface would be tedious work and likely yield less than satisfactory results. To get around this limitation, you can make use of the AutoCAD PFACE, or polyface, command, which enables you to create larger faces composed of many more individual faces by defining multiple faces which specify how the surface is created between points. Such a large non-planar surface is termed a polyface mesh. Here again, to construct a large polyface mesh manually would be tremendously tedious and time-consuming. The PFACE command is therefore usually used by other applications. The application “communicates” with the PFACE command through an application programming interface, such as AutoLISP, to automatically construct the mesh.