An extrusion, the simplest of all modeling transformations, is derived from a 2D loop (a closed chain or chains of line segments) drawn in a plane that has a thickness value. The thickness is always perpendicular to the work plane of the 2D lines (see Figure 7.7).

Figure 7.7. An extrusion is a 2D shape with thickness.

The element properties of an extrusion allow you to set the thickness of the extrusion as well as an offset from the work plane it was drawn on.

Figure 7.10 demonstrates how a furniture element can be constructed from two extrusions that create the entire shape.

Figure 7.10. A cozy lounge chair made of two simple extrusions.

Directions in a Reference Plane

The direction in which you draw a reference plane determines which way is positive and which way is negative.

These images demonstrate in plan view how the direction in which the reference plane was drawn affects the direction of the extrusion.

Because it is difficult to remember which way you drew each reference plane, we suggest that you name each reference plane you create. If you do that, you will see the name of the reference plane when it is selected, and the name is always placed at the end of the direction in which it was drawn.

When you use the face of an element as a work plane (rather than a reference plane), as shown in Figure 7.11, the direction of the extrusion will depend on whether you are making it a solid or a void shape.

Figure 7.11. A void extrudes into the solid face.

When you sketch a shape that is going to be a void, the void extrudes positively toward the interior of the object that you used to define the work plane.

When you add a solid shape, the shape extrudes positively away from the exterior of the object face, as shown in Figure 7.12.

Figure 7.12. A solid extrudes away from the solid face.

A revolve takes a 2D profile and rotates it around an axis. A revolve is composed of two elements: a 2D profile (think of this as a cross section) that will define the surface once it is revolved, and an axis that the profile revolves around. As with extrusions, the profile must be a closed loop of lines for the revolve to be valid. The bollard shown in Figure 7.13 is an example of a revolve form.

Figure 7.13. A revolve consists of a closed loop of lines and an axis of rotation.

By default, a revolve follows a full 360-degree path. In the element properties of a revolve, you can adjust this angle so that the profile will appear open, as shown in Figure 7.14. The parameters for the revolve will let you determine the start and end angles:

Figure 7.14. The end angle is set to 270 degrees.

When you select a revolve form, you'll see options similar to those available for an extrusion.

These are similar to the options available when selecting other modeling forms. You can change the 2D boundary that you used to execute the revolution with, set the visibility, edit the work plane, or re-host the form to another plane.

To understand how sketches create revolve shapes, we will look at three different results based on three different types of sketches. Figure 7.16 shows a standard revolve made out of one closed loop of lines; Figure 7.17 shows a revolve made out of two closed loops, where one loop is a subtractive element; and Figure 7.18 shows another revolve with two closed loops, where each loop is an additive element.

Figure 7.16. One closed loop of lines revolved around an axis.

Figure 7.17. In this example, the two closed loops of lines are placed one inside the other. Following the principle of sketch-based design, the inner loop creates a hole in the first loop.

Figure 7.18. The two loops are independent of one another and thus the resulting revolution creates two solid shapes.

A revolve can be composed of multiple closed loops to define the profile lines. Keep in mind that a profile must not intersect another profile; if it does, it will fail, and Revit will warn you when you try to finish the revolve. Depending on the positioning of the different profile loops, you can achieve different results, as shown in Figures 7.17 and 7.18.

A common example of a revolve form appears in Figure 7.19. The profile defines the cross section of a dome (cupola) roof. By rotating this profile around the center axis, we can generate a dome form.

Figure 7.19. The profile is revolved around the center axis, resulting in a dome form.

Another example of a revolve is this doorknob (Figure 7.20), where the cross section is drawn and then revolved around an axis.

Figure 7.20. A revolve can be used to create elements such as doorknobs as well.

A sweep is conceptually similar to an extrusion, but you must define a path for the extrusion profile to follow. The resulting form is generated by sweeping the profile along path segments. The profile is always swept perpendicular to the path segment it follows.

A sweep is defined by two sets of lines: the path, and the profile that you sweep along the path. The path can be drawn on any work plane, and the profile will always be drawn perpendicular to the path. The first segment of the path you draw determines the default work plane for the profile lines. To change the default location of the profile sketch plane, delete the line segment of the path that hosts it; the profile plane will jump to the next available line segment in the path. Figure 7.21 shows a swept form, along with the path and profile used to create it.

Figure 7.21. Example of a sweep. The dashed rectangle is the profile plane, which is always perpendicular to the path.

Defining the Sweep Path

With this method, you create a dependency between the swept element and the object that you selected for edges to define the path. Thus, when the geometry of the referenced object changes, so too will the swept element. We strongly recommend working in a 3D view when using this method, as it will make it easier to understand the relationships and effects of changing geometry (Figure 7.22).

Figure 7.22. Example of a sweeping path that is created by selecting existing edges of a solid geometry (in this case a wall opening). If the opening changes its size, the sweep will also change because of the dependency established with the Pick method.

Defining the Profile

A profile can be created using one of two methods: by drawing the profile directly or by using a loaded, predefined profile. These options are shown in the Ribbon when defining the profile for the sweep. The default, By Sketch, lets you draw the profile using standard draw tools. Click the Edit Profile button to bring up the Draw/Pick Gallery and start drawing lines. Once a profile has been loaded, it will show up in the Profile drop-down list.

This method is applicable in cases where you need to sweep the same profile on many paths. For example, you would first create a 2D profile family using the Profile (imperial) or M_Profile (metric) template. The profile has to be a closed loop of lines. Save the profile on your hard drive and then load it in your project or family.

Figure 7.23 shows the drop-down list and the Edit Profile and Load Profile buttons.

Figure 7.23. The default method for creating profiles is by sketching. You can also load premade profiles and use those.

If you use a loaded profile, you are given controls to position the profile relative to the X and Y axes (see Figure 7.24). These values are distances from the point of insertion of the profile (the intersection of the two reference planes in the Family Editor) and the point of reference of the profile and the path (the green cross). The values are editable in the Options bar, and will update if you move the profile interactively as well.

Figure 7.24. Offsetting the profile from the point of reference of the path. In this profile, the crossing of the references in the Family Editor was set to the lower-left corner of the profile. Thus, the offset values start from that point.

You can rotate the loaded profile by adding a value under the Angle option (see Figure 7.25). The Flip button mirrors the profile, if you need it. Click the Apply button on the Options bar to finish placing the profile.

Figure 7.25. The angle of rotation is applied to the profile that is to be swept.

Once a sweep is created, you can always change the values that are available in the Element Properties dialog box of the sweep, shown here:

The advantage to using premade profiles is enormous. Figure 7.26 illustrates a concrete example of a sunshading family that needs to be edited.

Figure 7.26. Sweeps can be used to create a sunshade.

If you had drawn your profile manually and then used the Copy Multiple command to repeat the shade, you would need to change each shade separately to apply a new profile. However, if you had drawn the shape as a loadable profile family and loaded it into the family, you would only need to change the profile family once and reload it. All instances of the profile will update automatically when reloaded.

Real World Scenario: Putting the Sweep Tool to Work

Here are some real-life scenarios in which the Sweep tool is very handy. We'll let the pictures do most of the talking.

First consider the modern shower fixture in the bathroom shown here:

This design was created as a simple sweep on a circular path:

The architectural example shown next demonstrates another use of sweeps: to create a highly specific wall shape. The retaining wall was created as an in-place wall using the Create function and the Sweep method.

Trajectory Segmentation

The Trajectory Segmentation parameter (see Figure 7.27) provides a way to segment smooth arcs into linear segments. This can be useful when you're rationalizing a form into planar, constructible elements.

Figure 7.27. Sweeps can be segmented using the Trajectory Segmentation parameter.

When the option is checked, it makes the parameter Maximum Segment Angle active. The value you set defines the maximum value of the angle between two segments of the arc. The smaller that value, the higher the number of linear segments that will replace the arc. Figure 7.28 shows the effects of this parameter.

Figure 7.28. An arc path can be segmented using the Maximum Segment Angle parameter.

A blend creates a form by connecting two different 2D shapes on two different planes along a linear path (see Figure 7.29). The distance between the two shapes—the base and top—is what defines the depth of the blend. The two shapes for the top and base of the blend do not need to be the same shape or have the same number of segments. Revit can even blend arcs into linear segments.

Figure 7.29. A blend is created from two sketches that blend together along a linear path.

When the top and base shapes are the same, the resulting form will look exactly like an extrusion or sweep along a single, linear segment. You can think of a blend as an extrusion with two sketches—one for the base, and one for the top.

The instance properties of a blend resemble those of an extrusion. You can set the blend depth as well as the offset from the work planes on which the shapes are drawn:

The total depth of the blend is the resulting value between the first end and the second end.

When a blended element is selected, look to the Modify Blend tab, where you'll be able to reedit the base and top, change the work plane, and re-host the blend:

As you can see, these are the same as the options you had for extrusions, with the addition of two other features that take you to the sketch of the base or top of the blend:

Whenever you select one of those two options, the Ribbon will update as shown in Figure 7.30. Click Finish Blend to complete the blend.

Figure 7.30. The contextual ribbon for editing a blend form (left and right sides shown).

When creating blends, the boundaries are drawn on a single work plane; this means that the two shapes will always be parallel to one another.

The Vertex Connect tool is specific to blends, and it shows up in the Ribbon when you're editing the blend boundary lines. This tool allows you to set the connection between the vertices that the two shapes have. Manipulating the vertices alters results in the final blend.

There are two different things you can do here:

Troubleshooting Blends

When the base shape of a blend has the same number of vertices as the top shape, the results are pretty much predictable. But often the number is not the same and you may not get the results you expect or desire. Here's a tip to consider when working with more complex blends: if one of the shapes is a circle, you might want to break it up into segments to get additional vertex points so that you arrive at a shape you need, as shown in Figures 7.33 and 7.34.

Figure 7.31. Various looks for the same element depending on the number of vertex connections or twists: the form shown can be either a tower shape or a table leg.

Figure 7.32. Changing vertex connections results in different blend forms.

Figure 7.33. A standard blend you will often see, with the base shape a square and the top shape a circle.

Figure 7.34. The same shapes with the circle split into four segments to add three more vertices. In example A, the vertex points are orthogonal; in B, they are rotated 45 degrees. Note the difference in the results.

Creating a Lamp with Blends and Sweeps

A real-world application of blends and sweeps can be seen in the Revit lamp family illustrated here:

The shade is constructed using a blend between a segmented circle for the top (A) and a more complex series of arcs for the base (B).

The top boundary has been divided into many segments to match the number of vertices of the base. This file can be found in the Chapter 7 folder on the book's companion web page if you want to explore it some more on your own (PS Lamp.rfa).

The legs of this lamp were made using sweeps. A void extrusion was used to cut the bottom of the legs horizontally so that the lamp sits flat on the floor.

Examples of Blends in Practice

The shapes in Figure 7.35 are inspired by the lamps of sculptor and designer Isamu Noguchi, but as you can imagine, they could just as easily represent a lamp, a column, or the shape of an entire building. They are constructed as simple combinations of connected blends.

Figure 7.35. A series of blends used to create a more complex form.

The blend boundaries cannot contain more than one loop of closed lines. If your intention is to create something like the lampshade shown in Figure 7.36, you need to add a void, or use a revolve. If you try to make such a shape using multiple loops, Revit will give you a warning message telling you that "more than one loop is not allowed."

Figure 7.36. This blend sketch with multiple loops is not allowed and would generate a warning message.

To obtain the result needed, you need to first create a solid blend and then a second one, done with a void that has a smaller radius, as shown in Figure 7.37.

Figure 7.37. A lampshade can be made by creating a void blend that is smaller than the solid blend.

The swept blend is a combination of a single-segment sweep with a blend. The principles for creating a swept blend follow from the sweep and blend workflow. With a blend, you have no control over the path: it is always perpendicular to the sketch plane. With the swept blend, you can create arc paths for the profiles to blend along. Examples of this include a roof or roof segment with a shape that changes over a nonstraight path, an elbow pipe that changes diameter between the two ends, funky furniture shapes or parts, and walls beneath curved stairs. Note that a swept blend can only contain a single line segment for the path. If you draw more than a single segment, Revit will issue this warning: "More than one curve not allowed."

Voids can be used in many ways to arrive at some surprising geometric forms. Think of a void as a way to carve away from a solid chunk of clay. You can truly sculpt with this tool!

Vaults

In Figure 7.38, complex vaults have been created out of a solid pentagonal extrusion and half-circle void extrusions are cutting through it.

Figure 7.38. Vaulted space created out of a combination of solid and void extrusions.

Join Geometry Tool Solves Geometry Connections

You will need to use the Join Geometry tool to have the void cut the solid.

Vertical Shapes

The basic shape of the table legs shown in Figure 7.39 is a simple blend. A plow shape is used at the base, and a triangular shape is defined for the top (A). Then a simple curved shaped void extrusion is used to carve out the blend (B). The result is an interesting shape that is far more complex than what you'd expect to be able to do with the four basic modeling forms (C-D).

Figure 7.39. A blend with a void extrusion results in some very interesting forms!

Complex Roofs

A shape like the one shown in Figure 7.40 could be created using two revolved shapes and then cut off using a void square around the edges.

Figure 7.40. Use of a simple void to cut out geometry from a revolved shape.

Where Can You Go from Here?

Mastering the modeling techniques in Revit and understanding the power of parametric relationships will open doors to some powerful modeling skills. In order to construct custom components, you'll need to become familiar with the toolset, and once you do, you'll be able to model nearly anything you can imagine. We've walked through the basic form-making tools for generating components. In the next chapter, "Concept Massing Study," we'll get into the new form-making tools that were added to Revit 2010 for making building mass forms. You'll see that making complex building forms in Revit is no longer science fiction, but right at your fingertips!