add hollow_out_holder

examples/hollow_out_holder.scad

@@ -0,0 +1,57 @@
+use <hull_polyline3d.scad>;
+use <util/rand.scad>;
+use <experimental/tri_bisectors.scad>;
+use <experimental/tf_bend.scad>;
+
+width = 5;
+columns = 30;
+rows = 15;
+radius = 30;
+angle = 360;
+thickness = 2;
+
+function h_lines_in_square(width) = 
+    let(
+        w = width / 2,
+        i = ceil(rand() * 10) % 2,
+        tris = i == 0 ? 
+                [
+                    [[0, 0], [width, 0], [width, width]],
+                    [[0, 0], [width, width], [0, width]]
+                ] 
+                :
+                [
+                    [[width, 0], [0, width], [0, 0]],
+                    [[width, 0], [width, width], [0, width]]
+                ]
+    )
+    concat(tri_bisectors(tris[0]), tri_bisectors(tris[1]));
+
+function hollow_out_square(size, width) =
+    let(
+        columns = size[0],
+        rows = size[1]
+    )
+    [
+        for(y = [0:width:width * rows - width])
+            for(x = [0:width:width * columns - width])
+                for(line = h_lines_in_square(width)) 
+                    [for(p = line) p + [x, y]] 
+    ];
+    
+lines = concat(
+    hollow_out_square([columns, rows], width),
+    [[
+        for(x = [0:width:width * columns]) [x, rows * width]
+    ]]
+);
+
+for(line = lines) {  
+   transformed = [for(pt = line) tf_bend([columns * width, rows * width], pt, radius, angle)];
+   hull_polyline3d(transformed, thickness, $fn = 4);
+}
+
+translate([0, 0, -thickness / 2])
+linear_extrude(thickness)
+rotate(180 / columns)
+    circle(radius + thickness / 2, $fn = columns);