added ellipse_extrude.scad

src/ellipse_extrude.scad

@@ -0,0 +1,45 @@
+/**
+* ellipse_extrude.scad
+*
+* Extrudes a 2D object along the path of an ellipse from 0 to 180 degrees.
+* The semi-major axis is not necessary because it's eliminated while calculating.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-ellipse_extrude.html
+*
+**/
+
+module ellipse_extrude(semi_minor_axis, height, center = false, convexity = 10, twist = 0, slices = 20) {
+    h = height == undef ? semi_minor_axis : (
+        // `semi_minor_axis` is always equal to or greater than than `height`.
+        height > semi_minor_axis ? semi_minor_axis : height
+    );
+    angle = asin(h / semi_minor_axis) / slices; 
+
+    module extrude(pre_z = 0, i = 1) {
+        if(i <= slices) {
+            f = cos(angle * i) / cos(angle * (i - 1));
+            z = semi_minor_axis * sin(angle * i);
+
+            translate([0, 0, pre_z]) 
+                rotate(-twist / slices * (i - 1)) 
+                    linear_extrude(
+                        z - pre_z, 
+                        convexity = convexity,
+                        twist = twist / slices, 
+                        slices = 1,
+                        scale = f
+                    ) children();
+
+            extrude(z, i + 1) 
+                scale(f) 
+                    children();
+        }
+    }
+    
+    translate([0, 0, center == true ? -h / 2 : 0]) 
+        extrude() 
+            children();
+}