Curve on Mesh¶

In one sentence: Interactively trace a B-spline curve along the surface of a mesh by picking points that snap to the mesh.

Overview¶

Workbench: Surface
Menu: Surface → Curve on Mesh
Shortcut: none

Enters an interactive curve-drawing mode where each click places a control point that snaps to the mesh surface. The result is a B-spline curve that follows the mesh topology. Used primarily in reverse-engineering workflows to extract guide curves from a scan mesh for subsequent surface reconstruction.

Intuition¶

Think of it like drawing with a pen on a 3-D sculpture: each click plants a dot on the surface, and FreeCAD connects those dots with a smooth B-spline that hugs the contours of the mesh. You're not drawing in free space — the pen always sticks to the surface.

This is the first step in a reverse-engineering workflow: scan → mesh → extract guide curves → build NURBS surfaces through those curves.

When to use it¶

You have an imported scan mesh (STL, OBJ) and want to extract feature curves (edges of surfaces, symmetry lines, section profiles) to guide surface reconstruction.
You need boundary curves for Filling or Sections that follow the actual scan geometry.
You are tracing a design intent curve on a physical prototype scan.

When NOT to use it¶

On CAD solid geometry — if you need curves on a B-rep solid, use Sketcher (sketch on face) or Draft tools. Curve on Mesh is specifically designed for mesh input.
For precise feature extraction — the B-spline approximation has tolerance; for exact geometric features, use dedicated scan-to-CAD software.

Step-by-step¶

Import or create a mesh object (use Mesh workbench → Import Mesh or Mesh from Shape).
Switch to the Surface workbench.
Select the mesh object.
Choose Surface → Curve on Mesh.
Click points on the mesh surface to trace the desired curve. Each click adds a control point that snaps to the mesh.
Press Enter or right-click to finish and accept the curve.
Press Escape to cancel without creating a curve.

Tip

Place more control points in regions with high curvature (tight bends, sharp features) and fewer points in flat or smoothly curved regions. Over-specifying flat regions makes the spline stiff and less fair.

Parameters¶

Parameter	Type	Default	Description
Mesh	Reference	—	The source mesh object that points snap to
Segment Size	Float (mm)	—	Approximation tolerance — how closely the B-spline follows the picked points
Angle Tolerance	Float (°)	—	Maximum angle between consecutive mesh face normals for a single curve segment

Resulting object properties¶

Property	Description
`Wire`	The B-spline curve draped on the mesh surface
`Mesh`	Reference to the source mesh
`SegmentSize`	Tolerance used during curve fitting

Common mistakes and pitfalls¶

Points don't snap to the mesh

Cause: The mesh may be offset from the origin, or snap is not active.
Fix: Ensure the mesh is the selected object before starting, and that the mesh is visible. If snapping doesn't work, try rebuilding the mesh normals (Mesh workbench → Harmonize Normals).

Curve drifts away from the mesh surface

Cause: Too few control points in a high-curvature region; the spline cuts corners.
Fix: Add more control points in high-curvature areas to pull the B-spline closer to the surface.

Curve on Mesh via Surface menu not the same as Sketcher On Surface

Cause: Part Design's "Sketcher On Surface" maps a 2D sketch onto a face, which is a different operation.
Fix: Confirm you are in the Surface workbench and using the correct menu entry. Curve on Mesh works with mesh objects; Sketcher On Surface works with B-rep faces.

Python API¶

The Curve on Mesh tool is primarily interactive. The resulting object can be accessed programmatically after creation:

import FreeCAD as App

doc = App.ActiveDocument

# After creating the curve interactively, access the result:
curve = doc.getObject("CurveOnMesh")   # actual name may differ
if curve:
    print("Wire:", curve.Wire)
    print("Segment size:", curve.SegmentSize)

# The wire can be used as a boundary edge for Filling:
fill = doc.addObject("Surface::Filling", "Fill")
fill.BoundaryEdges = [(curve, ["Edge1"])]
doc.recompute()