And then it was gone – Notes about developing my js13k entry of 2020

Start screen of my game.

This was the 3rd time I participated in the js13kGames game jam, so I was already aware of one thing: The size limit isn't my greatest problem, at least not to the extent of having to do code golfing right out of the gate. Instead I should make sure to have a good code base – multiple files for classes and namespaces as needed, named constants/variables instead of hardcoded values.

One really big advantage was, that I had already implemented things like an input system for keyboard and gamepad, and 2D collision detection in other projects of mine. I could just reuse some code with minor adjustments and save quite some time.

1. Improvement through little things

First some ideas on how to improve the general impression of the game, even though the player may not be actively aware of them.

Details to make the scene more alive. Especially animations can play a big role, and having a somewhat self-consistent art style – e. g. having a super detailed character would clash with uni-colored blocks as platforms. Some small details here are:

  • Standing on a platform and pressing down makes the character look down.
  • The character torso slowly bops up and down for breathing.
  • The character blinks.
  • Little dust clouds rise up when jumping.
  • Breaking platforms have little dust clouds.
  • The background moves slower than the foreground, giving a little sense of distance.

Coyote time. Directly after walking of a platform, the player still has a few frames time to jump even though the character isn't on ground anymore.

The game is paused when a gamepad is disconnected. If a gamepad was connected, it may have been used for playing the game. Let's assume it getting disconnected mid-game is an accident.

Pause screen.

Pausing means not doing anything (or not much). If a game is a bit more taxing and makes the CPU or GPU sweat, it's good to have the possibility to give it a break and let the temperature drop down again. If you have a pause option – which on its own is already good – it should truly pause and not keep rendering a complex scene.

I went a bit further. When paused, the game renders the pause screen once and then stops the main loop – no updates, no rendering. The game only keeps listening for key events and does a slower gamepad polling to know when to continue.

2. Watch your (time) step

It did not happen a lot, but some entries did not account for different monitor refresh rates. So when the game was developed for 60 FPS, and it maxed out at 144 FPS on my system, the game ran too fast. In the best case animations just looked weird, but mostly it meant having a hard time with the controls.

The solution is to develop for a certain frame rate and introduce a time factor to account for differences. This factor is then used when updating the physics and animation progresses. In the following example that factor is the variable dt. If the target FPS is 60 and the game runs at 120, dt would be 0.5. Because the update and draw functions are called twice as often as targeted, the progress has to be slowed down by half.

js13k.Renderer = {

	 * Start the main loop. Update logic, render to the canvas.
	 * @param {number} [timestamp = 0]
	mainLoop( timestamp = 0 ) {

		if( timestamp && this.last ) {
			// Time that passed between frames. [ms]
			const timeElapsed = timestamp - this.last;

			// Target speed of 60 FPS (=> 1000 / 60 ~= 16.667 [ms]).
			const dt = timeElapsed / ( 1000 / js13k.TARGET_FPS );

			// update and draw the level …
			this.update( dt );

		this.last = timestamp;

		requestAnimationFrame( t => this.mainLoop( t ) );


I am not saying my solution is the best way to do it. It is short and works well enough, though. An excellent article on the topic is: Fix Your Timestep! by Glenn Fiedler

3. Improving performance

When I started drawing the spikes with lineTo() calls, the performance went noticably down. But since most level objects did not change, they could be cached by pre-rendering them to an invisible canvas. This unchanging canvas is then used in the main loop with drawImage().

 * Render an object to an offscreen canvas.
 * @param  {js13k.LevelObject} o
 * @param  {number}            o.w - Width.
 * @param  {number}            o.h - Height.
 * @return {HTMLCanvasElement}
function toOffscreenCanvas( o ) {
	// This canvas is never added to the document body.
	const canvas = document.createElement( 'canvas' );
	canvas.width = o.w;
	canvas.height = o.h;

	const ctx = canvas.getContext( '2d' );
	ctx.imageSmoothingEnabled = false;
	o.draw( ctx );

	return canvas;

The best part: This pre-rendered image can still be used for the breaking animation. The shaking before breaking apart is just a randomized offset in the drawing position, and breaking it in half means two drawImage() calls – one for the left half and one for the right half, cutting the image in half.

Another optimization: Only render what is currently in the viewport. The levels in my game are long, there is never everything at once on the screen. So before drawing a platform or an effect, there is a check if at least some part of the bounding box is inside the current viewport.

 * Draw an object only if it is inside the visible viewport area.
 * @param {CanvasRenderingContext2d} ctx
 * @param {number}                   vpX - Viewport X offset.
 * @param {number}                   vpW - Viewport width.
 * @param {number}                   vpY - Viewport Y offset.
 * @param {number}                   vpH - Viewport height.
 * @param {js13k.LevelObject}        o
 * @param {number}                   o.x - X position in level.
 * @param {number}                   o.y - Y position in level.
 * @param {number}                   o.w - Width.
 * @param {number}                   o.h - Height.
function drawIfVisible( ctx, vpX, vpW, vpY, vpH, o ) {
		// outside the viewport to the right
		o.x > vpX + vpW ||
		// outside the viewport to the left
		o.x + o.w < vpX
	) {

		// outside the viewport to the bottom
		o.y > vpY + vpH ||
		// outside the viewport to the top
		o.y + o.y < vpY
	) {

	o.draw( ctx );

4. What could have been better

One of my main goals were good controls. Moving and jumping should feel amazing. I did not reach that goal and had to settle with good enough. I even reinstalled Celeste to compare how the character controls felt and what was possible: Could you still change direction mid-air when falling after a jump? (Yes.)

Falling of a block while steering in a direction still looks strange as the character more glides away than falls down.

Hanging on a wall.

One big-ish decision was to allow multiple wall jumps up the same wall. In a way it is a bad design decision: On one hand there is a time limit until the character loses grip and slides down; on the other hand they can just jump up and cling to the same wall higher up again. But it just felt better and didn't make the game that much easier. All levels are still solvable without using this “trick”.

An optimization area I neglected was memory and garbage collection. In Firefox and on a really old laptop I noticed some micro stutters and lost frames, making the scene suddenly skip. In a game about precise platforming this is a little disaster. From what I read here and there, these stutters could be caused by garbage collection. Some untested thoughts on what to improve:

  • Not deleting destroyed platforms, just keep them flagged as such so they are not updated anymore and skipped in rendering.
  • On level begin create a pool of effects – like the dust clouds from jumping – and reuse them instead of creating them when the jump happens.

The second level goes vertically upwards. When the player is already at a higher checkpoint and falls down, it is better for them to not land on a previous checkpoint and instead directly fall to their death. Otherwise they are back at this previous checkpoint. That is just frustrating. Older checkpoints should not overwrite later ones.

Player character.

The voting for this year's competition is still under way. Articles by other participants can be found on


js13kGames: Tricks applied in Risky Nav

Risky Nav

From the 13th August to the 13th September I participated in the js13kGames competition. My entry Risky Nav can be seen here and the source code is on GitHub here. In this blog post I will explain some of the tricks and techniques I used in making my game.

The game is tile based, so everything – player, monsters, goal – is always positioned at a (x, y) position on a 2D map.

About the background

The background is a single image which is created once right at the beginning. It is drawn on a canvas and each tile is 1px big. In the rendering loop it is then up-scaled to the desired tile size. To avoid a blurry image, it is necessary to disable anti-aliasing.

context.imageSmoothingEnabled = false;

let w = bgCanvas.width * tileWidth;
let h = bgCanvas.height * tileHeight;

function renderLoop() {
    context.drawImage( bgCanvas, 0, 0, w, h );

About the fog/shadow

The fog/shadow around the player is done in a similar way as the background. The image is pre-rendered with each tile being 1px and then up-scaled in the main loop. But it moves with the player. The darkness is determined by the euclidean distance from the player.

for( let y = 0; y < fogMapHeight; y++ ) {
    for( let x = 0; x < fogMapWidth; x++ ) {
        // Euclidean distance from origin.
        let de = Math.sqrt( x * x + y * y );

        // Darkness only starts 2 tiles away from the player.
        // f has to be a value between 0 and 1.
        let f = ( de < 2 ) ? 0 : Math.min( 1.15 - Math.min( 3 / de, 1 ), 1 );
        fogCtx.fillStyle = `rgba(0,0,0,${f})`;
        fogCtx.fillRect( x, y, 1, 1 );
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JavaScript source protection with NW.js

You can minify and uglify JavaScript files, but technically the source code of your distributed NW.js application is still readable. But NW.js also provides the means to compile JavaScript to a binary file and then load it as part of the application. The command line tool nwjc to create the binary file is included in the SDK version.

Assuming you have a JavaScript file js/private.js:

'use strict';

function secretFunction( foo ) {
    return foo * 4;

Then you can compile it like this to a file js/private.bin:

$ ./nwjs-sdk-v0.30.5-linux-x64/nwjc js/private.js js/private.bin

Internally the tool uses the V8 snapshot feature, which means the versions have to match. A binary file created with NW.js 0.30 can only be loaded by 0.30. Binary files also do not work cross-platform. For each platform it is necessary to compile its own binary file with the SDK for the same platform.

To then load the binary file in your application, it works like this:

let win = nw.Window.get();
win.evalNWBin( null, 'js/private.bin' );

let value = secretFunction( 4 ); // returns 16

Note however that the loading is per window. If you open another window in your application, the file has to be loaded there again.

Using the DevTools you can of course find the functions and variables which have been loaded from the file. The function implementation however is protected:

> String( secretFunction )
< "function secretFunction() { [native code] }"

DevTools issues

Update 2018-12-15: Since NW.js 0.34 this issue seems to be fixed. Loading binary files works even with the DevTools open.

There is an issue with loading binary files and the DevTools. Basically you cannot have the DevTools open and then load the file. There will be no error, but the contents will not be available. This is a known issue.

My temporary solution is to just close the DevTools. But just closing them right before is not enough, you also have to use a timeout before loading the file:

let win = nw.Window.get();

// Function is only available in the SDK build.
if( typeof win.closeDevTools === 'function' ) {

setTimeout( () => {
    win.evalNWBin( null, 'js/private.bin' );
}, 500 );

But why not check first if the DevTools are open? Then you could open them again afterwards. According to the API documentation there is win.isDevToolsOpen(). But it exists only in the documentation. Using the SDK build there is de facto no such function. This too is a known issue.

Wine for Windows

I successfully used Wine 3 to compile a binary file for the Windows version of a NW.js application and then load it there. So if you are on Linux or macOS you will not need Windows for your build process. You should of course still test your application to make sure it works on all targeted platforms.

Using NW.js to communicate with a DS4 controller

DS4 green light

NW.js still provides the Chrome Apps API which has been removed from Chrome, but not ChromeOS. This will allow us to access in a platform-independant manner devices which are connected with the PC per USB.

Without this API, a 3rd party Node.js module like node-hid could be used. This will however come with platform-dependant libraries and will have to be updated or rebuild each time the Node.js version changes.

This article concentrates on sending data to the controller. However it is also possible to retrieve data like pressed buttons using the established connection. Aside from using chrome.hid there is also the Gamepad API for read-only access.

Identifying the controller

First we need a way to identify the DS4. Devices come with a vendor Id and product Id. According to the Gentoo Wiki they are as follows:

Device Vendor Id Product Id
DS4 (1st gen) hex 054C / dec 1356 hex 05C4 / dec 1476
DS4 (2nd gen) hex 054C / dec 1356 hex 09CC / dec 2508

Having tested with both devices, I can also confirm the Ids.

Get the device

For all communication with the device, we will use the chrome.hid API. First we define a filter using the vendor and product Id, and then query the available devices:

var filter = {
    filter: [
        { vendorId: 1356, productId: 1476 },
        { vendorId: 1356, productId: 2508 }

chrome.hid.getDevices( filter, ( devices ) => {
    // Error handling.
    if( chrome.runtime.lastError ) {
        console.error( chrome.runtime.lastError );
    if( !devices ) {

    var device = devices[0];
    // Next: Connect to the device.
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Standalone-Anwendungen mit node-webkit

node-webkit demo window

Gestern habe ich ein wenig in node-webkit reingeschnuppert. Damit sollen sich ganz einfach Desktop-Anwendungen auf Basis von HTML, JavaScript und Node.js erstellen lassen. Die erstellte Anwendung lässt sich dann relativ einfach für verschiedene Betriebs­systeme verpacken. Für nicht allzu rechen-intensive Spiele scheint mir das recht interessant. Tatsächlich verwendet auch das kürzlich erschiene A Wizard's Lizard node-webkit [1].

Für eine kleine Demo habe ich das Beispiel von three.js genommen und als Anwendung verpackt. Das HTML und JavaScript lasse ich hier mal aus. Mein package.json für ein Fenster ohne Toolbar sieht wie folgt aus:

	"main": "index.html",
	"name": "nw-demo",
	"window": {
		"frame": true,
		"height": 600,
		"kiosk": false,
		"toolbar": false,
		"width": 900
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