Birail Property Editor

Creates a surface or polygon mesh by running the endpoints of an open profile curve along two rails (guide curves). Also sometimes called "guided extrude."

To apply: See Automatically Generating UV Coordinates [Modeling and Deformation Basics].

To redisplay: Select the object, choose Edit Properties Modeling Properties, and click the Birail tab.

Inputs

Hide/Unhide	Hides all the inputs for the generated object to simplify the display of your scene in the 3D views, or unhides the inputs when you need to modify them.
Delete	Freezes the generator and deletes the inputs. This simplifies the scene once you are satisfied that you won't need to modify the inputs again.

Automatic Uvs

Generate

Automatically generates explicit texture UV coordinates. The UVs created in this way are evenly spaced according to the number of segments (edges or knot curves) in each direction.

The UV generation is "live". If you change the number of subdivisions set in the generator operator, the UV coordinates are automatically updated. If you change the number of subdivisions after you have manually edited the UVs, you probably won't get desirable results because the UV indices have changed. If you freeze the UV coordinates, the generator operator is frozen as well.

Clicking this button multiple times does not generate multiple UV projections.

Subdivisions

Subdivision Type

Determines how you specify the number of subdivisions in the result.

Per Span uses the "natural" subdivisions as determined by the shapes of the inputs. You can subdivide each segment further using the U and V span sliders.

In this case, discontinuities resulting from multiknots in the inputs are preserved in the generated objects.
Absolute allows you to set the total number of subdivisions using the U and V abs sliders.

U, V span

The number of subdivisions corresponding to each segment of the inputs, when Subdivision Type is Per Span.

U, V abs

The total number of subdivisions, when Subdivision Type is Absolute.

Options

Align Profile To

Implicitly rotates the profile. Possible values are:

First or Second Rail. Aligns the profile with the specified rail (according to the order in which the rails were originally picked). The profile is rotated about the axis defined by its two end points until its normal at the corresponding end is perpendicular (or as close as possible) to the tangent of the rail.
None. The profile is not rotated.

Snap

Specifies how the generated surface is fitted to its inputs.

To Profile. The surface is fitted to the profile and the rails are implicitly adjusted.
To Rails. The surface is fitted to the rails and the profile is implicitly adjusted.