Lost in the Underworld – Day 1, js13kgames

Alright, I decided to participate for the first time on js13kgames!

The plan is to create a cavern exploration game with a huge world and multiple paths on it, where the player has to find treasures and powerups. I also hope this series of blog posts is more informative and useful than usual.

The idea is to do terrain generation on the fly using Voronoi diagrams and Cellular Automata, also I won’t use pixel art but instead vector graphics (at least for the terrain).

After checking some of the options in the tools page, I decided to use Florent Cailhol’s JS13k Boilerplate. It is probably the simpler starter project there… it has some game loop logic, seedable RNG and gulp scripts for packing and minifying. It also comes with a sample simple implementation for physics.

I started adding keyboard support so you could jump and move around.

const pressed = {};

module.exports = {
	init: function(){
		window.onkeydown = function(e) {pressed[e.keyCode] = true};
		window.onkeyup = function(e) {pressed[e.keyCode] = false};
	isDown: function(keyCode){
		return pressed[keyCode];

Also did some foundations of the “physics” system based on the original, since the terrain will be irregular, what I’m doing for the collisions is checking if the future position of the player will be located inside one of the polygons. I used the “inside” function from James Halliday Point in Polygon repo, an implementation of the PNPOLY algorithm. There’s still a lot of potential optimizations to be done.

  if (e.dx != 0){
    if (e.dx > 0){
      if (inside([tx+e.w,ty+e.h/2], s.vs)){
        hCollision = true;
    } else if (e.dx > 0){
      if (inside([tx,ty+e.h/2], s.vs)){
        hCollision = true;
  if (inside([tx+e.w/2,ty+e.h], s.vs)){
    vCollision = true;

Had to add a special case for high acceleration caused by gravity, which would make the player go though thing barriers because the updated position would just go through it. For this one I’m checking if the line traced from the current position to the future position intersects the polygon, based on a stack overflow response by Dan Fox.

if (e.dy > 100){
  //Prevent falling through thin borders at high acc
  if (polygonIntersects({
    a: {x: e.x+e.w/2, y: e.y+e.h},
    b: {x: tx+e.w/2, y: ty+e.h}},
   vCollision = true;

Added simple camera support, centered on the player, for this I have a camera object whose position I affect along with the player’s. Then I substract that position from the world position of all objects when it’s time to draw them, to translate them to viewport coordinates.


Then I added super simple Voronoi map gen using Raymond Hill’s JavaScript Voronoi, it adds quite a bit of weight to the package but works pretty nice. I may have to slim it down for the final dist.


For now, I’m just randomly picking Voronoi cells, next up will use CA for the caverns to look less random.

var voronoi = new Voronoi();
var bbox = {xl: -300, xr: 1450, yt: 0, yb: 3550};
var sites = [];
for (var i = 0; i < 450; i++){
    x: rand.range(bbox.xl, bbox.xr),
    y: rand.range(bbox.yt, bbox.yb)
var diagram = voronoi.compute(sites, bbox);
const stones = [];
  if (rand.bool())
  let vs = [];
  cell.halfedges.forEach(function (halfedge){
    vs.push([halfedge.getStartpoint().x, halfedge.getStartpoint().y]);
  stones.push({vs: vs});

Navigating around the map made evident that my physics system still needs a lot of work. I will post it here once it’s more polished so it can be useful for someone.

I found out the core of the RNG module of the boilerplate isn’t working, so made it use Math.random instead for now. It also seems the current gulp tasks don’t work well with some ES6 syntax, I’ll take care of that later.

Current package size is: 7.13 KB

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