added crystal_ball

README.md

@@ -43,6 +43,7 @@ Too many dependencies? Because OpenSCAD doesn't provide namespace management, I
 - 3D
 	- [rounded_cube](https://openhome.cc/eGossip/OpenSCAD/lib-rounded_cube.html)
     - [rounded_cylinder](https://openhome.cc/eGossip/OpenSCAD/lib-rounded_cylinder.html)
+    - [crystal_ball](https://openhome.cc/eGossip/OpenSCAD/lib-crystal_ball.html)
 	- [line3d](https://openhome.cc/eGossip/OpenSCAD/lib-line3d.html)
 	- [polyline3d](https://openhome.cc/eGossip/OpenSCAD/lib-polyline3d.html)
 	- [hull_polyline3d](https://openhome.cc/eGossip/OpenSCAD/lib-hull_polyline3d.html)

docs/lib-crystal_ball.md

@@ -0,0 +1,49 @@
+# crystal_ball
+
+Uses spherical coordinate system to create a crystal ball.  
+
+![Spherical coordinates (r, θ, φ) often used in mathematics](https://upload.wikimedia.org/wikipedia/commons/d/dc/3D_Spherical_2.svg)
+
+Dependencies: `rotate_p`, `cross_sections`, `polysections`, `ring_extrude`, `shape_pie`.
+
+## Parameters
+
+- `radius` : The radial distance r. 
+- `theta` : The azimuthal angle. It defaults to 360. It also accepts a 2 element vector. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.
+- `phi` : The polar angle. It defaults to 180. It also accepts a 2 element vector. The first element of the vector is the beginning angle in degrees, and the second element is the ending angle.
+- `$fa`, `$fs`, `$fn` : Check [the circle module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Using_the_2D_Subsystem#circle) or [the sphere module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#sphere) for more details. The final fragments will be a multiple of 4 to fit edges.
+
+## Examples
+
+	include <rotate_p.scad>;
+	include <cross_sections.scad>;
+	include <polysections.scad>;
+	include <ring_extrude.scad>;
+	include <shape_pie.scad>;
+	include <crystal_ball.scad>;
+	
+	crystal_ball(radius = 6);
+	
+	translate([12, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = 270,
+	        $fn = 12
+	    );
+	
+	translate([24, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = 270,
+	        phi = 90,
+	        $fn = 12
+	    );    
+	    
+	translate([36, 0, 0]) 
+	    crystal_ball(
+	        radius = 6, 
+	        theta = [-30, 270],
+	        phi = [30, 60]
+	    );        
+
+![crystal_ball](images/lib-crystal_ball-1.JPG)

src/crystal_ball.scad

@@ -0,0 +1,35 @@
+/**
+* crystal_ball.scad
+*
+* Uses Spherical coordinate system to create a crystal ball. 
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-crystal_ball.html
+*
+**/ 
+
+include <__private__/__nearest_multiple_of_4.scad>;
+include <__private__/__is_vector.scad>;
+
+module crystal_ball(radius, theta = 360, phi = 180) {
+    phis = __is_vector(phi) ? phi : [0, phi];
+    
+    frags = __frags(radius);
+
+    shape_pts = shape_pie(
+        radius, 
+        [90 - phis[1], 90 - phis[0]], 
+        $fn = __nearest_multiple_of_4(__frags(frags))
+    );
+                    
+    // _hole_r = 0.0005 for avoiding warnings
+    _hole_r = 0.0005;
+    ring_extrude(
+        shape_pts, 
+        angle = theta, 
+        radius = _hole_r, 
+        $fn = frags
+    );
+}