1's angle approx or recalculate
                const p_prev = this.points[i-1];
                const angle = Math.atan2(p1.y - p_prev.y, p1.x - p_prev.x); // Angle from prev to current
                const dxBottom = -Math.sin(angle) * this.thickness / 2;
                const dyBottom = Math.cos(angle) * this.thickness / 2;
                ctx.lineTo(p1.x + dxBottom, p1.y + dyBottom);
            }
             ctx.closePath();

            const [r,g,b] = hslToRgb(this.hueStart, 0.7, 0.6); // (饱和�