add doc

docs/lib2x-footprints2.md

@@ -7,7 +7,7 @@ Drive a turtle with `["forward", length]` or `["turn", angle]`. This function is
 ## Parameters
 
 - `cmds` : A list of `["forward", length]`s and `["turn", angle]`s.
-- `start` : Set the start point of the turtle.
+- `start` : Set the start point of the turtle. Default to `[0, 0]`.
 
 ## Examples
 	    
@@ -50,5 +50,5 @@ Drive a turtle with `["forward", length]` or `["turn", angle]`. This function is
 
 	polyline2d(poly, width = 1);
 
-![t3d](images/lib2x-footprints2-1.JPG)
+![footprints2](images/lib2x-footprints2-1.JPG)
 

docs/lib2x-footprints3.md

@@ -0,0 +1,55 @@
+# footprints3
+
+A 3D verion of [footprint2](https://openhome.cc/eGossip/OpenSCAD/lib2x-footprints2.html). 
+
+**Since:** 2.4
+
+## Parameters
+
+- `cmds` : A list of `["forward", length]`s, `["turn", angle]`s, `["roll", angle]`s and `["pitch", angle]`s. 
+- `start` : Set the start point of the turtle. Default to `[0, 0, 0]`.
+
+## Examples
+	    
+	use <hull_polyline3d.scad>;
+	use <turtle/footprints3.scad>;
+	
+	function xy_arc_cmds(radius, angle, steps) = 
+		let(
+			fa = angle / steps,
+			ta = fa / 2,
+			leng = sin(ta) * radius * 2
+		)
+		concat(
+			[["turn", ta]],
+			[
+				for(i = [0:steps - 2])
+				each [["forward", leng], ["turn", fa]]
+			],
+			[["forward", leng], ["turn", ta]]
+		);
+
+	poly = footprints3(
+		concat(
+			[
+				["forward", 10],
+				["turn", 90],
+				["forward", 10] 
+			], 
+			xy_arc_cmds(5, 180, 12),
+			[
+				["pitch", 90],
+				["forward", 10],
+				["roll", 90]
+			],
+			xy_arc_cmds(5, 180, 12),
+			[
+				["forward", 10]
+			]
+		)
+	);
+
+	hull_polyline3d(poly, thickness = 1);
+
+![footprints3](images/lib2x-footprints2-1.JPG)
+