Unrolling Surfaces in Rhino 8: UnrollSrf, Smash and Squish

When converting a 3D surface into a flat pattern in Rhino 8, choose the command based on the directions of the surface’s curvature. McNeel’s Rhino 8 documentation describes UnrollSrf for developable surfaces with curvature in one direction, and Smash and Squish for approximating the flattening of doubly curved surfaces. This guide covers the commands’ use cases, the effects of their settings, and steps for evaluating the resulting flat pattern before production.

1. Determine whether the surface can be flattened

Developable surfaces can be formed by bending a flat sheet without stretching, tearing or wrinkling it. Cylinders and cones are examples. In Rhino 8, UnrollSrf flattens surfaces of this kind, as well as suitable polysurfaces. Curves, text dots and points on the surface can also be included in the selection when running the command.

Spheres, tori and surfaces that curve in two directions cannot be transferred to a flat pattern without error unless material behavior is taken into account. Before starting, use Gaussian curvature analysis to inspect doubly curved regions and identify areas that are not developable. The Curvature command, described in Rhino’s documentation, also provides visual feedback about curvature as you move the cursor over a surface: a developable surface is represented by a single curvature arc, while a doubly curved surface is represented by two.

You can run UnrollSrf from the command line. The menu path is Surface Tools › Surface › Surface Flattening › Unroll Developable Surface. The Explode option determines whether the flattened surfaces remain separate or are rejoined along the edges of the original polysurface. If the flattened surface needs to be split along specific edges, select them with UnjoinEdge before running the command.

2. Evaluate Smash and Squish for double curvature

Smash produces an approximate 2D flattening without requiring the surface to be linear in one direction. Because a doubly curved form cannot be flattened onto a non-stretchable surface such as paper without error, the result is necessarily approximate to some degree. McNeel describes the command as an option to consider for fabric-like materials that can stretch to some extent. For a developable surface, the documentation recommends using UnrollSrf instead of Smash.

Squish converts a doubly curved 3D mesh or NURBS surface into a flat pattern. In the output, clusters of points in different colors distinguish regions where compression or expansion is expected as the surface is flattened. Text markers can also be placed at points with significant changes in length; the percentage shown in each marker indicates the magnitude of the change. The Squish report also provides estimates of area change, compression and expansion.

Squish’s CustomA, CustomB and CustomC options have four parameters that determine stretching or compression behavior in boundary and interior regions: BndStretch, BndCompress, InteriorStretch and InteriorCompress. The documentation gives these parameters an initial value of 1; each can be set to a positive number. Increasing one parameter relative to the others reduces the effect of the deformation it specifies. For example, McNeel provides a settings example with InteriorCompress=100 to severely limit expansion in interior regions. This is not a universal recipe; the results of the selected values should be reviewed separately.

3. Follow a practical review process

1. Inspect the geometry. Use Gaussian curvature analysis to identify doubly curved regions. If the surface curves in one direction, start with UnrollSrf; if it has double curvature, consider Smash for approximate flattening or Squish for deformation information.

2. Account for material behavior. Determine whether the material used in production can stretch and whether compression is acceptable. Choose Squish’s Deformation and Material options accordingly; do not assume the command accounts for all of the material’s physical properties.

3. Check the report and area difference. When using Squish, review the color-coded regions, the area-change report and the text markers showing the most significant length changes together. For UnrollSrf, also check the warning about the area difference between the flattened and original surfaces.

4. Transfer markers back to the 3D model. Curves or points created on a 2D pattern with Squish can be transferred to the original 3D shape with SquishBack. This lets you inspect where markers defined on the flat pattern sit on the model.

What does this offer for architecture, interior design and archviz workflows?

These commands can help evaluate how geometry can be transferred to a plane in design processes that require cladding or cutting patterns from curved surfaces. For surfaces curved in one direction, UnrollSrf helps create a direct flattening. Results from Smash or Squish for doubly curved surfaces should be treated not as definitive production-ready patterns, but as preliminary studies that show where geometric changes are concentrated.

This distinction matters especially in architectural and interior design projects involving curved cladding or textiles. McNeel warns that the Squish algorithm does not adequately model the real physical properties of most sheet materials, particularly the effects of material thickness. A flattened pattern therefore does not replace an assessment of material properties and production conditions. The documentation covers Rhino 8 commands but does not provide information about hardware, licensing or performance requirements.

Common mistakes

• Expecting UnrollSrf to produce an accurate, stress-free flattening of a sphere or another doubly curved surface.

• Treating the output from Smash or Squish as a definitive production pattern without validating the material and manufacturing process.

• Using a pattern without reviewing Squish’s area and deformation report.

• Ignoring UnrollSrf’s warning about the area difference between the original surface and the flattened result.

• Choosing Smash for developable surfaces, even though the documentation says it is not intended for this type of surface.