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Sweep 1 Rail

 History enabled

 Crease splitting enabled

The Sweep1 command fits a surface through a series of profile curves that define the surface cross-sections and one curve that defines a surface edge.


  1. Select a single rail curve.
  2. Select cross-section curves in the order that the surface will pass through them.
    When multiple closed cross-section curves are selected, there will be an extra step for adjusting curve seams.


Command-line options

(rails only)

Select connected edge/curve segments based on the continuity between segments.

How to chain selection.

(cross‑sections only)

Creates a surface that begins or ends at a point.

Adjust seam options (Closed curves only)


Reverses the curve direction.


Attempts to align the seam points and directions without intervention.


Moves the seam points to the way they were at the beginning of the command.


The seam points are always at the knots. You can only move them among the knots.


The seam points may leave the knots. You can move them without restriction.

To adjust seams

  1. Select each seam point and move it along the curve to line up all curve seams.

  2. Use the Flip option to make all seam arrows point to the same side.

  3. Press Enter to continue.

Sweep 1 Rail Options

Frame Style

A frame is a 3-D point and three direction vectors. It can be drawn as something that looks like the Rhino world axes icon. It describes a unique coordinate system in space. Frames are calculated along the rail and are used to orient the cross-section curves at those locations. In a simple case with one cross-section, frames are made at the cross-section curve location and where the calculated cross-section is going to go. The 3-D rotation between those two frames determines the rotation of the cross-section curve at its new location.

Freefrom is the default Frame style. The selected Frame style will be remembered in the current Rhino for next use.


The cross-section curve rotates to maintain its angle to the rail throughout the sweep.


Specify an axis for calculating the 3-D rotation of the cross-section.

The default Roadlike axis will be different depending on the rail curve. For a planar rail curve, the default axis is perpendicular to the curve plane. For a non-planar rail curve, world-Z axis will be used.

Set axis

Sets the axis direction for the Roadlike style.

Align with surface (surface edge as rail only)

If the rail is a surface edge, the cross-section curve will twist with the surface edge. If the shapes are tangent to the surface, the new surface should also be tangent.

Align with surface on.
Align with surface off.

Sweep options

Closed sweep

Creates a closed surface, continuing the surface past the last curve around to the first curve.

This option is only available after you select two cross-section curves.

Global shape blending

The sweep is linearly blended from one end to the other, creating sweeps that taper from one cross-section curve to the other. Otherwise, the sweep stays constant at the ends and changes more rapidly in the middle.

Global shape blending on.
Global shape bending off.
Untrimmed miters

If the sweep creates a polysurface with kinks, the component surfaces will be untrimmed.

Untrimmed Miters on.
Untrimmed Miters off.

Curve options

Refit rail

Refits the rail curve before creating the sweep.

Align cross sections

Allows reversing the direction of the cross-section curves.

Do not change cross sections

Creates the sweep without altering the cross-section curves.

Rebuild cross sections with ___ control points

Rebuilds the cross-section curve's control points before creating the sweep.

Refit cross sections within ___

Refits the cross-section curves before creating the sweep.

Technical notes

To determine the movement of a cross-section, one frame is found at an existing cross-section location and another is calculated at the desired location along the rail. The difference between those frames defines the movement of the cross-section.

The frames are found like this:

  1. The rail tangent and the fixed direction vector (if they are not parallel) define a plane. The cross product of the tangent vector and the fixed direction produces the normal vector of that plane (frame x) perpendicular to both of the input vectors.
  2. The cross product of frame x and the rail tangent produces another vector, frame y, perpendicular to the tangent and frame x.
  3. Taking the rail tangent as frame z, there is a unique 3-D frame at a specific location along the rail with its mutually perpendicular x, y and z coordinate directions.
  4. The rotation and translation between two of those frames determines how a cross-section will be moved from one location on the rail to another.

If the rail tangent and the arbitrary vector are parallel, they do not define a plane and the cross product does not produce a vector, so the frame is under-defined, and it will twist around the rail tangent since that is the only defined information.

See also


Fit a surface through profile curves and two edge curves.


The SubDSweep1 command sweeps a shape curve along a rail curve to create a SubD.


The SubDSweep2 command sweeps a shape curve along two rail curves to create a SubD.


Fit a surface through profile curves that define the surface shape.


Fit a surface through a network of crossing curves.

Create surfaces

Wikipedia: Tangent




Rhinoceros 7 © 2010-2024 Robert McNeel & Associates. 10-Apr-2024