constructor(start, end, duration) {
this.start = start;
this.end = end;
this.direction = normalize(sub(this.end, this.start));
this.up = [0, 1, 0];
this.duration = duration || 1.0;
}
ribbonPath.points.forEach(function (_, i) {
let [x, y, z] = ribbonPath.points[i];
let [dx, dy, dz] = ribbonPath.slopes[i];
let v = sub(
normalize([x, y, z]),
normalize(ribbonPath.center),
);
let u = normalize(ribbonPath.slopes[i]);
let w = cross(u, v);
let [a, b, c] = add([x, y, z], scale(w, +0.1));
let [d, e, f] = add([x, y, z], scale(w, -0.1));
ribbon.addPoint([x, y, z], w);
});