add doc

README.md

@@ -185,7 +185,7 @@ See [examples](examples).
 - [surface/sf_ring](https://openhome.cc/eGossip/OpenSCAD/lib2x-sf_ring.html)
 - [surface/sf_sphere](https://openhome.cc/eGossip/OpenSCAD/lib2x-sf_sphere.html)
 - [surface/sf_torus](https://openhome.cc/eGossip/OpenSCAD/lib2x-sf_torus.html)
-- surface/sf_solidify
+- [surface/sf_solidify](https://openhome.cc/eGossip/OpenSCAD/lib2x-sf_solidify.html)
 
 ### Noise (2.3 Preview)
 - noise/nz_perlin1

docs/lib2x-sf_solidify.md

@@ -0,0 +1,46 @@
+# sf_solidify
+
+It solidifies two square surfaces, described as a set of points `[x, y, z]`. 
+
+**Since:** 2.3
+
+## Parameters
+
+- `surface1` : A list of numbers (0 ~ 255).
+- `surface1` : The thickness of the model.
+- `slicing` : Given a rectangle, we have two ways to slice it into two triangles. Using this parameter to determine the way you want. It accepts `"SLASH"` (default) and `"BACK_SLASH"`.
+
+## Examples
+
+    use <surface/sf_solidify.scad>;
+
+    function f(x, y) = 
+    30 * (
+        cos(sqrt(pow(x, 2) + pow(y, 2))) + 
+        cos(3 * sqrt(pow(x, 2) + pow(y, 2)))
+    );
+
+    thickness = 2;
+    min_value =  -200;
+    max_value = 200;
+    resolution = 10;
+
+    surface1 = [
+        for(y = [min_value:resolution:max_value])
+            [
+                for(x = [min_value:resolution:max_value]) 
+                    [x, y, f(x, y) + 100]
+            ]
+    ];
+
+    surface2 = [
+        for(y = [min_value:resolution:max_value])
+            [
+                for(x = [min_value:resolution:max_value]) 
+                    [x, y, -f(x, y) - 100]
+            ]
+    ];
+
+    sf_solidify(surface1, surface2);
+
+![sf_solidify](images/lib2x-sf_solidify-1.JPG)