add nautilus_shell

examples/nautilus_shell.scad

@@ -0,0 +1,49 @@
+include <line2d.scad>;
+include <rotate_p.scad>;
+include <hull_polyline2d.scad>;
+include <bezier_curve.scad>;
+include <ellipse_extrude.scad>;
+
+
+$fn = 24;
+
+chambered_section_max_angle = 300;
+steps = 25;
+thickness = 1;
+slices = 5;
+semi_minor_axis = 10;
+height = 5;
+
+module nautilus_shell(chambered_section_max_angle, steps, thickness) {
+    function r(a) = pow(2.71828, 0.0053468 * a);
+    
+    a_step = chambered_section_max_angle / steps;
+    spiral = [
+        for(a = [a_step:a_step:chambered_section_max_angle + 450])  
+            rotate_p([r(a), 0], a)
+    ];
+
+    render() {
+        hull_polyline2d(spiral, thickness);
+
+        for(a = [a_step:a_step * 2:chambered_section_max_angle]) {
+            a2 = a + 360;
+            a3 = a + 420;
+            p1 = rotate_p([r(a), 0], a);
+            p2 = rotate_p((p1 + rotate_p([r(a2), 0], a2)) * .6, -5);
+            p3 = rotate_p([r(a3), 0], a3);
+            
+            hull_polyline2d(bezier_curve(0.1, 
+                [p1, p2, p3]
+            ), thickness);
+        }
+
+    }
+}
+
+ellipse_extrude(semi_minor_axis, height = height, slices = slices)
+    nautilus_shell(chambered_section_max_angle, steps, thickness);
+
+mirror([0, 0, 1])    
+ellipse_extrude(semi_minor_axis, height = height, slices = slices)
+    nautilus_shell(chambered_section_max_angle, steps, thickness);