Rhino 3D Tip: ExtractIsocurve — Extract U/V Isocurves for Precise Trims, Edits, and Construction

ExtractIsocurve turns a surface’s U/V parameter lines into editable curves you can trim with, build from, and analyze—fast.

Core idea

• An isocurve follows either the U or V direction of a NURBS surface. It lies exactly on the surface, which makes it ideal for precise splits, trims, and construction guides.
• Use Dir to visualize U (usually red) and V (usually green) directions before extracting.

How to use ExtractIsocurve effectively

• Sub-object select the face you want (Ctrl+Shift-click), then run ExtractIsocurve.
• Pick Direction = U or V (use Both when you need a crosshair at a point).
• Click on the face where you want the curve; leverage Osnaps (End, Mid, Near) and SmartTrack for precise placement.
• On multi-face polysurfaces, extract per face. If you need longer continuous isocurves, try MergeAllFaces first (when topology allows) to increase the single-face span.

Best-practice setup

• Reparameterize surfaces to 0–1 before extracting. This makes picking at exact fractions (0.25, 0.5, 0.75) intuitive and repeatable.
• When a surface’s parameterization is skewed, consider Rebuild or ChangeDegree to improve isocurve distribution. Evaluate the geometric impact before committing.
• For dense grids, use ExtractWireframe to pull all visible isocurves, then Cull or Simplify what you don’t need.

High-value workflows

• Clean trims and splits: Extract an isocurve, then Trim or Split. Because the curve lies on the surface, you avoid tolerance surprises common with projected curves.
• Surface edits with confidence: Use extracted isocurves as edit references for CageEdit regions, panel seams, or controlled offsets (OffsetSrf after splitting).
• Construction curves for surfacing: Combine multiple extracted isocurves as inputs for NetworkSrf, Loft, or Sweep2. They carry the surface’s parameter intent, improving continuity.
• Detail placement: Use isocurves as reliable rails or guides for features (vents, grooves) that must align with the underlying flow of the form.
• Analysis helpers: Run CurvatureGraph or Zebra along extracted isocurves to inspect continuity and smoothness in the direction that matters.

Troubleshooting and tips

• If extracted curves stop at face borders, reassess topology. Try MergeSrf or MergeAllFaces where appropriate, or extract per face and Join.
• If an extracted curve appears overly wiggly, the surface parameterization may be uneven. Rebuild the surface or use Reparameterize, then extract again.
• Need evenly spaced layout lines? Reparameterize to 0–1, then use object snaps and percentage picks to maintain consistent spacing.
• For features that must align to design intent rather than parameterization, pair ExtractIsocurve with Project, Pull, or FlowAlongSrf to validate positioning.

Related commands to keep handy

• Dir (check U/V), Reparameterize (0–1 domains), Rebuild/ChangeDegree (parameter quality), ExtractWireframe (bulk isocurves), DupEdge (border curves), Match (curve continuity), Split/Trim (edit operations).

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