add simple_seashell

examples/simple_seashell.scad

@@ -0,0 +1,31 @@
+include <circle_path.scad>;
+include <rotate_p.scad>;
+include <polysections.scad>;
+
+r1 = 15;
+r2 = 50;
+
+a1 = 1;
+a2 = 450;
+
+steps = 40;
+
+module simple_seashell(r1, r2, a1, a2, steps) {
+    rd = (r2 - r1) / steps;
+    ad = (a2 - a1) / steps;
+
+    sections = [
+        for (i = [0:steps])
+        let(
+            r = r1 + rd * i,
+            a = a1 + i * ad
+        )
+            [for(p = concat(circle_path(r), circle_path(r * 0.9))) 
+                rotate_p([p[0], p[1], 0] + [r, 0, 0], [0, a, 0])]    
+            
+    ];
+
+    rotate([90, 0, 0]) polysections(sections, "HOLLOW");
+}
+
+simple_seashell(r1, r2, a1, a2, steps);