add hollow_out_cylinder

examples/hollow_out/hollow_out_cylinder.scad

@@ -0,0 +1,42 @@
+use <hull_polyline3d.scad>;
+use <bend_extrude.scad>;
+use <experimental/tri_delaunay.scad>;
+use <experimental/tri_bisectors.scad>;
+use <experimental/ptf_bend.scad>;
+
+size = [100, 40];
+pt_nums = 20;
+thickness = 1;
+radius = 15;
+fn = 12;
+
+xs = rands(0, size[0], pt_nums);
+ys = rands(0, size[1], pt_nums);
+half_fn = fn / 2;
+dx = size[0] / fn;
+dy = size[1] / half_fn;
+points = concat(
+    [
+        [0, 0], [size[0], 0], 
+        [size[0], size[1]], [0, size[1]]
+    ],
+    [for(i = [1:fn - 1]) [i * dx, 0]],
+    [for(i = [1:fn - 1]) [i * dx, size[1]]],
+    [for(i = [1:half_fn - 1]) [0, dy * i]],
+    [for(i = [1:half_fn - 1]) [size[0], dy * i]],    
+    [for(i = [0:len(xs) - 1]) [xs[i], ys[i]]]
+);   
+
+bisectors = [
+    for(tri = tri_delaunay(points)) 
+    each tri_bisectors([points[tri[0]], points[tri[1]], points[tri[2]]])
+];
+
+for(line = bisectors) {
+    pts = [for(p = line) ptf_bend(size, p, radius, 360)];
+    hull_polyline3d(
+        concat(pts, [pts[0]]), 
+        thickness = thickness,
+        $fn = 4
+    );
+}