Rotational extrusion spins a 2D shape around the Z-axis. It's similar to the built-in `rotate_extrude`

; however, it supports `angle`

, `twist`

and `scale`

options.

Because we cannot retrieve the shape points of built-in 2D modules, it's necessary to provide `shapt_pts`

and `triangles`

.

If your 2D shape is not solid, indexes of triangles are required. See polysections for details.

## Parameters

`shape_pts`

: A list of points represent a shape. See the example below.`radius`

: The circle radius.`angle`

: Defaults to 360. Specifies the number of degrees to sweep, starting at the positive X axis. It also accepts a 2 element vector which defines the central angle. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.`twist`

: The number of degrees of through which the shape is extruded.`scale`

: Scales the 2D shape by this value over the length of the extrusion. Scale can be a scalar or a vector.`triangles`

:`"SOLID"`

(default),`"HOLLOW"`

or user-defined indexes. See polysections for details.`$fa`

,`$fs`

,`$fn`

: Check the circle module for more details.

## Examples

```
include <rotate_p.scad>;
include <cross_sections.scad>;
include <polysections.scad>;
include <ring_extrude.scad>;
shape_pts = [
[2, -10],
[2, 10],
[-2, 10],
[-2, -10]
];
ring_extrude(shape_pts, radius = 50, twist = 180);
```

```
include <rotate_p.scad>;
include <cross_sections.scad>;
include <polysections.scad>;
include <ring_extrude.scad>;
shape_pts = [
[2, -10],
[2, 10],
[-2, 10],
[-2, -10]
];
ring_extrude(shape_pts, radius = 50, angle = 180, scale = 2);
```