# What is SubD friendly?

## A NURBS curve is SubD friendly if

- it is cubic (degree 3)
- it is uniform
- it is non-rational
- it is periodic or has zero 2nd derivative at the start and end.

## A NURBS surface is SubD friendly if

- it is bicubic (U and V degrees are 3)
- it is uniform (U and V knots are uniform)
- it is non-rational
- has no poles (singular points)
- in the "u" parameter direction it is periodic or has zero 2nd partial derivative with respect to u derivative along its "East" and "West" edges.
- in the "v" parameter direction it is periodic or has zero 2nd partial derivative with respect to v along its "north" and "south" edges.

## The difference SubD friendly makes

When the ToSubD command converts a SubD friendly surface into a SubD object, the SubD edges will match to the surface isocurves.

When the input surface is not SubD friendly, extra rows of SubD faces will be added to get a closer match.

Rhinoceros 7 © 2010-2023 Robert McNeel & Associates. 28-Jun-2023