What is a ʼPlanarʼ curve in Rhino?

ByCADbrief Staff

Short Answer

In Rhino, a planar curve is any curve whose control points all lie on a single flat plane. The fastest professional way to verify this is with What, which reports whether the selected curve is planar. A curve can still look flat in one viewport but fail if even one point is off-plane.

What You Need to Know Before

Warning: Imported DWG, IGES, or traced geometry often appears 2D but is slightly misaligned in Z, so Rhino may treat it as non-planar. This commonly breaks surface creation commands like PlanarSrf, trimming, and closed-profile workflows.

How to Check if a Curve Is Planar

  • Command: What

  • Shortcut: what

  • Quick Steps:

    1. Select the curve in the viewport or from the layers panel.
    2. Type What in the command line and press Enter.
    3. Read the command history for the object properties and confirm whether Rhino reports the curve as planar.

  • Use the curve as a single selected object, and make sure it is not part of an unintended group or block reference.

Variables & Settings

  • Key Setting: Project
    When drawing or editing curves, the Project Osnap option can help keep picked points on the active construction plane. This reduces the risk of accidentally creating slightly non-planar curves during 2D drafting.

Why it Fails

  • Cause 1 (Geometry): One or more control points or edit points are slightly off the plane, often from imported data or 3D object snaps.

  • Cause 2 (Layers/Locks): The curve may be inside a locked block or on a locked layer, preventing edits needed to make it planar.

  • Cause 3 (Command/Logic): The curve may be open, self-intersecting, or only visually flat in a viewport, which leads users to assume it is planar when Rhino does not.

Quick Fix & Best Practice

  • Quick Fix: Use SetPt, enable only the Z option, and flatten the curve points to a common elevation.
  • Manager’s Verdict: Check planarity before using PlanarSrf, extrusions, or CNC profile prep. In production workflows, always validate imported 2D curves instead of trusting their appearance.

FAQ

How do I make a curve planar in Rhino?
Use SetPt and align all points to the same Z value.

Can a closed curve still be non-planar?
Yes, closed does not guarantee planar.

Why won’t PlanarSrf work on my curve?
The curve is usually open, self-intersecting, or not truly planar.

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