added stereographic_extrude

docs/lib-stereographic_extrude.md

@@ -0,0 +1,31 @@
+# stereographic_extrude
+
+Takes a 2D polygon as input and extends it onto a sphere. If you light up a lamp on the north pole of the sphere, the shadow will return to the original 2D polygon. For more information, take a look at [Stereographic projection](https://en.wikipedia.org/wiki/Stereographic_projection).
+
+The 2D polygon should center at the origin and you have to determine the side length of a square which can cover the 2D polygon. Because the 2D polygon will be extended onto a sphere, you can use `$fa`, `$fs` or `$fn` to controll the sphere resolution.
+
+## Parameters
+
+- `shadow_side_leng` : The side length of a square which can cover the 2D polygon.
+- `$fa`, `$fs`, `$fn` : Check [the sphere module](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Primitive_Solids#sphere for more details.
+
+
+## Examples
+    
+	dimension = 100;
+	
+	render() stereographic_extrude(shadow_side_leng = dimension)
+	   text(
+            "M", size = dimension, 
+            valign = "center", halign = "center"
+       );
+	   
+	color("black") 
+	   text(
+            "M", size = dimension, 
+            valign = "center", halign = "center"
+       );
+
+![stereographic_extrude](images/lib-stereographic_extrude-1.JPG)
+
+For more advanced examples, take a look at [my stereographic_projection collection](https://www.thingiverse.com/JustinSDK/collections/stereographic-projection).