string_tetrahedron.scad

use <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);
}