### mountains

mountains

```// You can find the Turtle API reference here: https://turtletoy.net/syntax
Canvas.setpenopacity(1);
const polygons = Polygons();

// Global code will be evaluated once.
const turtle = new Turtle();
turtle.penup();

const sun = (x, y, r, a, s) => {
const p = polygons.create();
for (let a = 0; a < 2 * Math.PI; a += Math.PI / 36) {
p.addPoints([x + r * Math.cos(a), y + r * Math.sin(a)]);
}
if (s) p.addHatching(a, s);
polygons.draw(turtle, p);
};

const mountains = ( h, a, s ) => {
const p = polygons.create();
let ht = h;
for (let i = -100; i <= 100; i += 10) {
const d = Math.random() > 0.5 ? 1 : -1;
ht += d * (Math.random() * Math.random() * (Math.random() > 0.8 ? 80 : 40));
if (ht > 90) ht = 90;
if (ht < h - 60) ht += Math.random() * 40;
k++;
hts += ht;
}
if (s) p.addHatching(a, s);
polygons.draw(turtle, p);
};

let k = 0, hts = 0;
mountains(90, 0, 0.1);
mountains(60, Math.random() * 2 * Math.PI, 0.6);
mountains(30, Math.random() * 2 * Math.PI, 1);
mountains(0, Math.random() * 2 * Math.PI, 2);
sun(-35, hts / k - 50, 50, 0, 0.1);

////////////////////////////////////////////////////////////////
// reinder's occlusion code parts from "Cubic space division #2"
// Optimizations and code clean-up by ge1doot
////////////////////////////////////////////////////////////////

function Polygons() {
const polygonList = [];
const Polygon = class {
constructor() {
this.cp = [];       // clip path: array of [x,y] pairs
this.dp = [];       // 2d line to draw
this.aabb = [];     // AABB bounding box
}
for (let i = 0; i < points.length; i++) this.cp.push(points[i]);
this.aabb = this.AABB();
}
addOutline(s = 0) {
for (let i = s, l = this.cp.length; i < l; i++) {
this.dp.push(this.cp[i], this.cp[(i + 1) % l]);
}
}
draw(t) {
if (this.dp.length === 0) return;
for (let i = 0, l = this.dp.length; i < l; i+=2) {
const d0 = this.dp[i];
const d1 = this.dp[i + 1];
t.penup();
t.goto(d0);
t.pendown();
t.goto(d1);
}
}
AABB() {
let xmin = 2000;
let xmax = -2000;
let ymin = 2000;
let ymax = -2000;
for (let i = 0, l = this.cp.length; i < l; i++) {
const x = this.cp[i][0];
const y = this.cp[i][1];
if (x < xmin) xmin = x;
if (x > xmax) xmax = x;
if (y < ymin) ymin = y;
if (y > ymax) ymax = y;
}
// Bounding box: center x, center y, half w, half h
return [
(xmin + xmax) * 0.5,
(ymin + ymax) * 0.5,
(xmax - xmin) * 0.5,
(ymax - ymin) * 0.5
];
}
a += Math.PI / 2;
const tp = new Polygon();
const x = this.aabb[0], y = this.aabb[1];
const w = this.aabb[2], h = this.aabb[3];
const l = Math.sqrt((w * 2) ** 2 + (h * 2) ** 2) * 0.5;
tp.cp.push([x - w, y - h], [x + w, y - h], [x + w, y + h], [x - w, y + h]);
const cx = Math.sin(a) * l, cy = Math.cos(a) * l;
let px = x - Math.cos(a) * l;
let py = y - Math.sin(a) * l;
for (let i = 0; i < l * 2; i += d) {
tp.dp.push([px + cx, py - cy], [px - cx, py + cy]);
px += Math.cos(a) * d;
py += Math.sin(a) * d;
}
tp.boolean(this, false);
for (const dp of tp.dp) this.dp.push(dp);
}
inside(p) {
// find number of i ntersection points from p to far away
const p1 = [0.1, -1000];
let int = 0;
for (let i = 0, l = this.cp.length; i < l; i++) {
if ( (p[0]-this.cp[i][0])**2 +  (p[1]-this.cp[i][1])**2 <= 0.001) return false;
if (
this.vec2_find_segment_intersect(
p,
p1,
this.cp[i],
this.cp[(i + 1) % l]
) !== false
) {
int++;
}
}
return int & 1;
}
boolean(p, diff = true) {
// polygon diff algorithm
const ndp = [];
for (let i = 0, l = this.dp.length; i < l; i+=2) {
const ls0 = this.dp[i];
const ls1 = this.dp[i + 1];
// find all intersections with clip path
const int = [];
for (let j = 0, cl = p.cp.length; j < cl; j++) {
const pint = this.vec2_find_segment_intersect(
ls0,
ls1,
p.cp[j],
p.cp[(j + 1) % cl]
);
if (pint !== false) {
int.push(pint);
}
}
if (int.length === 0) {
// 0 intersections, inside or outside?
if (diff === !p.inside(ls0)) {
ndp.push(ls0, ls1);
}
} else {
int.push(ls0, ls1);
// order intersection points on line ls.p1 to ls.p2
const cmpx = ls1[0] - ls0[0];
const cmpy = ls1[1] - ls0[1];
for (let i = 0, len = int.length; i < len; i++) {
let j = i;
const item = int[j];
for (
const db = (item[0] - ls0[0]) * cmpx + (item[1] - ls0[1]) * cmpy;
j > 0 && (int[j - 1][0] - ls0[0]) * cmpx + (int[j - 1][1] - ls0[1]) * cmpy < db;
j--
) int[j] = int[j - 1];
int[j] = item;
}
for (let j = 0; j < int.length - 1; j++) {
if (
(int[j][0] - int[j + 1][0]) ** 2 + (int[j][1] - int[j + 1][1]) ** 2 >= 0.01
) {
if (
diff ===
!p.inside([
(int[j][0] + int[j + 1][0]) / 2,
(int[j][1] + int[j + 1][1]) / 2
])
) {
ndp.push(int[j], int[j + 1]);
}
}
}
}
}
this.dp = ndp;
return this.dp.length > 0;
}
//port of http://paulbourke.net/geometry/pointlineplane/Helpers.cs
vec2_find_segment_intersect(l1p1, l1p2, l2p1, l2p2) {
const d = (l2p2[1] - l2p1[1]) * (l1p2[0] - l1p1[0]) - (l2p2[0] - l2p1[0]) * (l1p2[1] - l1p1[1]);
if (d === 0) return false;
const n_a = (l2p2[0] - l2p1[0]) * (l1p1[1] - l2p1[1]) - (l2p2[1] - l2p1[1]) * (l1p1[0] - l2p1[0]);
const n_b = (l1p2[0] - l1p1[0]) * (l1p1[1] - l2p1[1]) - (l1p2[1] - l1p1[1]) * (l1p1[0] - l2p1[0]);
const ua = n_a / d;
const ub = n_b / d;
if (ua >= 0 && ua <= 1 && ub >= 0 && ub <= 1) {
return [
l1p1[0] + ua * (l1p2[0] - l1p1[0]),
l1p1[1] + ua * (l1p2[1] - l1p1[1])
];
}
return false;
}
};
return {
create() {
return new Polygon();
},
draw(turtle, p) {
let vis = true;
for (let j = 0; j < polygonList.length; j++) {
const p1 = polygonList[j];
// AABB overlapping test - still O(N2) but very fast
if (
Math.abs(p1.aabb[0] - p.aabb[0]) - (p.aabb[2] + p1.aabb[2]) < 0 &&
Math.abs(p1.aabb[1] - p.aabb[1]) - (p.aabb[3] + p1.aabb[3]) < 0
) {
if (p.boolean(p1) === false) {
vis = false;
break;
}
}
}
if (vis) {
p.draw(turtle);
polygonList.push(p);
}
}
};
}
```