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Filling

In one sentence: Create a smooth NURBS surface that fills a closed boundary of selected edges with controlled G0, G1, or G2 continuity.

Overview

Workbench: Surface
Menu: Surface → Filling
Shortcut: none

Fills a closed loop of edges with a parametric NURBS surface. Boundary continuity can be set independently per edge (G0 = touching, G1 = tangent, G2 = curvature-matched). Internal edges and vertices can pull the fill shape towards desired interior geometry.

Intuition

Imagine stretching a soap film across a wire frame that you've bent into a closed loop. The soap film fills the hole with the smoothest possible surface that touches every wire. Filling does the same thing mathematically: given a closed boundary of edges, it solves for the smoothest NURBS patch that satisfies all boundary conditions.

The "continuity" settings control whether the patch just touches the adjacent face (G0), lies tangent to it (G1, like two sheets of paper meeting in a flat join), or matches its curvature (G2, like a perfectly blended rolling hill with no visible crease).

When to use it

  • You need to close an open region on a surface model with a smooth patch (e.g. filling the top face of a complex bottle shape).
  • You need G1 or G2 continuity across the boundary — for aesthetic product surfaces where highlights must flow smoothly across joins.
  • You have more than 4 boundary edges (Fill Boundary Curves only handles 2–4).

When NOT to use it

  • For 2, 3, or 4 boundary curves, consider Fill Boundary Curves first — it is simpler to use and uses a different algorithm that may give better results for simple patches.
  • For solid filling, use Part Design Pad or Part Loft — Filling produces a surface, not a solid.

Step-by-step

  1. Select a closed boundary loop of edges (hold Ctrl to select multiple edges). Every edge must share an endpoint with the next.
  2. Choose Surface → Filling.
  3. In the task panel under Boundary, verify the listed edges.
  4. For each boundary edge, set:
  5. Order — polynomial degree along that edge (3 = cubic, 5 = quintic)
  6. Continuity — None (G0), G1, or G2
  7. Optionally switch to the Unbound tab and add internal edges or vertices to constrain the interior shape.
  8. Click OK. A Surface::Filling object appears in the model tree.

Tip

Start with G1 continuity on all edges. Switch to G2 only if the adjacent faces are smooth NURBS surfaces — G2 on a planar face will be ignored.

Parameters

Parameter Type Default Description
Boundary Edges Reference list — Edges forming the closed boundary loop
Order Integer (per edge) 3 Polynomial degree along boundary edge (higher = can match more complex boundary curvature)
Continuity Enum (per edge) G0 Tangency condition at this edge: None (G0), G1 (tangent), G2 (curvature)
Max Degree Integer 5 Maximum NURBS degree of the result surface
Max Segments Integer 9 Maximum number of NURBS spans per boundary edge
Internal Edges Reference list — Edges inside the boundary that guide the surface interior
Internal Vertices Reference list — Points the surface must pass through

Continuity in depth

Mode What it enforces Requires
G0 (None) Surface passes through the boundary edge Nothing special
G1 Surface is tangent to the adjacent face Adjacent face must exist
G2 Surface matches curvature of the adjacent face Adjacent face must be a smooth (non-planar) NURBS surface

Common mistakes and pitfalls

Filling fails: boundary is not a closed loop

Cause: One or more edges have a gap at their junction — the endpoint coordinates do not match exactly.
Fix: Use Part → Check Geometry to find gaps. Add short connecting edges with the Draft or Sketcher workbench to close the loop.

G2 continuity has no visible effect

Cause: G2 requires the adjacent face to have a well-defined curvature. Planar faces (curvature = 0 everywhere) cannot donate curvature to the fill.
Fix: Use G1 for boundaries adjacent to planar faces; reserve G2 for boundaries on curved freeform surfaces.

Surface looks faceted, not smooth

Cause: Max Degree or Max Segments is too low for the complexity of the boundary.
Fix: Increase Max Degree (up to 8) and Max Segments to give the solver more freedom.

Python API

import FreeCAD as App

doc = App.ActiveDocument

# Create the filling surface object
surf = doc.addObject("Surface::Filling", "Fill")

# Set boundary edges — list of (object, [edge names]) tuples
body = doc.getObject("Body")
surf.BoundaryEdges = [
    (body, ["Edge1"]),
    (body, ["Edge2"]),
    (body, ["Edge3"]),
    (body, ["Edge4"]),
]
surf.BoundaryOrder = [3, 3, 3, 3]     # cubic along each edge
# Continuity: 0=G0, 1=G1, 2=G2
surf.BoundaryFaces = [...]             # TODO: verify exact continuity API

surf.MaxDegree = 5
surf.MaxSegments = 9
doc.recompute()

See also