Is a ʼNURBSʼ curve different from a standard vector line in Rhino?

ByCADbrief Staff

Short Answer

Yes. In Rhino, a NURBS curve is different from a standard vector line because most Rhino curve geometry is mathematically defined by control points, degree, and knots, not just two endpoints. Use What to verify the object type quickly. A line is simpler and cannot carry the same shape complexity as a NURBS curve.

What You Need to Know Before

Warning: A curve that looks like a straight vector line may still be a NURBS curve in Rhino, especially after import from DWG, AI, or STEP. This often causes unexpected results in filleting, rebuilding, cnc prep, or when checking continuity and edit behavior.

How-to

  • Command: What

  • Shortcut: What

  • Quick Steps:

    1. Select the curve in the viewport, then type What in the command line.
    2. Read the command history or dialog output to confirm whether the object is a Line, Polyline, or NURBS Curve.
    3. If needed, use the Properties panel or run PointsOn to inspect editable control points and confirm NURBS behavior.

  • Use the fastest professional method: object inspection with What before editing or converting geometry.

Variables & Settings

  • Key Setting: Control Points On (PointsOn)

  • Expert Setting: If PointsOn shows multiple control points along the object, it is behaving as a NURBS curve rather than a basic Rhino line. A true line usually has only the minimal definition and far less editable shape data.

Why it Fails

  • Cause 1 (Geometry): Imported “lines” are often converted into degree-1 or higher NURBS curves, so they are no longer true Rhino line objects.

  • Cause 2 (layers/Locks): The object may be on a locked layer, preventing inspection edits like control-point testing and making it harder to confirm the curve type.

  • Cause 3 (Command/Logic): Users assume visual appearance defines object type, but Rhino commands treat Line, Polyline, Arc, and NURBS Curve differently based on actual geometry class.

Quick Fix & Best Practice

  • Quick Fix: Run What first, then use Convert, Rebuild, or redraw with Line if you need a true simple line object.
  • Manager’s Verdict: Use true lines for drafting, snapping, and clean exports; use NURBS curves when you need controlled shaping, continuity, or downstream surface modeling.

FAQ

Can a straight-looking Rhino curve still be a NURBS curve?
Yes, it can be mathematically straight while still being stored as a NURBS curve.

Does a Rhino line have control points like a NURBS curve?
Not in the same editable way; NURBS curves expose control-point structure more fully.

Should I convert imported curves before fabrication work?
Yes, especially if you need clean linework, reliable offsets, or predictable CAM output.

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