Blender Spline Types: Poly vs Bezier vs NURBS

In Blender, a single Curve object can contain multiple independent splines, each utilizing a different mathematical method to generate its shape: Poly, Bezier, or NURBS. While Poly splines create straight, angular connections between vertices, Bezier splines use adjustable tangent handles for manual curvature control, and NURBS splines rely on mathematical weighting to generate ultra-smooth, continuous curves. Understanding how these three spline types function within the same curve data-block allows for greater precision and flexibility when modeling paths, profiles, and surfaces.

Poly Splines

Poly splines are the simplest curve type in Blender. They operate via linear interpolation, connecting control points with straight line segments without any inherent curvature.

  • Control Mechanism: Vertices behave similarly to standard mesh vertices. There are no direction handles or complex weight calculations.
  • Characteristics: Produces sharp corners and faceted paths. Smoothness can only be simulated by adding more vertices or applying subdivision.
  • Best Use Cases: Architectural outlines, hard-surface path layouts, simple laser-cutting profiles, and blocking out 2D shapes before conversion to other spline types.

Bezier Splines

Bezier splines are the most commonly used curve type in computer graphics. They generate smooth curves that pass directly through control points, with the shape defined by adjustable handles extending from each point.

  • Control Mechanism: Each control point features two tangent handles. Blender offers several handle types:
    • Free: Handles move independently for sharp corners.
    • Aligned: Handles remain locked in a straight line for smooth continuity.
    • Vector: Handles point directly toward adjacent vertices, creating sharp segments.
    • Automatic: Blender automatically calculates the smoothest tangent angle.
  • Characteristics: Highly intuitive and localized. Adjusting a handle affects only the curve segment immediately adjacent to that control point.
  • Best Use Cases: Typography, organic modeling, animation motion paths, cables, pipes, and detailed illustration profiles.

NURBS Splines

NURBS (Non-Uniform Rational B-Splines) calculate curves using complex mathematical formulas based on control point weights and order values. Unlike Bezier curves, NURBS curves generally do not pass directly through control points (except at the endpoints, depending on cyclic and clamp settings); instead, the points act as a magnetic hull pulling the curve toward them.

  • Control Mechanism: Points possess a "Weight" property that determines how strongly they pull the curve toward their position. The overall curve smoothness is governed by the curve's "Order" setting.
  • Characteristics: Delivers mathematically continuous, non-faceted curvature that is exceptionally smooth. Changes to a single control point smoothly distribute influence across a wider portion of the curve.
  • Best Use Cases: Industrial and product design, automotive modeling, aerodynamic surfaces, and scenarios requiring absolute mathematical smoothness.

Mixing Spline Types in a Single Curve Object

Blender allows you to mix all three types within one active Curve object in Edit Mode. While a single continuous segment must use one spline type, a Curve object can contain multiple disconnected spline islands with distinct types.

To change the type of any individual spline segment:

  1. Enter Edit Mode with your Curve object selected.
  2. Select at least one point on the target spline segment.
  3. Open the Curve menu in the viewport header (or press the context menu shortcut).
  4. Navigate to Set Spline Type and choose between Poly, Bezier, or NURBS.

This workflow enables you to combine rigid structural frameworks (Poly), intricate custom-shaped details (Bezier), and perfectly smooth transition hulls (NURBS) under a single modifier stack, origin point, and bevel configuration.