Rhino 8 Surface Analysis Guide: Zebra and Curvature Checks

This guide is based on the Zebra, Curvature, CurvatureAnalysis, and CurvatureGraph commands described in the official Rhino 8 documentation. These tools can be used to assess the smoothness of curves and surfaces, curvature behavior, and transitions at surface joins visually or numerically. The steps here cover the options described in the Rhino 8 documentation; these sources do not explain interface details in other versions.

1. Evaluate surface joins with Zebra

The Zebra command applies a stripe map to a surface, making the transition between adjacent surfaces visible at their join. You can adjust the stripes’ direction, size, and color. For an easier-to-read pattern, choose a stripe color that contrasts with the surface’s base color.

1. Select the surfaces to inspect and run the Zebra command.

2. Adjust the stripe direction, thickness, and color. Stripe options include horizontal and vertical directions, as well as sizes ranging from thickest to thinnest.

3. Check how the stripes continue across the surface join. If the pattern shifts sideways or breaks, this indicates a G0 (position only) condition: the surfaces meet, but there is a kink at the join.

4. If the stripes align but change direction sharply at the join, position and tangency may match while curvature does not. This behavior is described as G1 continuity.

5. If the stripes flow continuously and smoothly across the join, the appearance is consistent with G2 behavior, where position, tangency, and curvature match.

How easy the stripe pattern is to read can depend on the analysis mesh detail. Rhino documentation notes that freeform NURBS surfaces generally need a detailed mesh for appropriate inspection. Open the Analysis Mesh Options window from the Zebra options and increase the mesh density to improve analysis accuracy. Showing edges or isoparametric curves can also provide additional information about the geometry being inspected.

2. Examine curvature with Curvature and CurvatureAnalysis

The Curvature command evaluates curvature at a point on a curve using the radius of a circle. When you select a curve, the radius at the cursor location appears in the status bar; a circle tangent to the curve and a tangent line are also displayed. You can mark a point on the curve to make the evaluation explicit, or end the command with Esc.

When you select a surface, two half-circles showing minimum and maximum curvature appear as you move the cursor. When you select a point on the surface, the command area displays the 3D point and normal, as well as the maximum and minimum principal curvature, Gaussian curvature, and Mean curvature. These values are intended to help assess curvature behavior around a single point.

CurvatureAnalysis visualizes surface curvature using color mapping. The color analysis works in display modes other than Raytraced. The Gaussian style shows the value based on the product of the principal curvatures: positive values are associated with bowl-like surface behavior, negative values with saddle-like behavior, and a value of zero indicates flatness in at least one direction. The Mean style shows the absolute value of the mean curvature and helps identify sudden curvature changes relative to the surrounding surface.

Min Radius can be used for operations such as offsetting a surface by a distance of r or machining it with a spherical cutter of radius r. The documentation states that red areas are not suitable for these operations, while blue areas may be safe; areas approaching red from green should also be assessed carefully. These colors alone do not constitute approval for an operation. Max Radius can be used to search for flattened areas.

3. Read continuity with a curvature graph

The CurvatureGraph command displays curvature graphs on curves, surface edges, or SubD edges. Sudden jumps in a graph may indicate that curvature does not transition continuously from one section to another. If the graph continues along the connection without a jump, curvature continuity may be G2; however, the rate of change in curvature may still differ. So look not only for continuity, but also at the flow of the graph.

The scale, density, and color of graph indicators can be adjusted in the Curvature Graph window. For surfaces, indicators in the U and V directions can be displayed separately. The documentation notes that surface curvature indicators appear on isoparametric curves; if this display is turned off, they appear only on surface boundaries. When the analysis is complete, you can turn off the graphs with the CurvatureGraphOff command.

Use in architecture and modeling

These commands help assess surface geometry independently of visual production. For example, at a curved surface join, you can use Zebra to inspect how the stripes continue and CurvatureAnalysis to see where curvature changes. If offsetting or milling for production is involved, Min Radius can help identify areas of concern in relation to the specified radius; these uses are among the examples explicitly given in the documentation.

1. Start with Zebra to assess relative continuity at joins.

2. Use Curvature to understand curvature values in specific areas, and CurvatureAnalysis to see their distribution across a surface.

3. If the color distribution does not look appropriate for the surface, start with Auto Range and reassess the value range.

4. If there is not enough detail, increase the analysis mesh density and inspect the result again.

5. Turn off the display when the analysis is complete. According to Rhino documentation, surface analysis meshes can be stored in the file and may be large. The RefreshShade command or the Save geometry only option in the Save and SaveAs commands can remove existing analysis meshes.

Common mistakes

Do not interpret every change in the stripes at a join as the same kind of problem: a shift is associated with G0, alignment followed by a sharp turn with G1, and smooth continuation with G2 behavior. Evaluating CurvatureAnalysis colors without considering the range settings can also be misleading; when the geometry changes significantly, using Auto Range first is recommended. Finally, do not overlook analysis mesh detail on freeform NURBS surfaces: the documentation says that a detailed mesh is generally needed for accurate inspection.