half_tube.scad

$fn = 48;      // 圓的邊數
r1 = 15;       // 半徑 1.5 cm
r2 = 15;       // 也就是直徑 3 cm
thickness = 5; // 厚度 0.5 cm
length = 50;   // 長度 5 cm

function __frags(radius) = $fn > 0 ? 
            ($fn >= 3 ? $fn : 3) : 
            max(min(360 / $fa, radius * 6.28318 / $fs), 5);

function shape_ellipse(axes) =
    let(
        frags = __frags(axes[0]),
        step_a = 360 / frags,
        shape_pts = [
            for(a = [0:step_a:360 - step_a]) 
                [axes[0] * cos(a), axes[1] * sin(a)]
        ]
    ) shape_pts;

linear_extrude() difference() {
    polygon(
        shape_ellipse([r1, r2])
    );

    offset(r = -1) polygon(
        shape_ellipse([r1, r2])
    );
    
    translate([0, -r2/2]) square([r1 * 2, r2], center = true);
}