added rotate_p

README.md

@@ -36,6 +36,9 @@ Some modules may depend on other modules. For example, the `polyline2d` module d
 	- [hollow_out](https://openhome.cc/eGossip/OpenSCAD/lib-hollow_out.html)
 	- [bend](https://openhome.cc/eGossip/OpenSCAD/lib-bend.html)
 
+- Functon
+	- [rotate_p](https://openhome.cc/eGossip/OpenSCAD/lib-rotate_p.html)
+
 - Path
     - [circle_path](https://openhome.cc/eGossip/OpenSCAD/lib-circle_path.html)
     - [bezier](https://openhome.cc/eGossip/OpenSCAD/lib-bezier.html)

docs/lib-rotate_p.md

@@ -0,0 +1,57 @@
+# rotate_p
+
+Rotates a point `a` degrees around an arbitrary axis. The rotation is applied in the following order: `x`, `y`, `z`. 
+
+## Parameters
+
+- `point` : The point `[x, y, z]`.
+- `a` : An array `[deg_x, deg_y, deg_z]`. The same as the `a` parameter of the built-in `rotate`.
+
+## Examples
+    
+You can use the code below to create a line.
+
+	hull() {
+	    sphere(1);
+	    rotate([0, -45, 45]) 
+	        translate([20, 0, 0]) 
+	            sphere(1);   
+	}  
+
+The following code have the same effect.
+
+	point = [20, 0, 0];
+	a = [0, -45, 45];
+	
+	hull() {
+	    sphere(1);
+	    translate(rotate_p(point, a))    
+	       rotate(a)  
+	           sphere(1);   
+	}  
+
+![rotate_p](images/lib-rotate_p-1.JPG)
+
+The `rotate_p` function is useful in some situations. For examples, you probably want to get all points on the path of a spiral around a sphere. 
+
+	radius = 40;
+	step_angle = 10;
+	z_circles = 20;
+	
+	points_angles = [for(a = [0:step_angle:90 * z_circles]) 
+	    rotate_p(
+	        [radius, 0, 0], 
+	        [0, -90 + 2 * a / z_circles, a]
+	    )
+	];
+	
+    // Once you get all points on the path, you can place anything at each point.
+    // I just place a sphere as a simple demonstration.
+	for(pa = points_angles) {
+	    translate(pa[0]) 
+	        sphere(1);
+	}
+
+    %sphere(radius);
+
+![rotate_p](images/lib-rotate_p-2.JPG)

src/rotate_p.scad

@@ -0,0 +1,50 @@
+/**
+* rotate_p.scad
+*
+* Rotates a point 'a' degrees around an arbitrary axis. 
+* The rotation is applied in the following order: x, y, z. 
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-rotate_p.html
+*
+**/ 
+
+module arc(radius, angles, width, width_mode = "LINE_CROSS") {
+    w_offset = width_mode == "LINE_CROSS" ? [width / 2, -width / 2] : (
+        width_mode == "LINE_INWARD" ? [0, -width] : [width, 0]
+    );
+    
+    difference() {
+        circular_sector(radius + w_offset[0], angles);
+        circular_sector(radius + w_offset[1], angles);
+    }
+}
+
+function _rotx(pt, a) = 
+    let(cosa = cos(a), sina = sin(a))
+    [
+        pt[0], 
+        pt[1] * cosa - pt[2] * sina,
+        pt[1] * sina + pt[2] * cosa
+    ];
+
+function _roty(pt, a) = 
+    let(cosa = cos(a), sina = sin(a))
+    [
+        pt[0] * cosa + pt[2] * sina, 
+        pt[1],
+        -pt[0] * sina + pt[2] * cosa, 
+    ];
+
+function _rotz(pt, a) = 
+    let(cosa = cos(a), sina = sin(a))
+    [
+        pt[0] * cosa - pt[1] * sina,
+        pt[0] * sina + pt[1] * cosa,
+        pt[2]
+    ];
+
+function rotate_p(point, a) =
+    _rotz(_roty(_rotx(point, a[0]), a[1]), a[2]);