$fn = 50; // For smooth curves

// --- Helper Functions ---
function add(a, b) = [a[0] + b[0], a[1] + b[1]];
function sub(a, b) = [a[0] - b[0], a[1] - b[1]];
function scale(a, s) = [a[0] * s, a[1] * s];
function length(a) = sqrt(a[0] * a[0] + a[1] * a[1]);
function normalize(a) = let(n = length(a)) (n == 0 ? [0, 0] : [a[0] / n, a[1] / n]);

// --- Arc Generation ---
function arc(center, r, start_angle, end_angle, segments) =
    [for (i = [0:segments - 1])
        let(a = start_angle + i * (end_angle - start_angle) / (segments - 1))
        [center[0] + r * cos(a), center[1] + r * sin(a)]];

// --- Fillet at a Vertex ---
function fillet(v, v_prev, v_next, wt, seg) =
    let(
        u1 = normalize(sub(v, v_prev)),
        u2 = normalize(sub(v_next, v)),
        n1 = [u1[1], -u1[0]], // Outward normal for incoming edge
        n2 = [u2[1], -u2[0]], // Outward normal for outgoing edge
        T1 = add(v, scale(n1, wt)), // Tangent point on incoming edge
        T2 = add(v, scale(n2, wt)), // Tangent point on outgoing edge
        angle1 = atan2(T1[1] - v[1], T1[0] - v[0]),
        angle2 = atan2(T2[1] - v[1], T2[0] - v[0]),
        angle2_mod = (angle2 < angle1) ? angle2 + 360 : angle2,
        arc_pts = arc(v, wt, angle1, angle2_mod, seg)
    )
    arc_pts;

// --- Fillet Polygon ---
function fillet_polygon(poly, wt, seg) =
    [for (i = [0:len(poly) - 1])
        let(
            v = poly[i],
            v_prev = poly[(i + len(poly) - 1) % len(poly)],
            v_next = poly[(i + 1) % len(poly)],
            arc_pts = fillet(v, v_prev, v_next, wt, seg)
        )
        arc_pts];

// --- Main Function ---
function fillet_polygon_points(points_list1, wt, arc_segments) =
    let(
        rn = fillet_polygon(points_list1, wt, arc_segments)
    )
    [for (sublist = rn, point = sublist) point];

// --- Example Usage ---
points_list1 = [[-10, -10], [30, -5], [10, 10], [-10, 10]]; // Base polygon 
wt = 15; // Fillet (wall thickness) radius at each vertex
arc_segments = 50; // Number of points to generate along each fillet arc

pl2 = fillet_polygon_points(points_list1, wt, arc_segments);
echo(pl2 = pl2);

// Draw the final rounded polygon.
difference() {
    polygon(pl2);
    polygon(points_list1);
}