added bend

docs/lib-bend.md

@@ -0,0 +1,51 @@
+# bend
+
+Bend a 3D object into an arc shape.
+
+## Parameters
+
+- `size` : The size of a cube which can contain the target object.
+- `angle` : The central angle of the arc shape. The radius of the arc is calculated automatically.
+- `fn` : Number of fragments. The target object will be cut into `fn` fragments and recombined into an arc shape.
+
+## Examples
+
+The containing cube of the target object should be laid down on the x-y plane. For examples.
+
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	       
+	%cube(size = [x, y, z]);
+	linear_extrude(z) text("A");
+
+![bend](images/lib-bend-1.JPG)
+
+Once you have the size of the containing cube, you can use it as the `size` argument of the `bend` module.
+
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	       
+	*cube(size = [x, y, z]);
+	
+	bend(size = [x, y, z], angle = 270)
+	    linear_extrude(z) text("A");
+
+![bend](images/lib-bend-2.JPG)
+
+The arc shape is smoother if the `frags` is larger. 
+	
+	x = 9.25;
+	y = 9.55;
+	z = 1;  
+	
+	bend(size = [x, y, z], angle = 270, frags = 360)
+	    linear_extrude(z) 
+	        text("A");
+
+![bend](images/lib-bend-3.JPG)
+
+This module is especially useful when you want to create things such as [zentangle bracelet](https://www.thingiverse.com/thing:1569263).
+
+[![zentangle bracelet](http://thingiverse-production-new.s3.amazonaws.com/renders/eb/93/4f/62/1f/3bd1f628e1e566dcb5313035e4f3345b_preview_featured.JPG)](https://www.thingiverse.com/thing:1569263)

src/bend.scad

@@ -0,0 +1,53 @@
+/**
+* bend.scad
+*
+* Bend a 3D object into an arc shape. 
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-bend.html
+*
+**/ 
+
+
+module bend(size, angle, frags = 24) {
+    x = size[0];
+    y = size[1];
+    z = size[2];
+    frag_width = x / frags;
+    frag_angle = angle / frags;
+    half_frag_width = 0.5 * frag_width;
+    half_frag_angle = 0.5 * frag_angle;
+    r = half_frag_width / sin(half_frag_angle);
+    h = r * cos(half_frag_angle);
+    
+    module triangle_frag() {
+        translate([0, -z, 0]) 
+            linear_extrude(y) 
+                polygon(
+                    [
+                        [0, 0], 
+                        [half_frag_width, h], 
+                        [frag_width, 0], 
+                        [0, 0]
+                    ]
+                );    
+    }
+    
+    module get_frag(i) {
+        translate([-frag_width * i - half_frag_width, -h + z, 0]) 
+            intersection() {
+                translate([frag_width * i, 0, 0]) 
+                    triangle_frag();
+                rotate([90, 0, 0]) 
+                    children();
+            }
+    }
+
+    for(i = [0 : frags - 1]) {
+        rotate(i * frag_angle + half_frag_angle) 
+            get_frag(i) 
+                children();  
+    }
+}