remove swap_surface param

docs/lib3x-sf_thicken.md

@@ -0,0 +1,78 @@
+# sf_thicken
+
+It thickens a surface, described by a m * n list of `[x, y, z]`s.
+
+## Parameters
+
+- `points` : A m * n list of `[x, y, z]`s. See examples below.
+- `thickness` : The depth of the thickening.
+- `direction` : The direction of thickening. It accepts `"BOTH"` (default), `"FORWARD"` or `"BACKWARD"`. Thickening is applied in both directions from the surface, the direction of the surface normals or the opposite direction to the surface normals. It also accept a direction vector `[x, y, z]`. Thickening is only applied in the direction you give.
+- `swap_surface` : The direction of thickening. 
+
+## Examples
+
+	use <sf_thicken.scad>;
+	
+	points = [
+	    [[0, 0, 1], [1, 0, 2], [2, 0, 2], [3, 0, 3]],
+	    [[0, 1, 1], [1, 1, 4], [2, 1, 0], [3, 1, 3]],
+	    [[0, 2, 1], [1, 2, 3], [2, 2, 1], [3, 2, 3]],
+	    [[0, 3, 1], [1, 3, 3], [2, 3, 1], [3, 3, 3]]
+	];
+	
+	thickness = 0.5;
+	
+	sf_thicken(points, thickness);
+
+![sf_thicken](images/lib3x-sf_thicken-1.JPG)
+
+	use <sf_thicken.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;
+	
+	points = [
+	    for(y = [min_value:resolution:max_value])
+	        [
+	            for(x = [min_value:resolution:max_value]) 
+	                [x, y, f(x, y)]
+	        ]
+	];
+	
+	sf_thicken(points, thickness);
+
+![sf_thicken](images/lib3x-sf_thicken-2.JPG)
+
+	use <sf_thicken.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;
+	style = "LINES"; 
+	
+	points = [
+	    for(y = [min_value:resolution:max_value])
+	        [
+	            for(x = [min_value:resolution:max_value]) 
+	                [x, y, f(x, y)]
+	        ]
+	];
+	
+	sf_thicken(points, thickness, style);
+
+![sf_thicken](images/lib3x-sf_thicken-3.JPG)

src/surface/sf_thicken.scad

@@ -11,7 +11,7 @@
 use <../util/sum.scad>;
 use <sf_solidify.scad>;
 
-module sf_thicken(points, thickness, direction = "BOTH", swap_surface = false) {
+module sf_thicken(points, thickness, direction = "BOTH") {
     function tri_normal(tri) =
         let(v = cross(tri[2] - tri[0], tri[1] - tri[0])) v / norm(v);    
 
@@ -45,7 +45,12 @@ module sf_thicken(points, thickness, direction = "BOTH", swap_surface = false) {
                     dir_v
             ]
         ];
-        if(swap_surface) {
+
+        midy = len(points) / 2;
+        midx = len(points[0]) / 2;
+        nv = tri_normal([points[midy][midx], points[midy + 1][midx], points[midy][midx + 1]]);
+
+        if(nv * dir_v > 0) {
             sf_solidify(surface_bottom, points);
         }
         else {