Wires and Walls

31 Aug, 2002 By: John E. Wilson

This month we continue our exploration of form•Z, a powerful yet moderately priced 3D modeler from auto-des-sys, Inc. For more information about form•Z's pricing and its requirements, visit on the Internet.

Last month we focused on form•Z's tools for creating basic, or primitive, 3D geometric shapes. form•Z's Boolean Union, Difference, and Intersection tools enable you to create complex objects through a combination of these basic shapes, but if your model must have smooth, sculpted surfaces and edges that cannot be defined by lines and arcs, you must use other tools to construct them. And, generally these tools depend on wire-like objects. A tool may, for example, create a 3D object by sweeping a wire that defines the model's cross-section along a second wire that defines the object's lengthwise profile.

Object Types
form•Z lines and curves assume one of five different geometric configurations at the time they are drawn. A simple straight line, for example, can also assume the shape of a rectangle, a vertical plane, a vertical isosceles triangle, or a vertical wall that has thickness. A closed object, such as a circle, can also assume the shape of two concentric circles, a cylinder, a cone, or a circular wall, as shown in Figure 1. The form•Z names of these object types are 2D Surface/Wire, 2D Enclosure, 3D Extrusion, 3D Converged, and 3D Enclosure. Your selection in the Object Type palette of the Modeling toolbar determines the type to be used.

Figure 1. The objects you draw in form•Z assume one of five forms, depending on your selection in the Object Type palette.

As you are constructing objects that will serve as profiles and sweep paths, you will generally set the object type to 2D Surface/Wire, and occasionally to 2D Enclosure. You will use the 3D object types as a convenient way to make 3D solid and surface objects. 3D Extrusion and 3D Converged object types create a 3D surface when you draw an open object, and a 3D solid when you draw a closed object. A 3D Enclosure is always a 3D solid. There are only subtle differences between the 3D primitive objects described last month and the closed direct-drawn 3D objects, and you can freely mix the two types in Boolean operations.

The height of 3D objects is controlled by your selection in the Heights menu, which is located in the menu bar at the top of the form•Z screen. This menu offers 12 to 14 height values in metric or Imperial units, depending on the form•Z working preferences you have set, and you can modify these preset heights or switch to another set of pre-supplied heights to fit your current requirements. You can also select Graphic/Keyed in this menu to have form•Z prompt you for a height as you draw an object.

Direct-Drawn Objects
After specifying an object type, you will select a tool from either the Polygons and Circles palette or the Lines, Splines, and Arcs palette to draw an object. Appropriate options for the tool you select are displayed in the Tool Options palette, and prompts directing you to specify point locations appear in the Prompts palette. The points you specify with your pointing device are on the xy plane, which form•Z calls the Reference plane, unless you snap to points on existing objects. You can also create objects or portions of objects that are off the Reference plane by entering non-zero z-coordinate values in the Prompts palette.

You will use the tools in the Polygons and Circles palette to create closed objects, such as circles, ellipses, and rectangles, and these tools generally work in the same way the corresponding tools in other CAD programs do. An exception, though, is the versatile Polygon tool. You can specify any number larger than two for the number of sides of your polygon, and you can also add a pattern to the normally flat sides of the polygon to create shapes that resemble stars or flower petals.

The Line and the Point tools in the Lines, Splines, and Arcs palette always create an open object, as do the Arc tools when used by themselves. The four tools that create Bezier and spline curves can create either open or closed objects. They depend on control points and basis equations to define their shapes, and the words "Cubic" and "Quadratic" in the tool names refer to the size of the largest exponent in the curve's basis equations. (See the sidebar "There's Something about NURBS" in "Third Dimension, An Introduction to Rhino," December 2001, for information on spline curves.) form•Z will prompt you to specify point locations as you draw these curves. In all of these curves, the first and last points of open curves will be on the curve, but depending on the curve type, the intermediate points may or may not be on the curve. Double-clicking ends a curve and triple-clicking both closes and ends a curve.

The Vector Line tool can create a single open or closed object comprising segments made of lines, arcs, and splines, and Bezier curves. In using the Vector Line tool you specify its initial point, and then you draw straight segments by specifying other points, or you select any of the curve or arc tools to draw non-linear segments. Double-click to end a Vector Line or triple-click to close and end it.

Wire-Creation Tools
The C-Curve (which stands for Controlled-Curve) tool in form•Z's NURBS and Patches palette transforms vector lines into spline curves. The vertices of the vector line are used as control points. This is a very useful tool because it gives you a wide variety of options for establishing the curve's parameters. You will be prompted to select an object to smooth and the Tool Options pallet offer five choices in specifying a spline or Bezier curve type. You can also specify the degree of the curve.

You create helix curves with the Helix tool of the Parametric Derivatives palette. In using this tool, you specify the centerline of the helix by picking a point on the reference plane, by selecting one of the world coordinate system axes, or by selecting a previously drawn line. You can specify the parameters--such as length, radius, and pitch--of the helix in form•Z's Tool Options palette. You can apply length- and width-scale factors to create helixes having non-constant pitch and/or radius. You can also create helixes that follow a previously drawn path, which enables you to model such things as a coiled telephone cord, as shown in Figure 2.

Figure 2. form•Z's Helix tool can easily create helix curves that have non-constant pitch and/or radius. It can also create a helix with a centerline that follows a previously drawn path.

Editing the Objects
You can use your pointing device to modify the geometry of selected direct-drawn objects through form•Z's Edit Controls tool of the Pick palette. Grip points and arrows will appear at key points on the object you select, and you can drag them to change such things as length, width, radius, and height, as well as the center-to-end-point angles of arcs and the shape of curves.

You can also edit a direct-drawn object in form•Z's Query tool--select an object, and click Edit in the dialog box that is displayed. A second dialog box containing data relevant to the object's geometry that you can change will appear. Refer to the August 2002 Third Dimension column ( for more information about the use of the Query and Edit Controls tools.

You can use the C-Curve tool to edit spline and Bezier curves, even if the curve was not created with the C-Curve tool. You can change such things as control-point location and weight, equation degree, and even the curve type. The Line Editing palette contains tools for joining, trimming, and breaking wire objects.

About the Author: John E. Wilson