Flow and FlowAlongSrf in Rhino 8: A Guide to Mapping Objects to Curves and Surfaces

Rhino 8’s Flow and FlowAlongSrf commands are used to fit objects to curved paths or surfaces. McNeel’s Rhino 8 documentation describes Flow as mapping from a base curve to a target curve, and FlowAlongSrf as mapping from a base surface to a target surface. Both commands let you choose whether objects are simply repositioned or deformed to match the target geometry. The right command depends on whether the mapping follows a curve or takes place across a surface.

Which command should you use?

Flow moves and realigns an object or group of objects from a base curve to a target curve. It can be easier to create a flat shape along a straight path and then fit it to a curve than to model a complex shape around a curve from the outset. During the operation, select points near corresponding ends of the base and target curves.

FlowAlongSrf maps objects from a base surface to a target surface. With this command, an edge near a corner of the base surface is matched to the edge near the corresponding corner of the target surface. The surfaces’ parameterization and directions can affect the result just as much as the selection points.

Step-by-step workflow

1. Choose the command based on the geometry type. For mapping to a curve, use Transform › Flow along Curve; for mapping across a surface, use Transform › Flow along Surface. These are the menu paths specified in the documentation.

2. Select the objects to map. Flow can be used with a single object or a group. Then specify the base and target curves for Flow, or the base and target surfaces for FlowAlongSrf.

3. Match the corresponding start points. For Flow, select a point near one end of the base curve and the corresponding end of the target curve. For FlowAlongSrf, pay attention to matching corners and edges on the base and target surfaces. This is especially important with AutoAdjust=Yes, because the selection location defines the start of the mapping and the U direction.

4. Choose between rigid mapping and deformation. Rigid determines whether objects simply move along the target or change shape as they are mapped. If you want to reposition objects without changing their shape, consider Yes; if they need to deform to fit the target, consider the effect of No. Set whether to create a copy with Copy.

5. Check the orientation and structure. In FlowAlongSrf, AutoAdjust builds the mapping based on the selection location and edge direction, or on the surfaces’ UV directions. When working with 3D target curves in Flow, you can use Roadlike axis definitions. After the operation, inspect the objects’ orientation and shape on the target.

What do the options change?

The documentation does not recommend one ideal setting for every job. The values below explain what each option does and should be chosen according to the intended result; they do not imply default values for any setting.

In FlowAlongSrf, AutoAdjust=Yes derives the start and U direction from the selected corner and edge; the other direction is set as V. If the selection points on the object, base surface, and target surface do not correspond, the object may be mapped to the opposite side of the target surface. AutoAdjust=No uses the surfaces’ UV directions, so the location of the selected points is not critical. The two options also use different references with trimmed surfaces: Yes uses the shrunken surface, while No uses the underlying surface for mapping.

PreserveStructure does not refer to the same structure in both commands. The Flow documentation describes this option as preserving the curve or surface’s knot structure after deformation. The FlowAlongSrf documentation refers to the curve or surface’s control-point structure. In FlowAlongSrf, preserving the structure can reduce deformation accuracy if the object does not have enough control points; with No, objects are reorganized as needed for more accurate deformation. The option does not apply to polysurfaces and is not displayed when those objects are selected.

In Flow, the TargetSurface option uses a surface along with the target curve to map the objects’ “up” direction relative to the surface normal. If objects are oriented toward the wrong side of the surface, use the Flip or Dir commands to change the surface normal. Local uses a tube to define the area of effect around the base curve: geometry inside the tube flows, geometry outside remains fixed, and the effect falls off toward the tube wall. For 3D target curve orientation, Roadlike includes axes defined on the base and target curves in the rotation calculation.

Using Flow in architecture and archviz workflows

Flow can be used to fit a flat element to a curved path, while FlowAlongSrf can map objects to a surface form. This approach offers an alternative modeling workflow for architects, interior designers, and visualization specialists working with curved design elements or geometry placed along a surface. However, the documentation does not specify performance gains or hardware requirements for particular types of design work.

If you see stretching in a FlowAlongSrf result, don’t focus only on the geometry being mapped. According to McNeel’s documentation, matching the parameterization of the base and target surfaces can help prevent stretching, while uneven target-surface parameterization can stretch objects. The documentation associates a target surface rebuilt with dense isocurves and even parameterization with avoiding noticeable stretching. Also check the effects of the copy, rigidity, and orientation options against the needs of the design.

Common mistakes

• Selecting non-corresponding ends or edges on the base and target curves in Flow, or on the base and target surfaces in FlowAlongSrf.

• Overlooking the relative locations of selection points on the surfaces when using AutoAdjust=Yes.

• Ignoring surface parameterization when investigating stretching in a FlowAlongSrf result.

• Failing to check the direction of the target surface normal in Flow, or not considering Flip or Dir if objects are oriented to the wrong side.

• Assuming that PreserveStructure applies to polysurfaces.

• Assuming that RigidGroups is available when Rigid=No.