added sphere_spiral_extrude

docs/lib-sphere_spiral_extrude.md

@@ -0,0 +1,63 @@
+# sphere_spiral_extrude
+
+Extrudes a 2D shape along the path of a sphere spiral. 
+
+When using this module, you should use points to represent the 2D shape. You need to provide indexes of triangles, too. This module provides two prepared triangles indexes. One is `"RADIAL"`. See [polysections](https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html) for details.
+
+Dependencies: `rotate_p`, `sphere_spiral`, `cross_sections`, `polysections`.
+
+## Parameters
+
+- `shape_pts` : A list of points represent a shape. See the example below.
+- `radius` , `za_step`, `z_circles`, `begin_angle`, `end_angle`, `vt_dir`, `rt_dir` : See [sphere_spiral](https://openhome.cc/eGossip/OpenSCAD/lib-sphere_spiral.html) for details.
+- `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` : `"RADIAL"` (default), `"HOLLOW"` or user-defined indexes. See [polysections](https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html) for details.
+
+## Examples
+    
+	include <rotate_p.scad>;
+	include <cross_sections.scad>;
+	include <polysections.scad>;
+	include <sphere_spiral.scad>;
+	include <sphere_spiral_extrude.scad>;
+
+	function shape_pentagram(r) =
+		[
+			// shape points
+			[
+				[0, 1], [-0.224514, 0.309017], 
+				[-0.951057, 0.309017], [-0.363271, -0.118034], 
+				[-0.587785, -0.809017], [0, -0.381966], 
+				[0.587785, -0.809017], [0.363271, -0.118034], 
+				[0.951057, 0.309017], [0.224514, 0.309017]
+			] * r,
+			// triangles
+			[   
+				[0, 1, 9],
+				[2, 3, 1],
+				[4, 5, 3],
+				[6, 7, 5],
+				[8, 9, 7],
+				[1, 3, 5],
+				[1, 5, 7],
+				[1, 7, 9]
+			]
+		];
+
+	points_triangles = shape_pentagram(2);
+
+
+	sphere_spiral_extrude(
+		shape_pts = points_triangles[0],
+		radius = 40, 
+		za_step = 2, 
+		z_circles = 20, 
+		begin_angle = 90, 
+		end_angle = 450,
+		vt_dir = "SPI_UP",
+		scale = 5,
+		triangles = points_triangles[1]
+	);
+
+![sphere_spiral_extrude](images/lib-sphere_spiral_extrude-1.JPG)