Creating form•Z Skins

31 Oct, 2002 By: John E. Wilson

We continue exploring form•Z's tools for transforming wire objects into 3D objects this month. For more information on form•Z's hardware and software requirements and on its pricing, visit or refer to the July 2002 Third Dimension column.

The Skin tool, which is in form•Z's Parametric Derivatives palette, creates 3D objects from wireframe source and path objects, as does the Sweep tool I focused on in the October 2002 issue. The Skin tool, though, gives you more flexibility in creating 3D objects because you can utilize more than two sources and paths. As do all form•Z tools, the options for the Skin tool are displayed in the Tool Options palette. Two of the tool's options--Skinning Along Paths and Cross Skinning--act much like two entirely different tools; they have different requirements, and they work in different ways. Therefore, we will discuss the two options separately.

Figure 1. The Skinning Along Paths option of form•Z's Skin tool creates a 3D object by pushing and blending one or more sources along three or more paths. The object shown in a shaded viewing mode on the right in this figure was created from the two sources and four paths shown on the left.

Skinning Along Paths
Similar to the action of the Sweep tool, the Skinning Along Paths option of the Skin tool uses source objects to define the cross section of a 3D object and path objects to define its lengthwise shape, as shown in Figure 1. Unlike the Sweep tool, however, there are no restrictions on the maximum number of source and path objects. The minimum requirements are one source object and three path objects. Either or both can be open or closed. When your sources are closed and the paths are open, you can cap the ends of the resulting 3D object. A completely closed 3D object is a solid you can modify with form•Z's Boolean operations. On the other hand, when a 3D object has an opening, it is a surface that you will use Trim, Split, and Stitch operations to modify.

Unlike the Sweep tool, the Skin tool does not automatically move and orient source objects with path objects. Most of the time you will either draw or place the source objects in their final position relative to the path objects. This position does not have to be at the paths' end points; however, (and this is a very important, however) points--such as endpoints and vertex points--on the source must be within a distance tolerance to points on the paths. You can adjust this distance tolerance, but if you set a large tolerance value in an attempt to accommodate points that do not touch, form•Z may have problems determining which points to match. You can also use form•Z's Insert Point tool, which is in the Line Editing palette, to add points--such as at the midpoints of a rectangle's sides--to source and path objects.

When you have a single-source object, its overall shape is maintained as it is swept along the paths, and its size is adjusted to match the spacing of the paths. When you have multiple source objects, the cross-section shape of the 3D object blends to match the shapes of the sources. Source objects may have opposing directions. Path objects, though, should all have the same direction.

An alternative to placing source objects in their position relative to the path objects is to use the Skin tool's By Anchor Points option. To successfully use this option you must first use the Point Marker tool of the Topological Attributes palette to specify points on both the source and path objects that are to match.

Skinning Along Paths (continued)
To have form•Z display the points of objects, select Display from the menu bar. Then, select Display Options to bring up the Modeling Display Options dialog box, and choose Wireframe Options to bring up yet another dialog box. Click the Show Points checkbox, and, if you intend to use anchor points, click the Show Marked Points checkbox as well.

In using the Skin tool, you will specify the parameters for the operation, such as the number of source and path objects you intend to use, in the Tool Options palette. Then you will select the source object, or objects, and next you will select the path objects. Each source must consist of a single object. You cannot use, for instance, a rectangle made from four individual lines as a source. When source placement is by current position, the order in which you select source and path objects is not important--but be sure to select the source objects before the path objects.

Cross Skinning
The Cross Skinning option of the Skin tool revolves one or more sources about a closed path. When multiple sources are used, the surface shape of the resulting 3D object blends to match the shape of each individual source object. Even though the revolution path is always planar, the path object can be nonplanar. For a nonplanar path object the revolution plane is the average of the path's points.

The source object, or objects, can be either open or closed. You cannot mix the two types, though. A closed source must share two points with the path and, if there are multiple sources, two points with each of the other sources. An open source must share one point, and it can be an interior point, with the path. When multiple open sources are used, their endpoints that are not on the path must coincide.

Figure 2. The Cross Skinning option of form•Z's Skin tool revolves one or more sources about a closed path. For the most predictable results you will generally find it best to use just a single source and with its endpoints located so that the revolution axis passes through the centroid of the path.

When closed sources are used, the axis of revolution is between the source points on opposite sides of the path that are nearest the centroid of the path. When open sources are used, the axis of revolution is between their endpoints, or, if one source endpoint is on the path, the axis is the perpendicular line from the other endpoint to the revolution plane. For the most predictable results you will generally find it best to use just a single source with its endpoints located so that the revolution axis passes through the centroid of the path, as shown in Figure 2.

Smooth Skinned Objects
As do the other form•Z tools in the Parametric Derivatives palette that create 3D objects from wireframe objects, the Skin tool has options to make the surface of the resulting 3D model either facetted or smooth. Furthermore, when you select the smooth option, you can select NURBS as the type of smoothing to round sharp corners on source and path objects in addition to creating a smooth surface. This enables you to use rough-drawn vector lines as source and path objects and still create smooth 3D models.

You can create reasonably smooth-facetted skinned surfaces by using smoothly curved paths and sources, and you will sometimes use this technique because it gives you better control over the final shape of the 3D object than constructing it as a NURBS object. Often you will use the Vector Line tool to draw a set of source and path objects, and then smooth them with the C-Curve tool. When you do this, though, you should select that tool's Shallow, Normal, or Deep Tangent Quick Curve options because the other curve types do not preserve the original vertex and anchor points. Yet another way to create a smooth skinned 3D object is to make it as a facetted model, and then use the Patch Derive tool of the NURBS and Patches palette to smooth it.

About the Author: John E. Wilson