updated description

src/arc.scad

@@ -0,0 +1,29 @@
+/**
+* arc.scad
+*
+* Create an arc. You can pass a 2 element vector to define the central angle. 
+* It provides a fn parameter consistent with the $fn parameter of the circle module.
+* It depeonds on the circular_sector module so you have to include circular_sector.scad.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-arc.html
+*
+**/
+
+
+LINE_CROSS = 0;
+LINE_OUTWARD = 1; 
+LINE_INWARD = 2;
+
+module arc(radius, angles, width, width_mode = LINE_CROSS, fn = 24) {
+    w_offset = width_mode == LINE_CROSS ? [width / 2, -width / 2] : (
+	    width_mode == LINE_INWARD ? [0, -width] : [width, 0]
+	);
+	
+	difference() {
+	    sector(radius + w_offset[0], angles, fn);
+		sector(radius + w_offset[1], angles, fn);
+    }
+}