add doc

README.md

@@ -82,7 +82,7 @@ See [examples](examples).
 - [bezier_smooth](https://openhome.cc/eGossip/OpenSCAD/lib2x-bezier_smooth.html)	
 - [midpt_smooth](https://openhome.cc/eGossip/OpenSCAD/lib2x-midpt_smooth.html)
 - [in_polyline](https://openhome.cc/eGossip/OpenSCAD/lib2x-in_polyline.html)
-- contours (2.3 Preview)
+- [contours](https://openhome.cc/eGossip/OpenSCAD/lib2x-contours.html) (2.3 Preview)
 	
 ### Path
 - [arc_path](https://openhome.cc/eGossip/OpenSCAD/lib2x-arc_path.html)

docs/lib2x-contours.md

@@ -0,0 +1,51 @@
+# contours
+
+Computes contour polygons by applying [marching squares](https://en.wikipedia.org/wiki/Marching_squares) to a rectangular list of numeric values. 
+
+**Since:** 2.3
+
+## Parameters
+
+- `points` : 2 value array `[x, y]`, rectangle with dimensions `x` and `y`.
+- `threshold` : When applying a threshold value, the function returns isolines. When applying upper and lower threshold values, it returns isobands.
+
+## Examples
+
+    use <hull_polyline2d.scad>;
+    use <function_grapher.scad>;
+    use <contours.scad>;
+
+    min_value =  1;
+    max_value = 360;
+    resolution = 10;
+
+    function f(x, y) = sin(x) * cos(y) * 30;
+    
+    points = [
+        for(y = [min_value:resolution:max_value])
+            [
+                for(x = [min_value:resolution:max_value]) 
+                    [x, y, f(x, y)]
+            ]
+    ];
+
+    function_grapher(points, 1);
+
+    translate([max_value, 0, 0]) 
+    for(z = [-30:5:30]) {
+        translate([0, 0, z])
+        linear_extrude(1)
+        for(isoline = contours(points, z)) {
+            hull_polyline2d(isoline, width = 1);
+        }    
+    }
+
+    translate([0, max_value]) 
+    for(z = [-30:5:30]) {
+        linear_extrude(35 + z)
+        for(isoband = contours(points, [z, z + 5])) {
+            polygon([for(p = isoband) [p[0], p[1]]]);
+        } 
+    }
+
+![contours](images/lib2x-contours-1.JPG)

src/contours.scad

@@ -1,3 +1,13 @@
+/**
+* contours.scad
+*
+* @copyright Justin Lin, 2020
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib2x-contours.html
+*
+**/
+
 use <_impl/_contours_impl.scad>;
 
 function contours(points, threshold) =