Rhino 3D Tip: Controlled Curve Transitions with BlendCrv

Use BlendCrv when a transition needs more control than a constant-radius fillet. It creates a connecting curve with adjustable continuity at each end, making it useful for product profiles, architectural outlines, and curves that will drive sweeps or lofts.

Choose continuity deliberately. A connection can look smooth from a distance while still producing an unwanted highlight break in the resulting surface. Rhino’s blend settings let you specify how the new curve meets each neighboring curve:

• Position (G0): The endpoints meet, but their directions can differ. Use this when a visible corner is acceptable.
• Tangency (G1): The endpoints meet with aligned tangent directions. This removes the directional kink, although curvature can change abruptly.
• Curvature (G2): The connection matches tangent direction and curvature at the endpoint. This often produces a more refined transition for visible design profiles.

Build the blend with a controlled workflow:

• Prepare the opening. Trim or split the source curves so their intended endpoints leave enough room for the transition. A cramped gap can force an otherwise smooth blend into an awkward bulge.
• Run BlendCrv. Select the first curve near the end you want to connect, then select the second curve near its intended connection end. Selection location matters.
• Set continuity independently. Start with Tangency or Curvature at both ends. You can use different conditions when the design requires them; symmetry is not always necessary.
• Adjust the preview. Use the available blend handles and adjustment controls to refine fullness and balance. Make small changes while watching the entire transition, not just the endpoints.
• Inspect before accepting. Check the result in a suitable orthographic view and in Perspective, especially when the source curves are not coplanar.

Verify the geometry rather than trusting the silhouette. Run CurvatureGraph on the source curves and the blend. Look for unexpected spikes, excessive curvature, or abrupt changes near the connections. For a G2 connection, curvature should agree at the junction, but the overall graph can still reveal an unnecessarily lumpy transition.

Use GCon to evaluate continuity between the blend and each neighboring curve. This is particularly helpful when the profile will become an important surface boundary. A visually convincing connection is not a substitute for checking the geometric condition your downstream modeling requires.

Avoid these common shortcuts:

• Do not treat Join as a smoothing tool. It combines compatible curve segments into a polycurve; it does not improve their tangency or curvature.
• Do not assume G2 guarantees attractive shape. Endpoint continuity and overall fairness are different requirements.
• Do not rebuild the finished profile automatically. Rebuilding can alter carefully established endpoint conditions. Recheck continuity after any shape-changing edit.

For repeatable results, keep the original curves on a reference layer and compare alternatives before committing. Explore Rhino software and learning resources through NOVEDGE, and make continuity checks part of your everyday modeling workflow.

You can find all the Rhino products on the NOVEDGE web site at this page.