added bazier_surface

README.md

@@ -46,6 +46,7 @@ Some modules may depend on other modules. For example, the `polyline2d` module d
 - Path
     - [circle_path](https://openhome.cc/eGossip/OpenSCAD/lib-circle_path.html)
     - [bezier_curve](https://openhome.cc/eGossip/OpenSCAD/lib-bezier_curve.html)
+	- [bezier_surface](https://openhome.cc/eGossip/OpenSCAD/lib-bezier_surface.html)
     - [helix](https://openhome.cc/eGossip/OpenSCAD/lib-helix.html)
     - [archimedean_spiral](https://openhome.cc/eGossip/OpenSCAD/lib-archimedean_spiral.html)
     - [sphere_spiral](https://openhome.cc/eGossip/OpenSCAD/lib-sphere_spiral.html)

docs/lib-bezier_surface.md

@@ -0,0 +1,37 @@
+# bezier_surface
+
+Given a set of control points, the `bezier_surface` function returns points of the Bézier surface. Combined with the `function_grapher` module defined in my lib-openscad, you can create a Bézier surface.
+
+It depends on the `bezier_curve` function so remember to include bezier_curve.scad.
+
+## Parameters
+
+- `t_step` : The distance between two points of the Bézier path.
+- `points` : A set of control points. See examples below.
+
+## Examples
+
+If you have 16 control points and combine with the `function_grapher` module:
+
+	include <bezier_curve.scad>;
+	include <bezier_surface.scad>; 
+	include <function_grapher.scad>;
+
+	t_step = 0.05;
+	thickness = 0.5;
+
+	ctrl_pts = [
+		[[0, 0, 20],  [60, 0, -35],   [90, 0, 60],    [200, 0, 5]],
+		[[0, 50, 30], [100, 60, -25], [120, 50, 120], [200, 50, 5]],
+		[[0, 100, 0], [60, 120, 35],  [90, 100, 60],  [200, 100, 45]],
+		[[0, 150, 0], [60, 150, -35], [90, 180, 60],  [200, 150, 45]]
+	];
+
+	g = bezier_surface(t_step, ctrl_pts);
+	function_grapher(g, thickness);    
+
+![bezier_surface](images/lib-bezier_surface-1.JPG)
+
+The following figure shows controll points and bazier curves around the surface.
+
+![bezier_surface](images/lib-bezier_surface-2.JPG)

src/bezier_surface.scad

@@ -1,3 +1,19 @@
+/**
+* bezier_surface.scad
+*
+* Given a set of control points, the bezier_surface function returns points of the Bézier surface. 
+* Combined with the function_grapher module defined in my lib-openscad, 
+* you can create a Bézier surface.
+*
+* It depends on the bezier_curve function so remember to include bezier_curve.scad.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-bezier_surface.html
+*
+**/ 
+
 function bezier_surface(t_step, ctrl_pts) =
     let(pts =  [
         for(i = [0:len(ctrl_pts) - 1])