This function “turns” the curve (as on a lathe) to form a
solid of rotation along the x axis.

`turn3d(x, y = NULL, n = 12, smooth = FALSE, ...)`

## Arguments

- x, y
Points on the curve, in a form suitable for `xy.coords`

.
The `y`

values must be non-negative.

- n
How many steps in the rotation?

- smooth
logical; whether to add normals for a smooth appearance.

- ...
Additional parameters to pass to `tmesh3d`

.

## Value

A mesh object containing triangles and/or quadrilaterals.

## Author

Fang He and Duncan Murdoch