ToNURBS and Convert in Rhino 8: A Guide to Geometry Conversion

In Rhino 8, the ToNURBS command converts mesh and SubD objects into NURBS surfaces, and selected mesh or SubD edges into NURBS curves. The Convert command changes curve structure into arc or polyline segments. This guide is based on the options described in the Rhino 8 documentation dated 15 October 2026. The goal is not simply to run a command, but to choose the right option with an understanding of how the conversion may change the geometry and the way it can be edited.

1. Start a surface or curve conversion with ToNURBS

Select a mesh or SubD object and run ToNURBS to create NURBS surfaces. To convert edges into curves, select mesh or SubD edges while holding Shift and Ctrl. Double-click an edge to select an edge loop. This lets you work with the required edge geometry rather than the entire object.

The command’s DeleteInputObjects option determines whether the source object remains in the scene after conversion. If set to Yes, the NURBS object replaces the non-NURBS object. If set to No, the new object is added to the current layer and remains in the same position as the old object. If you may need to return to the source geometry, keeping it makes it easier to compare the two results. When the conversion is complete, check which objects are in the scene; old and new geometry occupying the same space can cause confusion.

2. Choose the right conversion structure for meshes and SubD

The mesh-specific MeshOptions include the TrimTriangularFaces option. Yes converts mesh triangles into trimmed NURBS surfaces, while No converts them into untrimmed NURBS surfaces. Which structure is required depends on the intended next use; the documentation does not specify one choice that suits every model. It is therefore important to inspect the resulting surfaces after conversion, especially in areas with many triangles.

SubD-specific options provide control over how surfaces are combined and continuity at extraordinary vertices. In the Faces section, Packed combines a quad face grid into a larger NURBS surface. With Unpacked, a separate NURBS surface is created for each quad SubD face. ShowFacePacks displays SubD faces in different colors according to the divisions in rectangular quad grids. ForceUpdateFacePacks can be used to rearrange the grids after a crease is added or a face is deleted; the documentation states that it is equivalent to the PackSubDFaces command.

The documentation does not recommend a single value for these options for a particular project. The table therefore summarizes what the settings do; it is not a ready-made choice to replace project requirements.

3. Evaluate continuity at extraordinary vertices

In a SubD-to-polysurface conversion, continuity at extraordinary vertices can be affected by the conversion options. According to the Rhino documentation, continuity at these vertices can only be approximated; the G0, G1, G1x, G1xx and G2 options offer different choices for the result in this area. Smooth SubD regions away from extraordinary vertices are always defined as G2 in the polysurface.

G0 aims to keep the shape as close as possible to the SubD. G1 targets G1 continuity around the vertex; in exchange, the shape may deviate a little more from the SubD. G1x is similar to G1 and aims to keep deviation in the edge normal at 1 degree or less. G1xx uses more control points to improve the smoothness of Zebra stripes by adding double knots to isoparametric curves near the edge. G2 provides G2 continuity at extraordinary vertices, but this choice may create bumps or dimples around them.

So when choosing an option, don’t focus only on the degree of continuity. How closely the converted surface matches the shape and how it looks around the vertex also matter. The documentation mentions Zebra analysis as a way to show G2 continuity near an extraordinary vertex in a SubD. When comparing different results, inspecting the same area helps you understand the effect of each choice.

4. Split curves into segments with Convert

The Convert command changes curve structure into arc or polyline segments. The Arcs option under Output creates arc segments; nearly straight sections may become straight line segments. The Lines option produces polyline segments. The command can also use the SimplifyInput option to join collinear line segments and arc segments that lie on the same circle. This simplification aims to make the conversion of NURBS curves made up of arc and line segments more accurate.

When SimplifyInput is set to Yes, simplification uses the absolute and angle tolerances; it does not use the custom tolerance settings specified in the command. The documentation notes that No can be used when simplification is too aggressive, converting normal NURBS curves into arcs or lines. DeleteInput controls whether the original curve is deleted after conversion. OutputLayer can place the result on the current layer, on the same layer as the input curve, or on the layer of the target surface. For an input object in a model added through Worksession, the result is placed on the current layer.

AngleTolerance sets the maximum angle at segment endpoints and overrides the system tolerance. Setting it to zero allows non-tangent arcs; this can result in kinks, but may use fewer segments than the Lines option. Tolerance sets the midpoint tolerance, overriding the system setting. MinLength and MaxLength control the minimum and maximum segment lengths; for both settings, zero means no limit is applied to the corresponding length.

Impact on architecture and visualization workflows

Architecture and interior design teams that need to transfer geometry to programs that read NURBS can use ToNURBS in Rhino 8 to convert meshes, SubD objects or some simple objects to NURBS. For visualization teams, the choices concern whether to keep the source object, whether to create surfaces individually or in groups, and what types of segments to convert curves into. Keeping the source geometry before conversion and inspecting the resulting surfaces makes it easier to spot unwanted changes.

The Rhino documentation does not specify hardware requirements, processing time, licensing costs or a compatibility list for other software for these commands. Offices should therefore test the geometry and required surface structure in their own target workflows, paying particular attention to continuity at SubD corners and the conversion of mesh triangles. For curves split into segments with Convert, it is also worth keeping in mind that tolerance and segment-length settings can affect the result.

Common mistakes

• Converting without checking the effect of DeleteInputObjects and losing the old geometry.

• Overlooking that Packed and Unpacked create different surface structures.

• Forgetting that choosing G2 may create bumps or dimples at extraordinary vertices.

• Expecting a result without considering how SimplifyInput handles tolerances during Convert.

• Setting AngleTolerance to zero without accounting for non-tangent arcs and possible kinks.