Relevant to Blender v2.31

The "Extrude along path" technique is a very powerful modelling tool. It consists of creating a surface by sweeping a given profile along a given path.

Both the profile and the path can be a Bézier or a NURBS curve.

Let's assume you have added a Bézier curve and a Bézier circle as separate objects to your scene (Figure 9.25, “Profile (left) and path (right).”).

Figure 9.25. Profile (left) and path (right).

Play a bit with both to obtain a nice 'wing-like' profile and a fancy path (Figure 9.26, “Modified profile (left) and path (right).”). By default, Béziers exist only on a plane, and are 2D objects. To make the path span in all three directions of space, as in the example shown above, press the 3D button in the Curve EditButtons (F9) Curve and Surface panel (Figure 9.27, “3D Curve button.”).

Figure 9.26. Modified profile (left) and path (right).

Figure 9.27. 3D Curve button.

Now look at the name of the profile object. By default it is "CurveCircle" and it is shown on the NKEY panel when it is selected. You can change it by SHIFT-LMB on the name, if you like (Figure 9.28, “Profile name.”).

Figure 9.28. Profile name.

Now select the path. In its EditButtons locate the BevOb: Text Button in the Curve and Surface panel and write in there the name of the profile object. In our case "CurveCircle" (Figure 9.29, “Specify the Profile on the path.”).

Figure 9.29. Specify the Profile on the path.

The result is a surface defined by the Profile, sweeping along the path (Figure 9.30, “Extrusion result.”).

Figure 9.30. Extrusion result.

To understand the results, and hence obtain the desired effects it is important to understand the following points:

Figure 9.31. Path local plane.

With the y-axis constrained upwards, unexpected results can occur when the path is 3D and the profile being extruded comes to a point where the path is exactly vertical. Indeed if the path goes vertical and then continues to bend there is a point where the y-axis of the profile should begin to point downwards. If this occurs, since the y-axis is constrained to point upwards there is an abrupt 180° rotation of the profile, so that the y-axis points upwards again.

Figure 9.32, “Extrusion problems due to y-axis constraint.” shows the problem. On the left there is a path whose steepness is such that the normal to the local path plane is always upward. On the right we see a path where, at the point circled in yellow, such a normal begins to point down. The result of the extrusion presents an abrupt turn there.

Figure 9.32. Extrusion problems due to y-axis constraint.

The only solutions to this problems are: To use multiple - matching - paths, or to carefully tilt the path to ensure that a normal always points upwards.