Lockstep multiplayer first steps

In this Gamasutra article, Tundra developers explain their approach to minimize problems when developing a lockstep multiplayer platform used for their game Rapture - World Conquest. Based on that, my first approach was to create and understand a deterministic lockstep logic without even considering networking yet, to focus on one problem at a time.

My objective is to have a reproducible game that given the start state and all the players actions during time, the game can be played again and the final state will be the same.

Lockstep logic

The first step was start by creating a logic to encapsulate the fixed game state for physics and game logic, and a lockstep to perform player actions. As I started without considering networking yet, all the player actions are directly enqueued locally by the user input or by a saved replay.

A lockstep logic means that there are some conditions that, if not met, the game should pause the simulation and wait for them. In the case of multiplayer games this is when the game have to wait for other players actions that didn't arrive in time (and where the waiting for other players dialog is shown in some games).

My code looks something like this pseudo code:

update(dt) {
    if (lockstepLogic.IsLockstepTurn()) {
        if (!lockstepLogic.IsReady())
    // normal accumulator logic for the fixed gamestep

Since I create all player actions locally, the lockstep never happens right now but it is a good practice to simulate and test it anyways.

Testing it

My first prototype using this logic is a box that moves over the screen. When the right mouse button is pressed, the game enqueues a player action to move the box to the specified position, then when the lockstep logic is processed the box receives the command and start moving to that position.


The moving logic was pretty simple, based on start and destination positions and a speed, the box moves itself to the destination in a straight line. Since that logic is performed in each fixed gamestep, the player see the box jumping between positions, it works but it doesn't look so good. To improve that and make the movement smoother I created a simple interpolation code.

Interpolation depends a lot on what you are trying to interpolate, in some cases could be a simple as the code I used there but there are other cases like the bouncing ball which need more data to create a better interpolation.

Note: adding interpolation means we have now a delay of one fixed gamestep between the box position being rendered and the real position in the game since we are using previous and current positions info for the interpolation.

My first replays

By recording all the player actions with the fixed gamestep frame they were executed, I created a basic way to replay the "game" by just reseting the game state to the initial state and start enqueing all the recorded actions in each corresponding frame.

Here is a video of the progress so far:

Yeah, I know, it's not a great thing, but at least I am starting to understand some of the challenges of making multiplayer games.

Validating simulation

To validate the game is always performing the same simulation given the same input, I added a checksum calculation based on the game state (for now just the moving box state) which is saved from time to time to use later as validation when simulating the game again. The idea was to start defining an API to get the important game state to consider when validating the simulation, and also to start testing game state validation. The code looks like this:

update(frame) {
    if (IsChecksumFrame(frame)) {
        if (recording) {
            checksumRecorder.RecordState(frame, CalculateChecksum());
        } else {
            if (!checksumValidator.IsValid(frame, 
                   checksumRecorder.SavedChecksums, CalculateChecksum())) {
               throw Exception("current game state is not valid!");

In my first tests I wasn't reseting the game to the initial state properly so my game was producing different checksums when reproducing the saved player actions, checksums validation started to work after that was fixed.

How am I calculating the Checksum? Right now I am not sure exactly what algorithm to use nor which game state should I consider for the checksum, so what I did was to encapsulate that in some interfaces and implemented a basic way to get both. For the checksum I am using a simple MD5 over a string, and for the game state, well..., a string with all important values concatenated (the moving box position, destination, if it is moving or not, etc).

string CalculateChecksum() {
    string gameStateValue = game.GetGameState();
    return MD5.CalculateHash(gameStateValue);

string GetGameState() {
    string gameState = "";
    foreach (object in gameObjects)
        gameState += object.GetGameState();
    return gameState;

For a future implementation what I know is that the game state should be composed with important values that can affect the simulation, so I shouldn't care about about audiovisual stuff like particles, effects and sounds.

Also, the game state concept could be used to reset to the initial game state or even for saved game states (to easily replicate some bug for example), and finally, it could be used to synchronize and validate state if for some reason I end up using a client/server architecture.

Next frontier: Determinism?

For now I didn't explore determinism realm because the solution really depends on the game logic but at the same time I must have it clear before starting the game code. One of the next steps is probably start testing with fixed point math, not sure yet, the idea is try to follow an approach similar to the gamasutra article's of reducing non determinism problem to the minimum before going multiplayer.

If you want to take a look all the code used for this blog post, here is the link.

Other links

Example of a dynamic lockstep implementation for Unity

Lockstep Framework for Unity

Reddit post about that Lockstep Framework

Another framework named lockstep.io

Developing your own Replay System

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Exploring Remote Multiplayer

Some time ago I’ve started to prototype a multiplayer game, it is some kind of super simplified RTS game for mobile devices where macro decisions are encapsulated into one action through a button.

First versions were prototyped in a multiplayer hot seat fashion by using only one device (a tablet or a phone), that allowed me to quickly iterate between different ideas and find fun as soon as possible. That was a successful approach since the idea of the game proved that it was indeed fun to play with friends, so the next step was to go remote multiplayer with two or more devices.

Since networking is a whole new world for me, I started by reading lots of different articles about making multiplayer games.

The first thing to know is that the common solutions around depend on each kind of game, and knowing that beforehand helps you deciding the best solution for your game, and how your game must be adapted to support it.

In my case, I am developing a really simplified version of a RTS game for mobile, similar to Clash Royale. It is a 1v1 game where each player controls multiple units by giving high level orders like “everyone attack!” or “build new supply depot”.

The common networking approach for this kind of games, where synchronizing the whole world is a bit heavy given there are tons of units, is to do a synchronized simulation where only the main actions are transmitted (to save bandwidth) and each machine/device perform the same simulation. This implies the game should be deterministic in order to avoid desynchronization between players, so given one start state of the game and a list of actions over time, the final state should be always the same.

Making a game deterministic is a really hard problem, but it also has its rewards, like being able to replay the game given the actions of each player over time. This is a great debug tool for developers and at the same time a great freature for players since it is used to see other strategies or to share a great victory with your friends, everything with almost no storage cost.

The idea is to start writing my findings in the remote multiplayer journey to share all the lessons learned.


These are some of the articles I read:

What every programmer needs to know about game networking

1500 Archers on a 28.8: Network Programming in Age of Empires and Beyond

Fast-Paced Multiplayer

Why adding multiplayer makes game coding twice as much work

Core network structures for games

Making Fast-Paced Multiplayer Networked Games is Hard

How to create an online multiplayer game with Unity

Networking in Unity

Creating a Cross-Platform Multiplayer Game in Unity

Understanding Fighting Game Networking

Desyncs and FPU synchronization

Cross platform RTS synchronization and floating point indeterminism

Minimizing the Pain of Lockstep Multiplayer

The Tech of Planetary Annihilation: ChronoCam

Synchronous RTS Engines and a Tale of Desyncs

Don’t Store That in a Float

Developing your own Replay System

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How Google Play Game Services softly killed Vampire Runner

We want to talk a bit about how we migrated from our custom highscores system to use Google Play Game Services (GPGS) Leaderboards for Vampire Runner and the consequences of that change.

Before GPGS was released, we worked on our custom solution for highscores for Vampire Runner. That solution consisted in having scores for multiple time intervals (daily, weekly, monthly and all times scores) and letting guest users play and make scores without the need to log in. You could play the game, make scores and then, if you thought your scores were good, link those scores with your name, really simple and easy, it worked fine.

That custom solution was running on an amazon server and costed about 15 USD/month and since the game was making around that money with the ads, we thought it was acceptable.

A while after GPGS was released, we thought, why maintain and pay money for our own server when we can use GPGS instead? It was very logic, they have some of the features we had on our system and everybody was starting to use it. Also, we trust Google, so we changed Vampire Runner to use GPGS.

However, some time after we changed Vampire Runner, we started to see an important decrease in the number of people playing the game. The problem was something we were aware of but we thought it wouldn't be so significant. We had a lot of players that didn't have a Google account or even Google Play (they probably downloaded the game from other store), and they couldn't log in to compete with other players so the game was less interesting for them since part of our game is based on that, competing to be better than other players.

The truth is the game is not a good game by itself and the main thing that kept our players was being able to compete with others (for a while at least), now that we are forcing them to log in they feel it isn't worth it so they just go away or they play without logging in, hence without making scores.


We don't think Google Play Game Services is the bad guy here but it is not good if you want to capture users from outside Google, same example would apply if we had forced Facebook or Twitter accounts. So if you have a game where you need players to catch other players, consider that. Of course, if you have a great game maybe with only Facebook, Google or Twitter you could keep your game alive but if you consider the three of them, you will get more players. In our case, we had all the world and then we restricted our game to only G+ accounts and that is why we lost players.

That was part of the story of the dying Vampire Runner, hope you like it.

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Did you miss us?

Hi guys, long time no see.

It's been a while since we came here and write, and we want to share why.

What happened

By the end of 2012, after we finished Clash of the Olympians to Android and iOS working hand in hand with Ironhide Game Studio, we had no ideas to work on. We weren't really excited in working by ourselves anymore because we didn't feel we could create something of the quality of Clash. Also, we really enjoyed working with Ironhide's team. In addition, Ironhide was looking for programmers and they told us they enjoyed working with us as well, so they offered us a job and we couldn't refuse (and we are still glad we did not).

Working at Ironhide

Two words: Indescribable awesomeness.

We are part of an excellent and diverse team composed by artists, developers, game designers and more. The workplace is comfy and the game development workflow is pretty dynamic. We can participate in a lot of game decisions and that rocks!

Working really close with artists allowed us to validate how much we needed one at Gemserk to define ideas and visualize, in an easy way, how our games should look like. It also works as motivation since you really want to see that art in the game.

Game design was another flaw at Gemserk, and we now see and understand it much better.

We learn new stuff each day and we also collaborate and share our knowledge to make the game and the team better. And at the end of the day, that feels really good. We hope it lasts long.

What about Gemserk

As company, Gemserk is kinda dead. We never reached what we wanted with our creations, and in those terms, we weren't successful. However, we don't see Gemserk as a failure either since we accomplished a lot in terms of technical knowledge and relationship with other game developers and tool makers and we are really happy with that.

We have no plans to work on anything Gemserk related for now but we believe we still have some stuff to share.

Thanks for reading our blog and hope we could still write interesting posts in the future.

The Gemserk team.

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Our solution to handle multiple screen sizes in Android – Part three

In the previous posts of this series we talked about our solution to handle multiple screen sizes for game menus, in particular we showed the main menu of the game Clash of the Olympians. In this post we are going to talk about what we did inside the game itself. As a side note, the solution we used here is simple and specific for this game, hope it could help as example but don't expect a silver bullet.

Scaling to match the physics world

As we use Box2D in Clash of the Olympians, the first step was to use a proper scale between Box2D bodies and our assets. The basic approach was to consider that 1m (meter in MKS system) was 32px, so in our target resolution of 800x480 could show 25m x 15m. We picked that scale because it gives pretty numbers both in terms of the game area and in terms of our assets, for example, a character of 64px of height is 2m tall. In particular, Achilles has a height of approx 60px which is equivalent to 1.875m using our scale, that sounds pretty reasonable for that character.

The image shows the relation between screen size in pixels (800x480 in this case) and the game world in meters.

Defining a virtual area to show

We previously said that we could show 25m x 15m, in fact, the height is not so important in Clash of the Olympians since the game mainly depends in the horizontal distance. So, if we had an imaginary with a resolution of 800x400 (really wide, an aspect ratio of 2) we would show in that case 12.5m of height, we could assume that if we show at least that height the game balance would be not affected at all (enemies are never spawned too high). However, in terms of horizontal distance we want to show always the same area across all devices to avoid changing the game balance (for example, if you could see less area you couldn't react in the proper time to some waves), that is why we decided to show always 25m in terms of width.


The image shows how we still show the same game world width of 25m on a 800x600 device.

Scaling the world back to match the screen size

Finally, in order to show this virtual area of 25m x H (with H >= 12.5m), we have to calculate the proper scale to set our game camera in each device. For example, in the case of having a Nexus 7 (1280x720 resolution device) the scale to show 25m of horizontal size is 51.2x since we know that 1280 / scale = 25, then 1280 / 25 = 51.2. In the case of a Samsung Galaxy Y (480x320 resolution device) the scale would be 19.2x since 480 / 25 = 19.2. Translating this inside the game would be something as easy as:

camera.scale = screen.width / 25

Final thoughts

This is not a general solution, it depends a lot in the game we were making and the things we could assume like the game height doesn't matter.

Even though the solution is specific and not so cool as the previous posts, we hope it could be of help when making your own game.

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Our resources manager and how it helped in localizing Clash of the Olympians

In this post we want to share how our resources manager implementation helped us when we had to localize Clash of the Olympians.

Our resources manager

Some time ago we started a small java project named jresourcesmanager (yeah, the most creative name in the world) which provides a simple abstraction of what a resource is and how it is loaded. It consists in some basic concepts named Resource, DataLoader and ResourceManager.


Is the concept of an application resource/asset, and provides an API to know if the resource is loaded or not and to load and unload it. Here is the code:

public class Resource<T> {

	T data = null;

	DataLoader<T> dataLoader;

	protected Resource(DataLoader<T> dataLoader) {
		this(dataLoader, true);

	protected Resource(DataLoader<T> dataLoader, boolean deferred) {
		this.dataLoader = dataLoader;
		if (!deferred)

	 * Returns the data stored by the Resource.
	public T get() {
		if (!isLoaded())
		return data;
	public void set(T data) {
		this.data = data;

	public DataLoader<T> getDataLoader() {
		return dataLoader;
	public void setDataLoader(DataLoader<T> dataLoader) {
		this.dataLoader = dataLoader;

	 * Reloads the internal data by calling unload and load().
	public void reload() {

	 * Loads the data if it wasn't loaded yet, it does nothing otherwise.
	public void load() {
		if (!isLoaded())
			data = dataLoader.load();

	 * Unloads the data by calling the DataLoader.unload(t) method.
	public void unload() {
		if (isLoaded()) {
			data = null;

	 * Returns true if the data is loaded, false otherwise.
	public boolean isLoaded() {
		return data != null;

	public Resource<T> clone() {
		return new Resource<T>(dataLoader);



Provides an API to define a way to load and unload a Resource, here is the code:

public abstract class DataLoader<T> {

	 * Implements how to load the data.
	public abstract T load();

	 * Implements how to unload the data, if it is nod automatic and you need to unload stuff by hand.
	public void unload(T t) {


	 * Provides a way to return custom information about the data loader.
	 * @return An object with the custom information.
	public Object getMetaData() {
		return null;



It is just a map with all the resources identified by a key, which can be a String for example, and an API to store and get resources to and from, respectively.

That is our resources management code, it is really simple and fulfilled our needs in assets loading/unloading for Vampire Runner and Clash of the Olympians.

The interesting part of this library is that it allows you to store whatever you want to consider as a resource/asset, and that feature is what we used in order to solve the localization issue.

Declaring resources

Another important pillar is how we declare resources in an easy way through the code, and that is by using some builders that simplified the resource declaration. As we were using LibGDX library, we created a resource builder for LibGDX assets and more. This resource builder allowed us to do stuff like this:

splitLoadingTextureAtlas("MainTextureAtlas", "data/images/screens/mainmenu/pack");
resource("MainBackgroundTop", sprite2().textureAtlas("MainTextureAtlas", "mainmenu-bg", 1).center(0.5f, 0.5f).trySpriteAtlas());

That declares a texture atlas with the name of MainTextureAtlas and then a resource which is a sprite with the name of MainBackgroundTop from a texture atlas resource identified by the previous name. Also, allow us to do stuff like declare that the sprite is centered in the middle (the anchor point) and more.

This code is not important, just an example of some of our builders.

Declaring localized resources

As we have the power to create complex resource builders and we needed a simple way to switch between assets given the language selected, we created a resource builder which allowed us to declare different resources, depending the current locale, using the same identifier. So, for example, we can do something like this:

resource("CreditsButton", new MultilanguageResourceBuilder<Sprite>() //
	.defaultLocale(new Locale("en")) //
	.resource(new Locale("en"), sprite2().textureAtlas(TextureAtlases.MeinMenu, "mainmenu-but-credits-en", 1).trySpriteAtlas()) //
	.resource(new Locale("es"), sprite2().textureAtlas(TextureAtlases.MeinMenu, "mainmenu-but-credits-es", 1).trySpriteAtlas()) //

That declares a resource named CreditsButton which returns a sprite with index 1 and name “mainmenu-but-credits-en” from a texture atlas in case the locale is English and a sprite with index 1 and name “mainmenu-but-credits-es” in case the locale is Spanish. It also declares that the default locale (in case a resource for the current locale wasn’t found) is English.

This resource builder was very handy because it allowed us to declare any type of resource for different locales, and from the application side it was transparent, we just ask for the resource CreditsButton.

In case you are interested, the code of the MultilanguageResoruceBuilder is:

public class MultilanguageResourceBuilder<T> implements ResourceBuilder<T> {

	private Map<Locale, ResourceBuilder<T>> resourceBuilders = new HashMap<Locale, ResourceBuilder<T>>();
	private Locale defaultLocale;

	public boolean isVolatile() {
		if (defaultLocale == null)
			throw new IllegalStateException("Multilanguage resource builder needs a default locale");
		ResourceBuilder<T> resourceBuilder = resourceBuilders.get(defaultLocale);
		if (resourceBuilder == null)
			throw new IllegalStateException("Multilanguage resource builder needs a default resource builder");
		return resourceBuilder.isVolatile();

	public MultilanguageResourceBuilder<T> defaultLocale(Locale locale) {
		this.defaultLocale = locale;
		return this;

	public MultilanguageResourceBuilder<T> resource(Locale locale, ResourceBuilder<T> resourceBuilder) {
		resourceBuilders.put(locale, resourceBuilder);
		return this;

	public T build() {
		Locale locale = Locale.getDefault();

		if (defaultLocale == null)
			throw new IllegalStateException("Multilanguage resource builder needs a default locale");

		if (!resourceBuilders.containsKey(locale))
			locale = defaultLocale;

		ResourceBuilder<T> resourceBuilder = resourceBuilders.get(locale);

		if (resourceBuilder == null)
			throw new IllegalStateException("Multilanguage resource builder needs a default resource builder");

		return resourceBuilder.build();



That was the way we used to support multiple languages in a transparent way for the application, we just need to change the current locale and reload the assets.

Hope this blog post idea helps you in case you are about to support multiple languages in your game, and see you next time.

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Our solution to handle multiple screen sizes in Android – Part two

Continuing with the previous blog post, in this post we are going to talk about the code behind the theory. It consists in three concepts, the VirtualViewport, the OrthographicCameraWithVirtualViewport and the MultipleVirtualViewportBuilder.


It defines a virtual area where the game stuff is contained and provides a way to get the real width and height to use with a camera in order to always show the virtual area. Here is the code of this class:

public class VirtualViewport {

	float virtualWidth;
	float virtualHeight;

	public float getVirtualWidth() {
		return virtualWidth;

	public float getVirtualHeight() {
		return virtualHeight;

	public VirtualViewport(float virtualWidth, float virtualHeight) {
		this(virtualWidth, virtualHeight, false);

	public VirtualViewport(float virtualWidth, float virtualHeight, boolean shrink) {
		this.virtualWidth = virtualWidth;
		this.virtualHeight = virtualHeight;

	public float getWidth() {
		return getWidth(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());

	public float getHeight() {
		return getHeight(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());

	 * Returns the view port width to let all the virtual view port to be shown on the screen.
	 * @param screenWidth
	 *            The screen width.
	 * @param screenHeight
	 *            The screen Height.
	public float getWidth(float screenWidth, float screenHeight) {
		float virtualAspect = virtualWidth / virtualHeight;
		float aspect = screenWidth / screenHeight;
		if (aspect > virtualAspect || (Math.abs(aspect - virtualAspect) < 0.01f)) {
			return virtualHeight * aspect;
		} else {
			return virtualWidth;

	 * Returns the view port height to let all the virtual view port to be shown on the screen.
	 * @param screenWidth
	 *            The screen width.
	 * @param screenHeight
	 *            The screen Height.
	public float getHeight(float screenWidth, float screenHeight) {
		float virtualAspect = virtualWidth / virtualHeight;
		float aspect = screenWidth / screenHeight;
		if (aspect > virtualAspect || (Math.abs(aspect - virtualAspect) < 0.01f)) {
			return virtualHeight;
		} else {
			return virtualWidth / aspect;


So, if we have a virtual area of 640x480 and want to show it on a screen of 800x480 we can do the next steps in order to get the proper values that we have to use as the camera viewport for that screen:

VirtualViewport virtualViewport = new VirtualViewport(640, 480);
float realViewportWidth = virtualViewport.getWidth(800, 480);
float realViewportHeight = virtualViewport.getHeight(800, 480);
// now set the camera viewport values
camera.setViewportFor(realViewportWidth, realViewportHeight);


In order to simplify the work when using LibGDX library, we created a subclass of LibGDX's OrthographicCamera with specific behavior to update the camera viewport using the VirtualViewport values. Here is its code:

public class OrthographicCameraWithVirtualViewport extends OrthographicCamera {

	Vector3 tmp = new Vector3();
	Vector2 origin = new Vector2();
	VirtualViewport virtualViewport;
	public void setVirtualViewport(VirtualViewport virtualViewport) {
		this.virtualViewport = virtualViewport;

	public OrthographicCameraWithVirtualViewport(VirtualViewport virtualViewport) {
		this(virtualViewport, 0f, 0f);

	public OrthographicCameraWithVirtualViewport(VirtualViewport virtualViewport, float cx, float cy) {
		this.virtualViewport = virtualViewport;
		this.origin.set(cx, cy);

	public void setPosition(float x, float y) {
		position.set(x - viewportWidth * origin.x, y - viewportHeight * origin.y, 0f);

	public void update() {
		float left = zoom * -viewportWidth / 2 + virtualViewport.getVirtualWidth() * origin.x;
		float right = zoom * viewportWidth / 2 + virtualViewport.getVirtualWidth() * origin.x;
		float top = zoom * viewportHeight / 2 + virtualViewport.getVirtualHeight() * origin.y;
		float bottom = zoom * -viewportHeight / 2 + virtualViewport.getVirtualHeight() * origin.y;

		projection.setToOrtho(left, right, bottom, top, Math.abs(near), Math.abs(far));
		view.setToLookAt(position, tmp.set(position).add(direction), up);
		Matrix4.mul(combined.val, view.val);

	 * This must be called in ApplicationListener.resize() in order to correctly update the camera viewport. 
	public void updateViewport() {
		setToOrtho(false, virtualViewport.getWidth(), virtualViewport.getHeight());


This class allows us to build a better VirtualViewport given the minimum and maximum areas we want to support performing the logic we explained in the previous post. For example, if we have a minimum area of 800x480 and a maximum area of 854x600, then, given a device of 480x320 (3:2) it will return a VirtualViewport of 854x570 which is a good match of a resolution which contains the minimum area and is smaller than the maximum area and has the same aspect ratio of 480x320.

public class MultipleVirtualViewportBuilder {

	private final float minWidth;
	private final float minHeight;
	private final float maxWidth;
	private final float maxHeight;

	public MultipleVirtualViewportBuilder(float minWidth, float minHeight, float maxWidth, float maxHeight) {
		this.minWidth = minWidth;
		this.minHeight = minHeight;
		this.maxWidth = maxWidth;
		this.maxHeight = maxHeight;

	public VirtualViewport getVirtualViewport(float width, float height) {
		if (width >= minWidth && width <= maxWidth && height >= minHeight && height <= maxHeight)
			return new VirtualViewport(width, height, true);

		float aspect = width / height;

		float scaleForMinSize = minWidth / width;
		float scaleForMaxSize = maxWidth / width;

		float virtualViewportWidth = width * scaleForMaxSize;
		float virtualViewportHeight = virtualViewportWidth / aspect;

		if (insideBounds(virtualViewportWidth, virtualViewportHeight))
			return new VirtualViewport(virtualViewportWidth, virtualViewportHeight, false);

		virtualViewportWidth = width * scaleForMinSize;
		virtualViewportHeight = virtualViewportWidth / aspect;

		if (insideBounds(virtualViewportWidth, virtualViewportHeight))
			return new VirtualViewport(virtualViewportWidth, virtualViewportHeight, false);
		return new VirtualViewport(minWidth, minHeight, true);
	private boolean insideBounds(float width, float height) {
		if (width < minWidth || width > maxWidth)
			return false;
		if (height < minHeight || height > maxHeight)
			return false;
		return true;


In case the aspect ratio is not supported, it will return the minimum area.

Floating elements

As we explained in the previous post, there are some cases where we need stuff that should be always at fixed positions in the screen, for example, the audio and music buttons in Clash of the Olympians. In order to do that we need to make the position of those buttons depend on the VirtualViewport. In the next section where we explain how to use all together we show an example of how to do a floating element.

Using the code together

Finally, here is an example showing how to use these concepts in a LibGDX application:

public class VirtualViewportExampleMain extends com.badlogic.gdx.Game {

	private OrthographicCameraWithVirtualViewport camera;
	// extra stuff for the example
	private SpriteBatch spriteBatch;
	private Sprite minimumAreaSprite;
	private Sprite maximumAreaSprite;
	private Sprite floatingButtonSprite;
	private BitmapFont font;

	private MultipleVirtualViewportBuilder multipleVirtualViewportBuilder;

	public void create() {
		multipleVirtualViewportBuilder = new MultipleVirtualViewportBuilder(800, 480, 854, 600);
		VirtualViewport virtualViewport = multipleVirtualViewportBuilder.getVirtualViewport(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
		camera = new OrthographicCameraWithVirtualViewport(virtualViewport);
		// centers the camera at 0, 0 (the center of the virtual viewport)
		camera.position.set(0f, 0f, 0f);
		// extra code
		spriteBatch = new SpriteBatch();
		Pixmap pixmap = new Pixmap(64, 64, Format.RGBA8888);
		pixmap.fillRectangle(0, 0, 64, 64);
		minimumAreaSprite = new Sprite(new Texture(pixmap));
		minimumAreaSprite.setPosition(-400, -240);
		minimumAreaSprite.setSize(800, 480);
		minimumAreaSprite.setColor(0f, 1f, 0f, 1f);
		maximumAreaSprite = new Sprite(new Texture(pixmap));
		maximumAreaSprite.setPosition(-427, -300);
		maximumAreaSprite.setSize(854, 600);
		maximumAreaSprite.setColor(1f, 1f, 0f, 1f);
		floatingButtonSprite = new Sprite(new Texture(pixmap));
		floatingButtonSprite.setPosition(virtualViewport.getVirtualWidth() * 0.5f - 80, virtualViewport.getVirtualHeight() * 0.5f - 80);
		floatingButtonSprite.setSize(64, 64);
		floatingButtonSprite.setColor(1f, 1f, 1f, 1f);
		font = new BitmapFont();
	public void resize(int width, int height) {
		super.resize(width, height);
		VirtualViewport virtualViewport = multipleVirtualViewportBuilder.getVirtualViewport(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
		// centers the camera at 0, 0 (the center of the virtual viewport)
		camera.position.set(0f, 0f, 0f);
		// relocate floating stuff
		floatingButtonSprite.setPosition(virtualViewport.getVirtualWidth() * 0.5f - 80, virtualViewport.getVirtualHeight() * 0.5f - 80);
	public void render() {
		Gdx.gl.glClearColor(1f, 0f, 0f, 1f);
		// render stuff...
		font.draw(spriteBatch, String.format("%1$sx%2$s", Gdx.graphics.getWidth(), Gdx.graphics.getHeight()), -20, 0);

	public static void main(String[] args) {
		LwjglApplicationConfiguration config = new LwjglApplicationConfiguration();

		config.title = VirtualViewportExampleMain.class.getName();
		config.width = 800;
		config.height = 480;
		config.fullscreen = false;
		config.useGL20 = true;
		config.useCPUSynch = true;
		config.forceExit = true;
		config.vSyncEnabled = true;

		new LwjglApplication(new VirtualViewportExampleMain(), config);


In the example there are three colors, green represents the minimum supported area, yellow the maximum supported area and red represents the area outside. If we see red it means that aspect ratio is not supported. There is a floating element colored white, which is always relocated in the top right corner of the screen, unless we are on an unsupported aspect ratio, in that case it is just located in the top right corner of the green area.

The next video shows the example in action:

UPDATE: you can download the source code to run on Eclipse from here.


In these two blog posts we explained in a simplified way how we managed to support different aspect ratios and resolutions for Clash of the Olympians, a technique that could be used as an acceptable way of handling different screen sizes for a wide range of games, and it is not hard to use.

As always, we hope you liked it and that it could be useful for you when developing your games. Opinions and suggestions are always welcome if you want to comment 🙂 and also share it if you liked it and think other people could benefit from this code.

Thanks for reading.

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Our solution to handle multiple screen sizes in Android - Part one

Developing games for multiple devices is not an easy task. Given the variety of devices, one of the most common problem is having to handle multiple screen sizes, which means different resolutions and aspect ratios.

In this blog post we want to share what we did to minimize this problem when making Ironhide's Clash of the Olympians for Android.

In the next sections we are going to show some common ways of handling the multiple screens problem and then our way.

Stretching the content

One common approach when developing a game is making the game for a fixed resolution, for example, making the game for 800x480.

Based on that, you can have the next layout in one of your game's screens:

Main screen of Clash of the Olympians in a 800x480 device.

Then, to support other screen sizes the idea is to stretch the content to the other device screen:

Main screen on a 800x600 device, stretched from 800x480.

The main problem is that the aspect ratio is affected and that is visually unacceptable.

Stretching + keeping aspect ratio

To solve part of the previous problem, one common technique is stretching but keeping the correct aspect ratio by adding dead space to the borders of the screen so the real game area aspect ratio is the same on different devices. For example:

Main screen in a 800x600 device with borders.

Main screen in a 854x480 device with borders.

This is an easy way to attack this multiple screen size problem, you can even create some nice borders instead of the black borders shown in the previous image to improve how it looks.

However, in some cases this is not acceptable either since it doesn't look so good or it feels like the game wasn't made for that device.

Our solution: Using a Virtual Viewport

Our approach consists in adapting what is shown in the game screen area to the device screen size.

First, we define a range of aspect ratios we want to support, for example, in the case of clash we defined 4:3 (800x600) and 16:9 (854x480) as our border case aspect ratios, so all aspect ratios in the middle of those two should be supported.

Given those two aspect ratios, we defined our maximum area as 854x600 and our minimum area as 800x480 (the union and intersection between 800x600 and 854x480, respecively). The idea is to cover the maximum area with stuff, but the important stuff (buttons, information, etc) should be always included in the minimum area.

The red rectangle shows the minimum area while the blue rectangle shows the maximum area.

Then, given a device resolution we calculate an area that matches the device aspect ratio and is included in the virtual area. For example, given a device with a resolution of 816x544 (4:3), this is what is shown:

The green rectangle shows the matching area for 816x544.

This is how the main screen is shown in a 816x544 device.

In case we are on a bigger or lower resolution than the maximum or minimum area we defined, respectively, for example a screen of 480x320 (3:2), what we do is calculate the aspect ratio and find a corresponding match for that aspect ratio in the area we defined. In the case of the example, one match could be 800x534 since it is 3:2 aspect ratio and it is inside our virtual area. Then we scale down to fit the screen.

The green rectangle shows the calculated area for a resolution of 800x534 (matching the aspect of the 480x320 device).

This is what is shown of the main screen in a 480x320 device (click to enlarge the image).

Floating elements

For some elements of the game, such as buttons, maintaining their fixed world position for different screen sizes doesn't look good, so what we do is making them floating elements. That means they are always at the same screen position, the next images shows an example with the main screen buttons:

Main screen's buttons distribution for a 854x480 device.

Main screen's buttons distribution for a 800x600 device. As you can see, buttons are relocated to match the screen size.

Finally, we want to show a video of this multiple screen sizes auto adjustment in real time:

Adjusting the game to the screen size in real time.

Some limitations

As we are scaling up/down in some cases to match the corresponding screen, some devices could perceive some blur since we are using linear filtering and the final position of the elements after the camera transformations could be not integer positions. This problem is minimized with better density devices and assets.

Layouts could change between different devices, for example, the layout for a phone could be different to the layout of a tablet device.

Text is a special case, when rendering text just downscaling it is not a correct solution since it could be not readable. You may have to re-layout text for lower resolution devices to show it bigger and readable.


If you design your game screens follow this approach, it is not so hard to support multiple screen sizes in an acceptable way. However there is still a lot of detail to take care of, like the problems we talked in the previous section.

In the next part of this blog post we will show some code based on LibGDX for those interested in how we implemented all this.

Thanks for reading and hope you liked it.

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Clash of the Olympians for Android

For the last eight months approx we were working with Ironhide Game Studio on a port to Android of their game Clash of the Olympians originally made for Flash. We are happy to announce that it was released on Google Play on last December 6th.

It is not a direct port since it has new features like bonuses for making combos during the game, new enemy behaviors, a hero room to see your score when you finish the game and multiple save slots. Also, the game mechanics changed a bit since they were adapted to touch devices and the game was rebalanced to match the new controls.

If you didn't already, go and get it on Google Play:


QR code:

Hope you enjoy it.

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Decoupling game logic from input handling logic

In this post we want to share how we are decoupling our game logic from the input handling as we explained briefly in a previous post about different controls we tested for Super Flying Thing.


There are different ways to handle the input in a game. Basically, you could have a framework that provides a way to define event handlers for each input event, or to poll input values from the API. LibGDX provides both worlds so it is up to you what you consider best for your game. We prefer to poll for input values for the game logic itself.

When starting to make games, you probably feel tempted to add the input handling logic in one or more base concepts of your game, for example, if you were making Angry Birds you probably would add it to the Slingshot class to detect when to fire a bird or not. That is not totally bad if you are making a quick prototype but it is not recommended for long term because it would be harder to add or change between different control implementations.

Abstracting the input

To improve a bit this scenario in our games, we are using an intermediary class named Controller. That class provides values more friendly and related with the game concepts. A possible Controller class for our example could be:

class SlingshotController {
	boolean charging;
	Vector2 direction;

Now, we could process the input handling in one part of the code and update a common Controller instance shared between it and the game logic. Something like this:

class SlingshotMouseControllerLogic extends InputListener {

	Slingshot slingshot;
	SlingshotController controller;

	public boolean touchDown (InputEvent event, float x, float y, int pointer, int button) {
		slingshotPosition = slingshot.getPosition();
		controller.charging = slingshotPosition.isNear(x,y);
		return controller.charging;

	public void touchUp (InputEvent event, float x, float y, int pointer, int button) {
		if (!controller.charging) 
		controller.charging = false;

	public void touchDragged (InputEvent event, float x, float y, int pointer) {
		if (!controller.charging) 
		slingshotPosition = slingshot.getPosition();

Or if you are polling the input:

class SlingshotMouseControllerLogic implements Updateable {

	Slingshot slingshot;
	SlingshotController controller;

	boolean touchWasPressed = false;

	public void update(float delta) {
		boolean currentTouchPressed = Gdx.input.isPressed();

		slingshotPosition = slingshot.getPosition();
		x = Gdx.input.getX();
		y = Gdx.input.getY();

		if (!touchWasPressed && currentTouchPressed) {
			controller.charging = slingshotPosition.isNear(x,y);
			touchWasPressed = true;

		if(touchWasPressed && !currentTouchPressed) {
			controller.charging = false;
			touchWasPressed = false

		if (!controller.charging)


Now, the Slingshot implementation will look something like this:

class Slingshot { 

	// multiple fields
	// render logic 

	Controller controller;
	boolean wasCharging = false;

	update() {
		if (controller.charging && !wasCharging) {
			// starts to draw stuff based on the state we are now charging...
			// charges a bird in the slingshot.

			wasCharging = true;
		} else if (!controller.charging && wasCharging) {
			// stops drawing the slingshot as charging

			// fires the bird!!
			wasCharging = false;
		// more stuff...

As you can see, the game concept Slingshot doesn't know anything about input anymore and we could switch to use the keyboard, Xbox 360 Controller, etc, and the game logic will not notice the change.


Decoupling your game logic from the input by abstracting it in a class is a good way to keep your game logic depending mainly on game concepts making it easier to understand and improving its design. Also, it is a good way to create several controls for the game (input, AI, network, recorded input, etc), while the game logic don't even notice the change.

This post provides a really simple concept, the concept of abstraction, it is nothing new and probably most of the game developers are already doing this, even though we wanted to share it, maybe it is helpful for someone.

We tried to use simple and direct code in this post to increase understandability, however in our games, as we use an entity system framework, we do it a bit different using components, scripts and systems instead of direct classes for concepts like the class Slingshot we presented in this post, but that's food for another blog post.

Finally, we use another abstraction layer over the framework input handling which provides us a better and simplified API to poll info from, that's why we prefer to poll values, food for another post as well.

Hope you like it, as always.

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