add doc

README.md

@@ -150,7 +150,7 @@ See [examples](examples).
 	- [ptf/ptf_y_twist](https://openhome.cc/eGossip/OpenSCAD/lib2x-ptf_y_twist.html)
 	- [ptf/ptf_circle](https://openhome.cc/eGossip/OpenSCAD/lib2x-ptf_circle.html)
 	- [ptf/ptf_bend](https://openhome.cc/eGossip/OpenSCAD/lib2x-ptf_bend.html)
-	- ptf/ptf_ring
+	- [ptf/ptf_ring](https://openhome.cc/eGossip/OpenSCAD/lib2x-ptf_ring.html)
 	- ptf/ptf_sphere
 	- ptf/ptf_torus
 

docs/lib2x-ptf_ring.md

@@ -0,0 +1,45 @@
+# ptf_ring
+
+Transforms a point inside a rectangle to a point of a ring. It can create things such as [tiled_line_mobius](https://github.com/JustinSDK/dotSCAD/blob/master/examples/tiled_line_mobius.scad).
+
+![ptf_ring](images/lib2x-ptf_ring-1.JPG)
+
+**Since:** 2.3
+
+## Parameters
+
+- `size` : 2 value array `[x, y]`, rectangle with dimensions `x` and `y`.
+- `point` : The point to be transformed.
+- `radius` : The radius of the ring.
+- `angle` : The central angle of the arc.
+- `twist` : The number of degrees of through which the rectangle is twisted.
+
+## Examples
+
+    use <hull_polyline3d.scad>;
+    use <ptf/ptf_ring.scad>;
+
+    size = [20, 10];
+    radius = 5;
+
+    rows = [
+        for(y = [0:size[1]])
+            [for(x = [0:size[0]]) [x, y]]
+    ];
+
+    columns = [
+        for(x = [0:size[0]])
+            [for(y = [0:size[1]]) [x, y]]
+    ];
+
+    for(line = rows) {
+        twisted = [for(p = line) ptf_ring(size, p, radius, 360, 180)];
+        hull_polyline3d(twisted, thickness = .5);
+    }
+
+    for(line = columns) {
+        twisted = [for(p = line) ptf_ring(size, p, radius, 360, 180)];
+        hull_polyline3d(twisted, thickness = .5);
+    }
+
+![ptf_ring](images/lib2x-ptf_ring-2.JPG)

docs/lib2x-ptf_x_twist.md

@@ -39,4 +39,4 @@ Twist a point along the x-axis. You can use it to create something such as a [tw
         hull_polyline3d(twisted, thickness = .1);
     }
 
-![ptf_x_twist](images/lib2x-ptf_x_twist-1.JPG)
+![ptf_x_twist](images/lib2x-ptf_x_twist-2.JPG)

src/ptf/ptf_ring.scad

@@ -1,13 +1,16 @@
+/**
+* ptf_ring.scad
+*
+* @copyright Justin Lin, 2020
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib2x-ptf_ring.html
+*
+**/ 
+
 use <ptf/ptf_rotate.scad>;
 use <ptf/ptf_y_twist.scad>;
 
-/*
-    size: The size of a rectangle.
-    point: A point in the rectangle.
-    radius: ring radius.
-    angle: arc angle.
-    twist: The number of degrees of through which the rectangle is twisted.
-*/
 function ptf_ring(size, point, radius, angle = 360, twist = 0) = 
     let(
         yleng = size[1],