add doc

docs/lib2x-ptf_sphere.md

@@ -0,0 +1,45 @@
+# ptf_sphere
+
+Transforms a point inside a rectangle to a point of a sphere. It can create things such as [sphere maze](https://github.com/JustinSDK/dotSCAD/blob/master/examples/maze/sphere_maze.scad).
+
+![ptf_sphere](images/lib2x-ptf_sphere-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 sphere.
+- `angle` : [za, xa] mapping angles.
+
+## Examples
+
+    use <hull_polyline3d.scad>;
+    use <ptf/ptf_sphere.scad>;
+
+    size = [20, 10];
+    radius = 5;
+    angle = [180, 270];
+
+    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_sphere(size, p, radius, angle)];
+        hull_polyline3d(twisted, thickness = .5);
+    }
+
+    for(line = columns) {
+        twisted = [for(p = line) ptf_sphere(size, p, radius, angle)];
+        hull_polyline3d(twisted, thickness = .5);
+    }
+
+![ptf_sphere](images/lib2x-ptf_sphere-2.JPG)