From 4c7ef79c17f1bc29c95a4254cc469a9d300c2121 Mon Sep 17 00:00:00 2001
From: Justin Lin <caterpillar@openhome.cc>
Date: Thu, 19 Sep 2019 10:10:32 +0800
Subject: [PATCH] add string_tetrahedron

---
 examples/string_tetrahedron.scad | 94 ++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 examples/string_tetrahedron.scad

diff --git a/examples/string_tetrahedron.scad b/examples/string_tetrahedron.scad
new file mode 100644
index 00000000..c3fe7a4e
--- /dev/null
+++ b/examples/string_tetrahedron.scad
@@ -0,0 +1,94 @@
+include <hull_polyline3d.scad>;
+
+leng = 50;
+thickness = 5;
+segs_per_side = 20;
+line_fn = 5;
+model = "Tetrahedron"; // [Tetrahedron, Base, Both]
+
+module lines_between(side1, side2, thickness, segs) {
+    function pts(p1, p2, segs) =
+        let(
+             p = p2 - p1,
+             dx = p[0] / segs,
+             dy = p[1] / segs,
+             dz = p[2] / segs
+        ) [for(i = [0:segs]) p1 + [dx, dy, dz] * i];
+
+    pts1 = pts(side1[0], side1[1], segs);
+    pts2 = pts(side2[0], side2[1], segs);
+    
+    for(i = [0:len(pts1) - 1]) {
+        hull_polyline3d(points = [pts1[i], pts2[i]], thickness = thickness);
+    }
+}
+
+function height(leng) = 
+    leng * sqrt(1 - 4 / 9 * pow(sin(60), 2));
+
+function vts(leng) = 
+    let(
+        half_leng = leng / 2,
+        vt1 = [half_leng, 0, 0],
+        vt2 = [0, leng * sin(60), 0],
+        vt3 = [-half_leng, 0, 0],
+        vt4 = [0, half_leng * tan(30), height(leng)]
+    ) [vt1, vt2, vt3, vt4];
+
+module string_tetrahedron(leng, thickness, segs_per_side, line_fn) {
+    $fn = line_fn;
+    
+    half_leng = leng / 2;
+    
+    vts = vts(leng);
+
+    vt1 = vts[0];
+    vt2 = vts[1];
+    vt3 = vts[2];
+    vt4 = vts[3];
+
+    lines_between([vt1, vt2], [vt3, vt4], thickness, segs_per_side);
+    lines_between([vt2, vt3], [vt1, vt4], thickness, segs_per_side);
+    lines_between([vt3, vt1], [vt2, vt4], thickness, segs_per_side);
+}
+
+module base(leng, thickness, line_fn) {
+    vts = vts(leng);
+    r = leng / 4;
+    half_th = thickness / 2;
+
+    difference() {
+        sphere(r, $fn = 48);
+        
+        translate([0, 0, -r])
+            linear_extrude(r) 
+                square(r * 2, center = true);
+        
+        translate([0, 0, height(leng) + half_th])
+             rotate([0, 180, 0]) 
+                 translate([0, -leng / 2 * tan(30), 0]) 
+                     hull() {
+                        translate(vts[0])
+                            sphere(half_th, $fn = line_fn);
+                        translate(vts[1])
+                            sphere(half_th, $fn = line_fn);
+                        translate(vts[2])
+                            sphere(half_th, $fn = line_fn);
+                        translate(vts[3])
+                            sphere(half_th, $fn = line_fn);     
+                     }
+    }
+}
+
+if(model == "Tetrahedron") {
+    string_tetrahedron(leng, thickness, segs_per_side, line_fn);
+} else if(model == "Base") {
+    base(leng, thickness, line_fn);
+} else {
+    translate([0, 0, height(leng) + half_th])
+         rotate([0, 180, 0]) 
+             translate([0, -leng / 2 * tan(30), 0]) 
+                 string_tetrahedron(leng, thickness, segs_per_side, line_fn);
+    base(leng, thickness, line_fn);
+}
+