Blend Curve¶

In one sentence: Create a smooth NURBS curve that joins two edges with independently controlled G0–G3 geometric continuity at each end.

Overview¶

Workbench: Surface
Menu: Surface → Blend Curve
Shortcut: none

Generates a Surface::FeatureBlendCurve that connects a point on one edge to a point on another edge. Continuity order (G0 through G3) and tangent influence size are set independently at each end, giving fine control over how the blending curve transitions into the adjacent geometry.

Intuition¶

When two surfaces meet at an edge and you need a smooth transition curve between them, Blend Curve creates exactly that connector. The continuity settings determine how "smoothly" the curve arrives at each end:

G0 — the curve touches the edge endpoint (like two roads that meet at a junction but form a sharp corner).
G1 — the curve is tangent to the edge at the endpoint (the roads merge in a smooth, straight merge lane).
G2 — the curve matches the curvature at the endpoint (like a perfectly banked road where the curvature change is imperceptible).
G3 — the rate of curvature change also matches (the smoothest possible join, used in high-quality surface design).

The "size" parameter controls how far the tangent influence reaches — a larger size creates a longer, more gradual transition.

When to use it¶

You need a fair transition curve to guide a Sections surface between two edges with specific continuity requirements.
You are building a network of curves for Class-A surfacing and need blend curves at every join.
You want to connect two edges with a curve that transitions smoothly into both without a visible kink.

When NOT to use it¶

For filling a surface gap, use Filling — Blend Curve creates a curve, not a surface patch.
For a simple straight line between two points, use Draft Line — the continuity machinery in Blend Curve would be unnecessary overhead.

Step-by-step¶

Select two edges (Ctrl-click the second edge to add to selection).
Choose Surface → Blend Curve.
The blend curve appears connecting the nearest endpoints of the two edges.
In the Properties panel, set:
Start Continuity and End Continuity (0–3)
Start Size and End Size (tangent influence lengths)
Start Parameter and End Parameter (0.0–1.0) to move the attachment point along each edge.
Press Enter or recompute to see the updated curve.

Parameters¶

Parameter	Type	Default	Description
Start Edge	Reference	—	First edge (the blend starts here)
Start Parameter	Float (0–1)	1.0	Position along the start edge where the blend attaches
Start Continuity	Integer (0–3)	2	Continuity order at the start: 0=G0, 1=G1, 2=G2, 3=G3
Start Size	Float	1.0	Length of tangent influence at the start (larger = softer transition)
End Edge	Reference	—	Second edge (the blend ends here)
End Parameter	Float (0–1)	0.0	Position along the end edge
End Continuity	Integer (0–3)	2	Continuity order at the end
End Size	Float	1.0	Length of tangent influence at the end

Continuity order guide¶

Order	Symbol	Visual effect
0	G0	Meets the edge — sharp corner possible
1	G1	Tangent — no visible kink
2	G2	Curvature-matched — highlight flow is smooth
3	G3	Acceleration-matched — smoothest possible join

Common mistakes and pitfalls¶

G2 or G3 continuity has no effect on a straight edge

Cause: Straight edges have zero curvature; G2 requires non-zero curvature to be meaningful.
Fix: Use G1 for edges adjacent to planar faces. G2/G3 work on curved NURBS edges.

Blend curve has an unexpected S-shape

Cause: The Start Size and End Size are too large relative to the distance between the edges — the curve overshoots and doubles back.
Fix: Reduce Start Size and End Size until the curve takes a direct path between the two edges.

Python API¶

import FreeCAD as App

doc = App.ActiveDocument
blend = doc.addObject("Surface::FeatureBlendCurve", "Blend")

blend.StartEdge = (doc.getObject("Body"), ["Edge1"])
blend.EndEdge   = (doc.getObject("Body"), ["Edge5"])
blend.StartParameter = 1.0   # end of Edge1
blend.EndParameter   = 0.0   # start of Edge5
blend.StartContinuity = 2    # G2
blend.EndContinuity   = 2    # G2
blend.StartSize = 10.0
blend.EndSize   = 10.0
doc.recompute()