How we used Entity Component System (ECS) approach at Gemserk - 2/2

So, after our first attempt on using ECS, when we started to develop mobile games and moved to the LibGDX framework, we decided to abandon our ComponentsEngine and start over.

We were still reading about ECS while we were creating our set of tools and code over LibGDX. At some point in time, some of the Java community developers started Artemis, a lightweight ECS, and we decided to give it a try.

Artemis

This is a simplified core architecture diagram:

Note: we used a modified version of Artemis with some additions like enable/disable an Entity.

In this pure approach, Entities and Components are just data, and they are related by identifiers, like these tables in a relational database:

Where we have two entities, both with a PositionComponent and only one of them with MovementComponent.

An example of this components in code:

public class PositionComponent : Component {
    public float x, y;
}

public class MovementComponent: Component {
    public float speed;
}

EntitySystems perform the game logic. They normally work on a subset of Components from an Entity but they could need it in order to enable/disable/destroy it (if it is part of its logic).

An example of a System is LimitLinearVelocitySystem, used in multiple of our games to limit the physics velocity of an Entity with PhysicsComponent and LimitVelocityLimitComponent:

public void process(Entity e) {
    PhysicsComponent physicsComponent = 
                     Components.getPhysicsComponent(e);
    Body body = physicsComponent.getPhysics().getBody();

    LinearVelocityLimitComponent limitComponent = 
             e.getComponent(LinearVelocityLimitComponent.class);
    Vector2 linearVelocity = body.getLinearVelocity();

    float speed = linearVelocity.len();
    float maxSpeed = limitComponent.getLimit();

    if (speed > maxSpeed) {
        float factor = maxSpeed / speed;
        linearVelocity.mul(factor);
        body.setLinearVelocity(linearVelocity);
    }
}

There are some extra classes like the TagManager which allows assigning a unique string identifier to an Entity in order to find it, and the GroupManager which allows adding entities to groups identified by a name, which is more like how tags are used everywhere else.

Even though it could look similar to our ComponentsEngine where the Properties in that engine correspond to the Components in this one, and Components to Systems, there is an important difference: Systems are not part of an Entity and work horizontally over all entities with specific Components (data). So, in this approach, changing only data from entities indeed change their behaviour in the game, so it is really data driven.

Since Entities and Components are just data, their instances in memory are easier to reuse. An Entity is just an id, so it is obvious. A Component is a set of data that doesn't care too much which Entity it belongs to. So, after the Entity doesn't need it anymore, it could be reused elsewhere, improving a lot memory usage and garbage collection.

Artemis, and other pure ECS implementations, are really lightweight and performant frameworks. The real power comes from the Systems and Components built over them. In our case, we created a lot of Systems that we started to reuse between games, from SuperFlyingThing to VampireRunner and even in Clash of the Olympians.

Scripting

One of the most interesting ones is the Scripting framework we created over Artemis. It was really useful and gave us a lot of power to overcome some of the limitations we were facing with the pure approach when we need to create a specific logic for a specific moment during the game and (probably) never again and we didn't want a System for that.

They work similar to Unity MonoBehaviours and our logic Components from ComponentsEngine, and they also belong to the Entity lifecycle. One difference, however, is that they try to avoid storing data in their instances as much as possible and instead try to store and read data from Components, like Systems do.

Here is an example Script from one of our games:

public class MovementScript extends ScriptJavaImpl {
	
  @Override
  public void update(World world, Entity e) {

    MovementComponent movementComponent = Components.getMovementComponent(e);
    SpatialComponent spatialComponent = Components.getSpatialComponent(e);
    ControllerComponent controllerComponent = Components.getControllerComponent(e);
		
    Controller controller = controllerComponent.controller;
    Movement movement = movementComponent.getMovement();
    Spatial spatial = spatialComponent.getSpatial();
		
    float rotationAngle = controllerComponent.rotationSpeed * GlobalTime.getDelta();
		
    Vector2 linearVelocity = movement.getLinearVelocity();
		
    if (controller.left) {
        spatial.setAngle(spatial.getAngle() + rotationAngle);
        linearVelocity.rotate(rotationAngle);
    } else if (controller.right) {
        spatial.setAngle(spatial.getAngle() - rotationAngle);
        linearVelocity.rotate(-rotationAngle);
    }
		
  }
	
}

Note: GlobalTime.getDelta() is similar to Unity's Time API.

One problem about the Scripting framework is that it is not always clear when some logic should be in a Script or a System, and that made reusability a bit harder. In the case of the example above, it is obvious it should be moved to a System.

Templates

Another useful thing we did over Artemis was using templates to build entities in a specific way, similar to what we had in ComponentsEngine.

We used also a concept of Parameters in templates in order to customize parts of them. Similar to ComponentsEngine, templates could apply other templates and different templates could be applied to the same Entity.

Here is an example of an Entity template:

public void apply(Entity entity) {

    String id = parameters.get("id");
    String targetPortalId = parameters.get("targetPortalId");
    String spriteId = parameters.get("sprite", "PortalSprite");
    Spatial spatial = parameters.get("spatial");
    Script script = parameters.get("script", new PortalScript());

    Sprite sprite = resourceManager.getResourceValue(spriteId);

    entity.addComponent(new TagComponent(id));
    entity.addComponent(new SpriteComponent(sprite, Colors.darkBlue));
    entity.addComponent(new Components.PortalComponent(targetPortalId, spatial.getAngle()));
    entity.addComponent(new RenderableComponent(-5));
    entity.addComponent(new SpatialComponent(spatial));
    entity.addComponent(new ScriptComponent(script));

    Body body = bodyBuilder //
            .fixture(bodyBuilder.fixtureDefBuilder() //
                    .circleShape(spatial.getWidth() * 0.35f) //
                    .categoryBits(CategoryBits.ObstacleCategoryBits) //
                    .maskBits((short) (CategoryBits.AllCategoryBits & ~CategoryBits.ObstacleCategoryBits)) //
                    .sensor()) //
            .position(spatial.getX(), spatial.getY()) //
            .mass(1f) //
            .type(BodyType.StaticBody) //
            .userData(entity) //
            .build();

    entity.addComponent(new PhysicsComponent(new PhysicsImpl(body)));

}

That template configures a Portal entity in SuperFlyingThing which teleports the main ship from one place to another. Click here for the list of templates used in that game.

Interacting with other systems

Sometimes you already have a solution for something, like the physics engine, and you want to integrate it with the ECS. The way we found was to create a way to synchronize data from that system to the ECS and vice versa, sometimes having to replicate a bit of data in order to have it easier for the game to use it.

Finally

Even though we had to do some modifications and we know it could still be improved, we loved using a pure ECS in the way we did.

It was used to build different kind of games/prototypes and even a full game like the Clash of the Olympians for mobile devices, and it worked pretty well.

What about now?

Right now we are not using any of this in Unity. However we never lost interest nor feith in ECS, and there are starting to appear new solutions over Unity in the last years that caught our attention. Entitas is one of them. The Unity team is working towards this path too as shown in this video. Someone who din't want to wait started implementing that solution in his own way. We've also watched some Unite and GDC talks about using this approach in big games and some of them even have a Scripting layer too, which is awesome since, in some way, it validates we weren't so wrong ;).

I am exited to try ECS approach again in the near future. I believe it could be a really good foundation for multiplayer games and that is something I'm really interested in doing at some point in my life.

Thanks for reading.

References

 

 

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How we used Entity Component System (ECS) approach at Gemserk - 1/2

When we started Gemserk eight years ago, we didn't know which was the best way to make games. So before starting, we did some research. After reading some articles and presentations we were really interested in trying an Entity Component System (ECS) approach for our games. However, since we didn't find a clear guide or implementation at that point we had to create our own solution while exploring and understanding it. We named our engine ComponentsEngine.

Components Engine

The following image shows a simplified core architecture diagram:

Note: part of the design was inspired by a Flash ECS engine named PushButtonEngine.

An Entity is just a holder of state and logic. In an ECS, anything can be an Entity, from an enemy ship to the concept of a player or even a file path to a level definition. It depends a lot on the game you are making and how you want to structure it.

A Property is part of the state of an Entity, like the health or the position in the world, anything that means something for the state of the game. It can be accessed and modified from outside.

Components perform logic updating one or more Properties to change the Entity state. A component could for example change the position of an Entity given a speed and a moving direction.

They normally communicate with each other either by modifying common Properties (when on the same Entity) or by sending and receiving Messages through a MessageDispatcher (when on the same or on different Entities). They just have to register a method to handle a message. In some way, this is pretty similar to using SendMessage() method in Unity and having the proper methods in the MonoBehaviours that need to react to those messages.

EntityTemplates are an easy way to define and build specific game entities, they just add Properties and Components (and more stuff) to make an Entity behave in one way or another.

For example, a ShipTemplate could add position, velocity and health properties and some components to perform movement and to process damage from bullet hits:

public void build() {
        // ... more stuff 
        property("position", new Vector2f(0, 0));
        property("direction", new Vector2f(1, 0));

        property("speed", 5.0f);

        property("currentHealth", 100.0f);
        property("maxHealth", 100.0f);

        component(new MovementComponent());
        component(new HealthComponent());
}

An example of the MovementComponent:

public class MovementComponent() {

   @EntityProperty
   Vector2f position;

   @EntityProperty
   Vector2f direction;

   @EntityProperty
   Float speed;

   @Handles
   public void update(Message message) {
      Property<Float> dt = message.getProperty("deltaTime");
      position += direction * speed * dt.get();
   }

}

In some way, EntityTemplates are similar to Unity Prefabs or Unreal Engine Blueprints, represeting in some way a (not pure) Prototype pattern.

Some interesting stuff of our engine:

  • Since templates are applied to Entities, we can apply multiple templates to the same one. In this way, we could have common templates to add generic features like motion for example. So we could do something like OrcTemplate.apply(e) to add the orc properties and components and then MovableTemplate.apply(e), so now we have an Orc that moves.
  • Templates can apply other templates inside them. So we could do the same as before but inside OrcTemplate, we could apply MovableTemplate there. Or even use this to create specific templates, like OrcBossTemplate which is an Orc with a special ability.
  • Entities have also tags which are defined when applying a template too and are used to quickly identify entities of interest. For example, if we want to identify all bullets in the game, we could add the tag "Bullet" during the Entity creation and then, when processing a special power, we get all the bullets in the scene and make them explode. Note: in some ECS a "flag" component is used for this purpose.
  • The Property abstraction is really powerful, it can be implemented in any way, for example, an expression property like "my current health is my speed * 2". We used that while prototyping.

Some bad stuff:

  • Execution speed wasn't good, we had a lot of layers in the middle, a lot of reflection, send and receive messages (even for the update method), a lot of boxing and unboxing, etc. It worked ok in desktop but wasn't a viable solution for mobile devices.
  • The logic started to be distributed all around and it wasn't easy to reuse, and we started to have tons of special cases, when we couldn't reuse something we simply copy-pasted and changed it.
  • There was a lot of code in the Components to get/set properties instead of just doing the important logic.
  • Indirection in Properties code was powerful but we end up with stuff like this (and we didn't like it since it was too much code overhead):
e.getProperty("key").value = e.getProperty("key").value + 1.
  • We didn't manage to have a good data driven approach which is one of the best points of ECS. In order to have different behaviours we were forced to add both properties, components and even tags instead of just changing data.

Note: Some of these points can be improved but we never worked on that.

Even though we don't think the achitecture is bad, it guided us to do stuff in a way we didn't like it and didn't scale. We feel that Unity does the same thing with its GameObjects and MonoBehaviours, all the examples in their documentation go in that direction.

That was our first try to the ECS approach. In case you are interested, ComponentsEngine and all the games we did with it are on Github and even though they are probably not compiling, they could be used as reference.

This post will continue with how we later transitioned to use a more pure approach with Artemis, when we were more dedicated to mobile games.

References

Evolve your hierarchy - Classic article about Entity Component System.

PushButtonEngine - Flash Entity Component System we used as reference when developing our ComponentsEngine.

Game architecture is different - A quick transformation from normal game architecture to ECS.

Slick2D - The game library we were using during ComponentsEngine development.

 

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Android and Desktop games internationalization using Java and LibGDX

Recently, we had to add multiple language support for a game we are developing. As you may know, Java provides classes to simplify the task of making your application available in multiple languages. In this post we want to share a bit our experience when using Java localization classes in a LibGDX application to provide multiple language support for both Android and desktop platforms.

Quick introduction

Java provides a class named ResourceBundle which provides a way to store resources (mainly strings) for a given locale, so you can ask for a string identified by a key and it will return the text depending the current locale. You can read the article Java Internationalization: Localization with ResourceBundles if you want to know more about how to use Java classes for internationalization. The rest of the post assumes you know something about Locale and ResourceBundle classes.

Why we don't use Android resources

Android provides also a way to support multiple locale resources but it depends on Android API, so we prefer to use the Java API instead which should work on all platforms.

Our experience when using Java internationalization on Android

When letting ResourceBundle to automatically load resources bundles from properties files, Java expects them to be in ISO-8859-1 encoding. However, it seems Android behaves in a different way and expects another encoding by default. So, when resource bundles are automatically loaded in Android from an ISO-8859-1 properties file with special characters, it loads them wrong.

The first try

The first solution we tried to fix this was to call ResourceBundle.getBundle() method using a custom Control implementation which creates PropertyResourceBundles using an InputReader with the correct encoding. Here is a code example to achieve that:

public class EncodingControl extends Control {

	String encoding;

	public EncodingControl(String encoding) {
		this.encoding = encoding;
	}

	@Override
	public ResourceBundle newBundle(String baseName, Locale locale, String format, ClassLoader loader, boolean reload) 
			throws IllegalAccessException, InstantiationException, IOException {
		String bundleName = toBundleName(baseName, locale);
		String resourceName = toResourceName(bundleName, "properties");
		ResourceBundle bundle = null;
		InputStream inputStream = null;
		try {
			inputStream = loader.getResourceAsStream(resourceName);
			bundle = new PropertyResourceBundle(new InputStreamReader(inputStream, encoding));
		} finally {
			if (inputStream != null)
				inputStream.close();
		}
		return bundle;
	}
}

After that, we customized the Control class to work with LibGDX FileHandle in order to place the properties files in the assets folder. Here is the final code for our Control implementation:

public class GdxFileControl extends Control {

	private String encoding;
	private FileType fileType;

	public GdxFileControl(String encoding, FileType fileType) {
		this.encoding = encoding;
		this.fileType = fileType;
	}
	
	public ResourceBundle newBundle(String baseName, Locale locale, String format, ClassLoader loader, boolean reload) 
			throws IllegalAccessException, InstantiationException, IOException {
		// The below is a copy of the default implementation.
		String bundleName = toBundleName(baseName, locale);
		String resourceName = toResourceName(bundleName, "properties");
		ResourceBundle bundle = null;
		FileHandle fileHandle = Gdx.files.getFileHandle(resourceName, fileType);
		if (fileHandle.exists()) {
			InputStream stream = null;
			try {
				stream = fileHandle.read();
				// Only this line is changed to make it to read properties files as UTF-8.
				bundle = new PropertyResourceBundle(new InputStreamReader(stream, encoding));
			} finally {
				if (stream != null)
					stream.close();
			}
		}
		return bundle;
	}
}

And that can be called in this way:

	ResourceBundle.getBundle("messages", 
		new GdxFileControl("ISO-8859-1", FileType.Internal))

That worked really well until we discovered that Android API sucks and doesn't support ResourceBundle.Control before API level 9, that means our solution works only for users with Android 2.3+. That's a problem since we want to support 2.0+, so we had to think another way to solve this.

The second try

After some tests, we discovered that if we construct a PropertyResourceBundle using an InputStream, the expected encoding is ISO-8859-1 for both desktop and Android. That means that, if we use that specific PropertyResourceBundle constructor, we don't have to force the encoding. So, the new solution consists in building a PropertyResourceBundle for each locale and configuring the hierarchy ourselves by setting their parent ResourceBundle. Here is an example of what we do now:

	FileHandle rootFileHandle = Gdx.files.internal("data/messages.properties");
	FileHandle spanishFileHandle = Gdx.files.internal("data/messages_es.properties");
	ResourceBundle rootResourceBundle = new PropertyResourceBundle(rootFileHandle.read());
	ResourceBundle spanishResourceBundle = new PropertyResourceBundle(spanishFileHandle.read()) {{
		setParent(rootResourcebundle);
	}};

After that we created a map of ResourceBundles for each Locale we support, so we can call something like:

	ResourceBundle resourceBundle = getResourceBundle(new Locale("es"));

The good part is this solution works well for both Android and desktop despite the Android API level (PropertyResourceBudndle seems to be supported from API Level 1). The bad part is that we lost the ResourceBundle logic to automatically build the hierarchy of resources and we had to do that manually now.

UPDATE: The class we use for this stuff is available in our commons-gdx project, resources module with the name of ResourceBundleResourceBuilder.

Conclusion

Supporting multiple languages in an application is a way to say users of all around the world you care about them but translating text to several languages is not cheap at all, however, Java provides a good framework to simplify the job if you decide to support internationalization.

And as a side conclusion: never assume all the Java classes you are using are implemented for the minimum Android API you are targeting.

References

Animation4j - Interpolation Functions

Interpolation Functions are a key concept in animation4j project, they define how to a variable should change from one value to another. To define interpolation functions in animation4j, InterpolatorFunction interface should be implemented, the API looks like this:

public interface InterpolatorFunction {

	/**
	 * @param t
	 *            A real number in the interval [0,1]
	 * @return The interpolated value.
	 */
	float interpolate(float t);

}

So basically, functions works for values of t between interval [0,1]. An example implementation of this function is a linear Bézier:

/**
 * Linear Bézier curve implementation of an InterpolatorFunction.
 * 
 */
public class LinearBezierInterpolatorFunction implements InterpolatorFunction {

	private final float p0, p1;

	public LinearBezierInterpolatorFunction(float p0, float p1) {
		this.p0 = p0;
		this.p1 = p1;
	}

	@Override
	public float interpolate(float t) {
		if (t < 0)
			return p0;
		if (t > 1)
			return p1;
		return (1 - t) * p0 + t * p1;
	}

}

The library already provides some implementations, you could instantiate them using InterpolatorFunctionFactory factory class. The API of the factory looks like this:

	// returns a cubic Bézier interpolation function based on the four specified points.
	InterpolatorFunction cubicBezier(float p0, float p1, float p2, float p3);

	// returns a quadratic Bézier interpolation function based on the four specified points.
	InterpolatorFunction quadratic(float p0, float p1, float p2);

	// returns a cubic Bézier interpolation function with presetted values.
	InterpolatorFunction ease();

	// returns a linear Bézier interpolation function.
	InterpolatorFunction linear();

	// returns a cubic Bézier interpolation function with presetted values.
	InterpolatorFunction easeIn();

	// returns a cubic Bézier interpolation function with presetted values.
	InterpolatorFunction easeOut();

	// returns a cubic Bézier interpolation function with presetted values.
	InterpolatorFunction easeInOut();

In the previous post we talked about transitions of variables from one value to another using Transition interface. When building a transition using the Transitions factory, you could specify the interpolation functions you want to use, if you don't, then linear Bézier functions are used by default. One example of how to create a Transition specifying the interpolation functions looks like this:

Transition<Vector2f> transition = Transitions.transition(
	new Vector2f(0f, 0f), 					// the starting value
	vector2fConverter, 						// the type converter (using two variables)
	InterpolatorFunctionFactory.easeOut(),	// the interpolation function for the first variable
	InterpolatorFunctionFactory.easeIn());	// the interpolation function for the second variable

One thing to mention is that Transitions factory methods use a varargs to specify interpolation functions parameters because we don't know beforehand how many variables you need. In case you specify less functions than variables, then linear Bézier functions will be used for the variables without functions specified.

As we said on the previous post, animation4j API could change since this blog post was written. API used for this post is the one of the current released version 0.0.8 (already uploaded to maven central).

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Animation4j - Transitions

As we said on a previous post, we are going to talk a bit more about the current released version (0.0.8) of animation4j, remember that the API could change since we wrote this post.

In this case, we are going to talk about making a transition from one value to another in some time. Animation4j provides you one interface to make transitions, the Transition interface, the API by now looks like this:

public interface Transition<T> {

	/**
	 * Returns the current value of the transition.
	 * 
	 */
	T get();

	/**
	 * Start an interpolation from a to b in the specified default time.
	 * 
	 * @param t
	 *            The wanted new value.
	 */
	void set(T t);

	/**
	 * Start an interpolation from a to b in the specified time.
	 * 
	 * @param t
	 *            The wanted new value.
	 * @param time
	 *            The time to set the new value. If time is zero, then value is applied directly.
	 */
	void set(T t, int time);
}

The library uses generics so you can create transitions of any type. To do this, you can use the Transitions factory class, where you can create transitions for your types. The API of the factory class looks like this:

<T> Transition<T> transition(T startValue, TypeConverter<T> typeConverter);

The current Transition generic implementation internally work with a float[] in order to optimize memory and to simplify internal work. To use it, you will have to create a converter from your type to float[] and vice versa by implementing the TypeConverter<T> interface. Current API of TypeConverter<T> looks like this:

/**
 * Provides a way to convert an object in a float[] array and vice versa, for interpolation purposes.
 * 
 * @param <T>
 *            The type to convert.
 * @author acoppes
 */
public interface TypeConverter<T> {

	/**
	 * Returns the quantity of variables are used to convert the object to the float[] and vice versa.
	 * 
	 * @return the quantity of variables used.
	 */
	int variables();

	/**
	 * Copy the values of the object to the specified float array, if null it will create a new float array.
	 * 
	 * @param object
	 *            The object from where to get the values to fulfill the float array.
	 * @param x
	 *            The float array to copy the values of the object. If null it will create a new float array.
	 * @return The float array with the values of the object.
	 */
	float[] copyFromObject(T object, float[] x);

	/**
	 * Copy the values of the float array to the specified object.
	 * 
	 * @param object
	 *            The object which the float array values will be copied to. If null or object immutable, it will create a new object.
	 * @param x
	 *            The float array to get the values to fulfill the object.
	 * @return An object with the values of the float array.
	 */
	T copyToObject(T object, float[] x);

}

Type converters should be stateless, so you can reuse a single type converter for all your transitions of the same type. For the next Vector2f class example:

public class Vector2f {

	public float x,y;

	public Vector2f(float x, float y) {
		set(x,y);
	}

	public void set(float x, float y) {
		this.x = x;
		this.y = y;
	}

}

We could create the next type converter:

public class Vector2fConverter implements TypeConverter<Vector2f> {

	@Override
	public float[] copyFromObject(Vector2f v, float[] x) {
		if (x == null) 
			x = new float[variables()];  // don't worry about garbage generation, the transition implementation will cache these values.
		x[0] = v.x;
		x[1] = v.y;
		return x;
	}

	@Override
	public Vector2f copyToObject(Vector2f v, float[] x) {
		if (v == null)
			v = new Vector2f(0, 0); // don't worry about garbage generation, the transition implementation will cache these values.
		v.x = x[0];
		v.y = x[1];
		return v;
	}

	@Override
	public int variables() {
		// we are only using two variables.
		return 2;
	}
}

So, to create a transition, your code would look like:

TypeConverter<Vector2f> converter = new Vector2fConverter(); // could be reused
Transition<Vector2f> transition = Transitions.transition(new Vector2f(100, 100), converter);

// now, set a transition to (500,500) in five seconds.
transition.set(new Vector2f(500, 500), 5000);

// wait some time, and get the value interpolated
Vector2f v = transition.get();

For more information, there is an transitions example in the examples module.

The idea is to provide different TypeConverter implementations for different libraries as project modules so you don't have to implement a TypeConverter for a Slick2D vector2f, or libgdx Vector2. However it is really easy to implement a type converter and you only have to do it once. Also, you will probably use transitions only for some types.

In one of the next posts, we want to talk about interpolation functions (and how are they used for transitions) as they are key concepts in animation4j project.

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Animation4j Project Introduction

Some time ago I started an internal project to simplify working with animations and transitions when making games in Java.

It was used in some of our games, for example ZombieRockers to make different kind of effects like fade in/out of screens or the points messages when scoring, or in JSnakes to make camera effects like zoom in/out, camera movement, etc.

After using it on our games, it became more clear what kind of things are possible with it.

Now that we are using Github for our source code, we started a dedicated project named animation4j with all the stuff related with animations.

This video shows some working examples of the project:

You can download a runnable jar to test the features shown in the video.

The project is on development, so right now there are a lot of things to improve starting by making easy to understand examples.

I plan to add more posts explaining some design decisions and showing how to use the library.

Hope Java developers could find some use for it.

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JNLP Downloader Tool

Some time ago, Rubén wrote a Java based tool to download all JNLP resources and prepare executable files to run it based on the JNLP file values.

It downloads all the resources specified by the JNLP file and save them into /libs and /natives for jar resources and nativelib resources respectively, in the second case it creates sub folders for each platform (Windows, Mac and Linux). After that, it creates one executable script file to run the application for each platform, configuring classpath and java.library.path inside it.

For example, running the tool:

 
 java -jar jnlpdownloader.jar example http://www.example.com/application.jnlp

where the JNLP contents are:

 
<?xml version="1.0" encoding="utf-8"?>
<jnlp spec="1.0+" codebase="http://www.example.com/" href="application.jnlp">
    <information>
            <title>Some Title</title>
            <vendor>Some Vendor</vendor>
            <description>Some Description</description>
    </information>
    <resources>
            <jar href="slf4j-api-1.5.8.jar" />
            <jar href="google-collections-1.0.jar" />
            <jar href="lwjgl-2.4.2.jar" />
    </resources>
    <resources os="Windows">
            <nativelib href="lwjgl-2.4.2-natives-win.jar" />
    </resources>
    <resources os="Linux">
            <nativelib href="lwjgl-2.4.2-natives-linux.jar" />
    </resources>
    <resources os="Mac">
            <nativelib href="lwjgl-2.4.2-natives-mac.jar" />
    </resources>  
    <application-desc main-class="Main" />
</jnlp>

will create the next file structure:

 
./example
./example/natives
./example/natives/Linux
./example/natives/Linux/liblwjgl.so
./example/natives/Windows
./example/natives/Windows/lwjgl.dll
./example/natives/Mac
./example/natives/Mac/liblwjgl.jnilib
./example/libs
./example/libs/google-collections-1.0.jar
./example/libs/slf4j-api-1.5.8.jar
./example/libs/lwjgl-2.4.2.jar
./example/run-windows.bat
./example/run-macosx.sh
./example/run-linux.sh

More info at the project's home page.

We used this tool mainly to take a snapshot of a deployed Java Web Start application to make it run offline, for demo purposes.

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SVG Path Traversal in Java

Because some people asked, in this post we are going to explain in some detail how we do the balls movement in Zombie Rockers. All this work could be useful for other kind of games using paths like, for example, a tower defense.

For Zombie Rockers (we have to change that name), we defined a Path as an ordered collection of points. Simplifying our Path class, it could be something like this:

 
class Path {
    ArrayList<Vector2f> points;
}

note: we are using Vector2f provided by Slick2D

To easily traverse a path, we created the class PathTraversal, which let us move forward or backward through the path. For example, we could start in the beginning of a path built by three points: (100,100), (200, 100) and (300, 100). Then, if we advance 50 units, then we are going to be on the coordinate (150, 100), between first and second point.

note: when we say units, could be anything you need, pixels, meters, etc, it is not important for the example.

The idea of the PathTraversal in the future is to support any kind of paths, for example, one made using Bézier curves instead only a list of points.

This shows the API of the PathTraversal:

 
class PathTraversal {
    // the path we are traversing
    Path path; 
    // the distance to the origin we are right now
    float getCurrentDistance();
    // go back the specified distance
    void backward(float distance);
    // go forward the specified distance
    void forward(float distance);
    // returns a point based on the current distance
    Vector2f getPoint();
}

You can create the Path in the way you want, for Zombie Rockers we are using SVG files to store the paths for each level and we are loading it using SVG Salamander project. First we create a AWT Shape using the Path class provided by the SVG Salamander, then we use a PathIterator to build the points we need to create our Path.

 
public List<Vector2f> loadPointsFromSVG(URI fileUri) {
	ArrayList<Vector2f> points = new ArrayList<Vector2f>();
	SVGDiagram diagram = SVGCache.getSVGUniverse().getDiagram(fileUri);
	SVGElement element = diagram.getElement(pathName);
	List vector = element.getPath(null);
	com.kitfox.svg.Path pathSVG = (com.kitfox.svg.Path) vector.get(1); 
	// get the AWT Shape
	Shape shape = pathSVG.getShape();	 
	// iterate over the shape using a path iterator discretizing with distance 0.001 units	 
	PathIterator pathIterator = shape.getPathIterator(null, 0.001d);
	float[] coords = new float[2];
	while (!pathIterator.isDone()) {
	pathIterator.currentSegment(coords);
	points.add(new Vector2f(coords[0], coords[1]));
	pathIterator.next();
	}
}

To create the SVG path, you can use any program you want, in our case we are using mainly Gimp, we tried using Inkscape too but as we are creating the levels using Gimp it was easy to do all the work using the same application. So, open Gimp, create a path and then export it to a SVG file. Then, it should be ready to be loaded by SVG salamander inside your code.

A working example of this technique is shown in the next video of Zombie Rockers:

Conclusion

This solution works well while you avoid making the angle of two consecutive segments too sharp. For example, right now when balls traverse between two segments where the angle is too sharp, they will change the render direction too quickly and doesn't looks so good.

The problem of SVG Path when the angle of two consecutive segments is too sharp.
Figure 1: The problem of SVG Path when the angle of two consecutive segments is too sharp.

One solution to reduce this, is to increment the points of the path when making a discrete version of the path, so you will have more intermediate points, then less angle brusque changes.

Another solution is to use a smooth change for the rendering direction, so when you change the value of the render direction, it will change smoothly over time.

Hope it could be of help.

Update: Added figure for the problem of the angles between segments.

Update2: Added links to the current implementation of Path and PathTraversal classes.

Update3: Fixed the path example.

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Signing JARs for Applet and Webstart

We are developing our games using Java and deploying them as Applets and Webstart applications.

By default, applications launched with Java Webstart or as an Applet run in a restricted environment. We are using some technologies which require unrestricted access like Lwjgl, Jinput and custom ClassLoaders. In order to have access to these features, every jar must be signed with a certificate.

Once the jars have been downloaded on the client machine and the signature is validated, the user is requested whether he trust or not the provider of the certificate and if he wants to accept it permanently.

In the beginning, one option is to generate a new certificate every time we sign the jars of an application but this means that although the user might have accepted a certificate permanently, he will have to accept the new one.

A better option is to generate a certificate once and use it every time we sign an application, so that whenever the user accepts our certificate permanently he won’t be bothered again.

A way to create a certificate and manually sign an application with it is explained at Sun's Java Documentation.

In our case, we are using maven as our build tool with maven-webstart-plugin to automatically sign our jars. This plugin allow us to use both options.

In order to easily choose between them, we configure the plugin using properties instead of fixed values, so we can override them with profiles. Using the default values of these properties, the plugin generates a new certificate each time.
If we want to make a public build, we activate a maven profile overriding these properties to use an existent certificate used by all of our applications.

Here are some snippets of our configuration files:

pom.xml - maven-webstart-plugin configuration

  
<configuration>
	<sign>
		<keystore>${gemserk.keystore}</keystore>
		<keypass>${gemserk.keypass}</keypass>
		<storepass>${gemserk.storepass}</storepass>
		<alias>${gemserk.alias}</alias>

		<!-- default values if gen is true -->
		<validity>3560</validity>
		<dnameCn>Gemserk</dnameCn>
		<dnameOu>Gemserk</dnameOu>
		<dnameO>Gemserk</dnameO>
		<dnameL>Montevideo</dnameL>
		<dnameSt>Montevideo</dnameSt>
		<dnameC>UY</dnameC>

		<verify>true</verify>

		<keystoreConfig>
			<delete>${gemserk.keystore.delete}</delete>
			<gen>${gemserk.keystore.gen}</gen>
		</keystoreConfig>
	</sign>
</configuration>

We use our company name as a prefix for the property keys in order to have a common scope when setting the values.

pom.xml - default values

  
<properties>
	<!-- Properties for keystore generation  -->
	<gemserk.keystore>/tmp/keystore-gemserk</gemserk.keystore>
	<gemserk.keypass>m2m2m2</gemserk.keypass>
	<gemserk.storepass>m2m2m2</gemserk.storepass>
	<gemserk.alias>gemserk.com</gemserk.alias>
	<gemserk.keystore.delete>true</gemserk.keystore.delete>
	<gemserk.keystore.gen>true</gemserk.keystore.gen>
</properties>

settings.xml - profile declaration

  
<profile>
	<id>useDeploymentCertificate</id>
	<properties>
		<gemserk.keystore>/opt/gemserk-keystore</gemserk.keystore>
		<gemserk.keypass>password</gemserk.keypass>
		<gemserk.storepass>password</gemserk.storepass>
		<gemserk.alias>gemserk.com</gemserk.alias>
		<gemserk.keystore.delete>false</gemserk.keystore.delete>
		<gemserk.keystore.gen>false</gemserk.keystore.gen>
	</properties>
</profile>

In order to build an application for deployment, you can activate this profile from the command line using:

mvn package -PuseDeploymentCertificate

If you have any questions leave a comment.

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