Fill Boundary Curves¶

In one sentence: Create a NURBS surface patch from exactly 2, 3, or 4 boundary curves using the OpenCASCADE GeomFill algorithm.

Overview¶

Workbench: Surface
Menu: Surface → Fill Boundary Curves
Shortcut: none

Generates a smooth NURBS patch interpolated through 2, 3, or 4 boundary curves (edges or wires). Three fill styles control the interior shape: Stretched, Coons, and Curved. Simpler and faster than Filling for patches with a small number of boundary curves.

Intuition¶

Think of the boundary curves as the four edges of a window frame (or three edges of a triangular frame). Fill Boundary Curves stretches a smooth surface sheet across that frame, fitting it to all four boundary curves simultaneously.

The three fill styles are different mathematical recipes for deciding how the interior of the sheet behaves:

Stretched minimises surface normal variation — produces the most uniform- looking surface.
Coons uses the classic bilinear blending formula — a good all-round default for four-sided patches.
Curved minimises curvature — produces the smoothest possible interior but may drift away from the boundary more than the other modes.

When to use it¶

You have a clean 2-, 3-, or 4-sided boundary of curves and want the fastest route to a surface patch.
You are doing Class-A surfacing and need to choose between fill algorithms to see which one gives the best highlight reflection.
You need a triangular (3-sided) patch — the Coons algorithm handles this naturally.

When NOT to use it¶

More than 4 boundary curves — use Filling instead, which handles arbitrary closed loops.
When you need G1 or G2 continuity control — Fill Boundary Curves only provides positional (G0) continuity at the boundary. Use Filling for continuity control.

Step-by-step¶

Select exactly 2, 3, or 4 edges or wires (Ctrl+click).
Choose Surface → Fill Boundary Curves.
In the task panel, select the Fill Style (Stretched, Coons, or Curved).
Each curve appears in the list. If the surface appears twisted, click the reverse button (⇄) next to the offending curve to flip its direction.
Click OK. A Surface::GeomFillSurface object is created.

Parameters¶

Parameter	Type	Default	Description
Boundary Edges	Reference list	—	2, 3, or 4 boundary edges or wires
Fill Type	Enum	Stretched	Interior shape algorithm: Stretched, Coons, or Curved

Fill type in depth¶

Style	Code	Best for
Stretched	0	Minimises normal variation — use for uniform-looking patches
Coons	1	Classic bilinear blend — good general-purpose choice for 4-sided patches
Curved	2	Minimises curvature — smoothest interior, may deviate from boundary shape

Common mistakes and pitfalls¶

Selecting 5 or more curves causes the tool to fail

Cause: The GeomFill algorithm is defined only for 2, 3, or 4 inputs.
Fix: Use Filling for boundaries with more edges.

Surface appears twisted between two curves

Cause: One curve runs in the opposite direction to the adjacent curve, causing the surface to wrap around itself.
Fix: Use the reverse button (⇄) in the task panel to flip the offending curve's direction.

Curves don't connect at corners

Cause: The boundary curves do not share endpoints (there are gaps). GeomFill requires connected boundary curves.
Fix: Close the gaps in the boundary before applying. Add connecting segments with Draft or use the Sketcher to ensure end-to-end connectivity.

Python API¶

import FreeCAD as App

doc = App.ActiveDocument
surf = doc.addObject("Surface::GeomFillSurface", "FillSurface")

# Set boundary edges — list of (object, [edge_name]) tuples
surf.BoundaryEdges = [
    (doc.getObject("Wire1"), ["Edge1"]),
    (doc.getObject("Wire2"), ["Edge1"]),
    (doc.getObject("Wire3"), ["Edge1"]),
    (doc.getObject("Wire4"), ["Edge1"]),
]

# Fill type: 0=Stretched, 1=Coons, 2=Curved
surf.FillType = 1   # Coons
doc.recompute()