Adapt Geometry to Curves and Surfaces with Flow and FlowAlongSrf in Rhino 8

In Rhino 8, the Flow and FlowAlongSrf commands can reposition or deform objects to fit the shape of a curve or surface. As described in McNeel’s official Rhino 8 documentation, Flow maps objects from a base curve to a target curve, while FlowAlongSrf maps them from a base surface to a target surface. Choosing corresponding points near curve ends or surface corners, and checking surface parameterization, can help reduce unexpected changes in orientation and stretching.

Which command should you use?

Use Flow to move an object or group of objects from one curve to another and realign them to match the curve’s shape. This lets you create a flat shape first, then map it onto a curved form. In the Rhino menu, the path is Transform › Flow along Curve; the command is named Flow.

Use FlowAlongSrf when you need to map an arrangement of objects from a source surface to another surface. Its menu path is documented as Transform › Flow along Surface. The objects are mapped to the target surface according to the layout of the source surface.

Mapping along a curve with Flow

1. Prepare the geometry and base curve. Flow realigns objects according to their relationship to a base curve and a target curve. Preparing a shape in a flat layout first, rather than building it directly on the curve, is one of the workflows described in the documentation.

2. Select corresponding ends on the base and target curves. After running the Flow command, specify the base curve with a point near one of its ends. Then select a point near the corresponding end of the target curve. Check your selections carefully in case the starting points do not match, which can cause the object to map in an unexpected direction.

3. Decide whether deformation is needed. Rigid=Yes moves objects along the target without changing their shape. With Rigid=No, objects can deform to fit the target curve. When Rigid is enabled, RigidGroups determines whether groups are handled together or their objects are treated separately.

4. Consider curve-related orientation and length options. If a surface is used as the target for Flow, TargetSurface can determine the objects’ “up” direction based on the surface normal. If the mapping goes to the opposite side of the target surface, the source points to the Flip or Dir commands to change the normal direction. Stretch controls how objects are stretched or compressed along the curve. With Local, the area of influence is defined around the input curve: objects within it are mapped, while those outside remain fixed; the mapping effect decreases toward the boundary of the influence area.

Surface mapping with FlowAlongSrf

1. Select the objects and surfaces in order. Start FlowAlongSrf and select the objects to map. Then specify an edge near a corner of the base surface, followed by an edge near the corresponding corner of the target surface. This pairing helps define the direction on the source and target surfaces.

2. Check the parameterization of both surfaces. McNeel’s documentation says the base and target surfaces should have regular, evenly distributed parameterization to prevent stretching. This does not mean the parameterization of the two surfaces has to be identical; rather, the distribution within each surface should be even. Irregular parameterization can cause objects to stretch noticeably during mapping. The documentation shows an example where the target surface is rebuilt and given more evenly spaced isocurves to prevent stretching.

3. Choose the AutoAdjust behavior based on your selections. AutoAdjust=Yes uses the selected location and nearby edge to determine the start and U direction, rather than relying on the surfaces’ UV directions; the other direction becomes V. If the selection points on the base and target do not correspond, the object may map to the opposite side of the target surface. AutoAdjust=No uses the surfaces’ UV directions and is less dependent on the selection points matching exactly.

4. Account for trimmed surfaces. According to the documentation, with AutoAdjust=Yes, mapping uses the shrunken version of the surface; with No, it uses the underlying surface. If the result is unexpected, check the selection points, whether the surface is trimmed, and the AutoAdjust setting.

Options and their effects

Checklist for architectural and visualization workflows

Rhino users preparing repeated elements along a curved facade, patterns to fit a curved surface, or groups of objects to move to another curve can make use of these commands. The workflow is to first decide whether deformation is needed, then check the appropriate command, corresponding selection points, and surface parameterization. These steps make it easier to inspect the model’s shape and orientation after mapping.

This guide covers Rhino 8 as documented in the cited sources. The sources do not provide information about hardware requirements, licensing terms, or compatibility with other software. So, when adding these commands to a workflow, it makes sense to test them on a small piece of geometry and, especially on a production model, verify the results of the deformation and structure-preservation options.

Common mistakes

• Selecting points near different curve ends or non-corresponding surface corners can change the mapping direction.

• Failing to check the parameterization distribution within each surface can cause objects to stretch.

• Expecting deformation with Rigid=Yes misinterprets what the option does.

• Overlooking the AutoAdjust setting or trimmed-surface behavior can cause objects to appear on the wrong side or in the wrong position.

• Assuming that PreserveStructure=Yes applies to polysurfaces is incorrect; the option is not available when a polysurface is selected.