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25. September 2014

 

So far we’ve covered creating a simple simulation then how to create physics bodies and apply forces to them now we look at dealing with collisions.  Truth of the matter is, most of the work is just done for us.  Let’s just right in with an example, heavily copied and pasted from the previous example.

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input;
import com.badlogic.gdx.InputProcessor;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.BitmapFont;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.Matrix4;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.*;

public class Physics2 extends ApplicationAdapter implements InputProcessor {
    SpriteBatch batch;
    Sprite sprite;
    Texture img;
    World world;
    Body body;
    Body bodyEdgeScreen;
    Box2DDebugRenderer debugRenderer;
    Matrix4 debugMatrix;
    OrthographicCamera camera;
    BitmapFont font;


    float torque = 0.0f;
    boolean drawSprite = true;

    final float PIXELS_TO_METERS = 100f;

    @Override
    public void create() {

        batch = new SpriteBatch();
        img = new Texture("badlogic.jpg");
        sprite = new Sprite(img);

        sprite.setPosition(-sprite.getWidth()/2,-sprite.getHeight()/2);

        world = new World(new Vector2(0, -1f),true);

        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        bodyDef.position.set((sprite.getX() + sprite.getWidth()/2) /
                        PIXELS_TO_METERS,
                (sprite.getY() + sprite.getHeight()/2) / PIXELS_TO_METERS);

        body = world.createBody(bodyDef);

        PolygonShape shape = new PolygonShape();
        shape.setAsBox(sprite.getWidth()/2 / PIXELS_TO_METERS, sprite.getHeight()
                /2 / PIXELS_TO_METERS);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = shape;
        fixtureDef.density = 0.1f;
        fixtureDef.restitution = 0.5f;

        body.createFixture(fixtureDef);
        shape.dispose();

        BodyDef bodyDef2 = new BodyDef();
        bodyDef2.type = BodyDef.BodyType.StaticBody;
        float w = Gdx.graphics.getWidth()/PIXELS_TO_METERS;
        // Set the height to just 50 pixels above the bottom of the screen so we can see the edge in the
        // debug renderer
        float h = Gdx.graphics.getHeight()/PIXELS_TO_METERS- 50/PIXELS_TO_METERS;
        //bodyDef2.position.set(0,
//                h-10/PIXELS_TO_METERS);
        bodyDef2.position.set(0,0);
        FixtureDef fixtureDef2 = new FixtureDef();

        EdgeShape edgeShape = new EdgeShape();
        edgeShape.set(-w/2,-h/2,w/2,-h/2);
        fixtureDef2.shape = edgeShape;

        bodyEdgeScreen = world.createBody(bodyDef2);
        bodyEdgeScreen.createFixture(fixtureDef2);
        edgeShape.dispose();

        Gdx.input.setInputProcessor(this);

        debugRenderer = new Box2DDebugRenderer();
        font = new BitmapFont();
        font.setColor(Color.BLACK);
        camera = new OrthographicCamera(Gdx.graphics.getWidth(),Gdx.graphics.
                getHeight());
    }

    private float elapsed = 0;
    @Override
    public void render() {
        camera.update();
        // Step the physics simulation forward at a rate of 60hz
        world.step(1f/60f, 6, 2);

        body.applyTorque(torque,true);

        sprite.setPosition((body.getPosition().x * PIXELS_TO_METERS) - sprite.
                        getWidth()/2 ,
                (body.getPosition().y * PIXELS_TO_METERS) -sprite.getHeight()/2 )
        ;
        sprite.setRotation((float)Math.toDegrees(body.getAngle()));

        Gdx.gl.glClearColor(1, 1, 1, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        batch.setProjectionMatrix(camera.combined);
        debugMatrix = batch.getProjectionMatrix().cpy().scale(PIXELS_TO_METERS,
                PIXELS_TO_METERS, 0);
        batch.begin();

        if(drawSprite)
            batch.draw(sprite, sprite.getX(), sprite.getY(),sprite.getOriginX(),
                    sprite.getOriginY(),
                    sprite.getWidth(),sprite.getHeight(),sprite.getScaleX(),sprite.
                            getScaleY(),sprite.getRotation());

        font.draw(batch,
                "Restitution: " + body.getFixtureList().first().getRestitution(),
                -Gdx.graphics.getWidth()/2,
               Gdx.graphics.getHeight()/2 );
        batch.end();

        debugRenderer.render(world, debugMatrix);
    }

    @Override
    public void dispose() {
        img.dispose();
        world.dispose();
    }

    @Override
    public boolean keyDown(int keycode) {
        return false;
    }

    @Override
    public boolean keyUp(int keycode) {


        if(keycode == Input.Keys.RIGHT)
            body.setLinearVelocity(1f, 0f);
        if(keycode == Input.Keys.LEFT)
            body.setLinearVelocity(-1f,0f);

        if(keycode == Input.Keys.UP)
            body.applyForceToCenter(0f,10f,true);
        if(keycode == Input.Keys.DOWN)
            body.applyForceToCenter(0f, -10f, true);

        // On brackets ( [ ] ) apply torque, either clock or counterclockwise
        if(keycode == Input.Keys.RIGHT_BRACKET)
            torque += 0.1f;
        if(keycode == Input.Keys.LEFT_BRACKET)
            torque -= 0.1f;

        // Remove the torque using backslash /
        if(keycode == Input.Keys.BACKSLASH)
            torque = 0.0f;

        // If user hits spacebar, reset everything back to normal
        if(keycode == Input.Keys.SPACE|| keycode == Input.Keys.NUM_2) {
            body.setLinearVelocity(0f, 0f);
            body.setAngularVelocity(0f);
            torque = 0f;
            sprite.setPosition(0f,0f);
            body.setTransform(0f,0f,0f);
        }

        if(keycode == Input.Keys.COMMA) {
            body.getFixtureList().first().setRestitution(body.getFixtureList().first().getRestitution()-0.1f);
        }
        if(keycode == Input.Keys.PERIOD) {
            body.getFixtureList().first().setRestitution(body.getFixtureList().first().getRestitution()+0.1f);
        }
        if(keycode == Input.Keys.ESCAPE || keycode == Input.Keys.NUM_1)
            drawSprite = !drawSprite;

        return true;
    }

    @Override
    public boolean keyTyped(char character) {
        return false;
    }


    // On touch we apply force from the direction of the users touch.
    // This could result in the object "spinning"
    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        body.applyForce(1f,1f,screenX,screenY,true);
        //body.applyTorque(0.4f,true);
        return true;
    }

    @Override
    public boolean touchUp(int screenX, int screenY, int pointer, int button) {
        return false;
    }

    @Override
    public boolean touchDragged(int screenX, int screenY, int pointer) {
        return false;
    }

    @Override
    public boolean mouseMoved(int screenX, int screenY) {
        return false;
    }

    @Override
    public boolean scrolled(int amount) {
        return false;
    }
}

 

Here is the application running

Click to open in new Window (It's probably already run by the time you scrolled down)


You can control it using the following controls:

  • Left and Right Arrow:  Apply impulse along X axis
  • Up and Down Arrow: Apply force along Y axis.
  • [ and ]: Apply torque
  • \: Set torque to 0
  • SPACEBAR or '2': Reset all
  • Click, apply force from point of click
  • ESC or '1' toggles display of the sprite graphic on and off

 

This is basically the same application we created in the previous example, except now there is actually something to collide with.  In this case we created a physics body defined by an edge shape just 50 pixels above the bottom of the screen.  The key difference between our two bodies is bodyEdgeScreen has it’s physics type set to StaticBody.  A static body is exactly what it’s name implies, static, meaning that it doesn’t move.  It participates in the physics simulation ( stuff can hit it ), but it is un affected by the simulation itself.  If this body wasn’t static, gravity would have grabbed it and set it free falling in space.  As you can see, it is created the same way as our other physics body.  The only other major difference is instead of being defined in the physics engine by a box, it is instead represented as an edge defined by two points.  This means our EdgeShape has no volume.

 

Now that we have collisions occurring a few values became much more important, density, restitution (and not shown, friction).  These are set on the FixtureDef of your Body.  We touched briefly on density in the previous tutorials, it helps determine the overall mass of your object along with the shape.  Restitution can be thought of as, for the lack of a better term, as bounciness.  Restitution helps determine how one body will respond to contact with another physics object.  Rubber for example would have a high restitution, while rock would have a very low restitution.  Friction on the other hand determines how one body slides across another body.

 

What happens if you want to perform some kind of operation when a collision occurs?  Well fortunately this is quite simple, let’s take a look at another heavily cut and paste example!

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.MathUtils;
import com.badlogic.gdx.math.Matrix4;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.*;

public class Physics3 extends ApplicationAdapter {
    SpriteBatch batch;
    Sprite sprite,sprite2;
    Texture img;
    World world;
    Body body,body2;
    Body bodyEdgeScreen;

    Matrix4 debugMatrix;
    OrthographicCamera camera;

    final float PIXELS_TO_METERS = 100f;

    @Override
    public void create() {
        batch = new SpriteBatch();
        img = new Texture("badlogic.jpg");

        // Create two identical sprites slightly offset from each other vertically
        sprite = new Sprite(img);
        sprite.setPosition(-sprite.getWidth()/2,-sprite.getHeight()/2 +200);
        sprite2 = new Sprite(img);
        sprite2.setPosition(-sprite.getWidth()/2 + 20,-sprite.getHeight()/2 + 400);

        world = new World(new Vector2(0, -1f),true);

        // Sprite1's Physics body
        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        bodyDef.position.set((sprite.getX() + sprite.getWidth()/2) /
                        PIXELS_TO_METERS,
                (sprite.getY() + sprite.getHeight()/2) / PIXELS_TO_METERS);


        body = world.createBody(bodyDef);

        // Sprite2's physics body
        BodyDef bodyDef2 = new BodyDef();
        bodyDef2.type = BodyDef.BodyType.DynamicBody;
        bodyDef2.position.set((sprite2.getX() + sprite2.getWidth()/2) /
                        PIXELS_TO_METERS,
                (sprite2.getY() + sprite2.getHeight()/2) / PIXELS_TO_METERS);

        body2 = world.createBody(bodyDef2);

        // Both bodies have identical shape
        PolygonShape shape = new PolygonShape();
        shape.setAsBox(sprite.getWidth()/2 / PIXELS_TO_METERS, sprite.getHeight()
                /2 / PIXELS_TO_METERS);

        // Sprite1
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = shape;
        fixtureDef.density = 0.1f;
        fixtureDef.restitution = 0.5f;


        // Sprite2
        FixtureDef fixtureDef2 = new FixtureDef();
        fixtureDef2.shape = shape;
        fixtureDef2.density = 0.1f;
        fixtureDef2.restitution = 0.5f;

        body.createFixture(fixtureDef);
        body2.createFixture(fixtureDef2);

        shape.dispose();

        // Now the physics body of the bottom edge of the screen
        BodyDef bodyDef3 = new BodyDef();
        bodyDef3.type = BodyDef.BodyType.StaticBody;
        float w = Gdx.graphics.getWidth()/PIXELS_TO_METERS;
        float h = Gdx.graphics.getHeight()/PIXELS_TO_METERS;

        bodyDef3.position.set(0,0);
        FixtureDef fixtureDef3 = new FixtureDef();

        EdgeShape edgeShape = new EdgeShape();
        edgeShape.set(-w/2,-h/2,w/2,-h/2);
        fixtureDef3.shape = edgeShape;


        bodyEdgeScreen = world.createBody(bodyDef3);
        bodyEdgeScreen.createFixture(fixtureDef3);
        edgeShape.dispose();

        camera = new OrthographicCamera(Gdx.graphics.getWidth(),Gdx.graphics.
                getHeight());

        world.setContactListener(new ContactListener() {
            @Override
            public void beginContact(Contact contact) {
                // Check to see if the collision is between the second sprite and the bottom of the screen
                // If so apply a random amount of upward force to both objects... just because
                if((contact.getFixtureA().getBody() == bodyEdgeScreen &&
                        contact.getFixtureB().getBody() == body2)
                        ||
                        (contact.getFixtureA().getBody() == body2 &&
                                contact.getFixtureB().getBody() == bodyEdgeScreen)) {

                    body.applyForceToCenter(0,MathUtils.random(20,50),true);
                    body2.applyForceToCenter(0, MathUtils.random(20,50), true);
                }
            }

            @Override
            public void endContact(Contact contact) {
            }

            @Override
            public void preSolve(Contact contact, Manifold oldManifold) {
            }

            @Override
            public void postSolve(Contact contact, ContactImpulse impulse) {
            }
        });
    }

    @Override
    public void render() {
        camera.update();
        // Step the physics simulation forward at a rate of 60hz
        world.step(1f/60f, 6, 2);

        sprite.setPosition((body.getPosition().x * PIXELS_TO_METERS) - sprite.
                        getWidth()/2 ,
                (body.getPosition().y * PIXELS_TO_METERS) -sprite.getHeight()/2 );


        sprite.setRotation((float)Math.toDegrees(body2.getAngle()));
        sprite2.setPosition((body2.getPosition().x * PIXELS_TO_METERS) - sprite2.
                        getWidth()/2 ,
                (body2.getPosition().y * PIXELS_TO_METERS) -sprite2.getHeight()/2 );
        sprite2.setRotation((float)Math.toDegrees(body.getAngle()));

        Gdx.gl.glClearColor(1, 1, 1, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        batch.setProjectionMatrix(camera.combined);
        batch.begin();

        batch.draw(sprite, sprite.getX(), sprite.getY(),sprite.getOriginX(),
                sprite.getOriginY(),
                sprite.getWidth(),sprite.getHeight(),sprite.getScaleX(),sprite.
                        getScaleY(),sprite.getRotation());
        batch.draw(sprite2, sprite2.getX(), sprite2.getY(),sprite2.getOriginX(),
                sprite2.getOriginY(),
                sprite2.getWidth(),sprite2.getHeight(),sprite2.getScaleX(),sprite2.
                        getScaleY(),sprite2.getRotation());
        batch.end();
    }

    @Override
    public void dispose() {
        img.dispose();
        world.dispose();
    }
}

 

And when you run it

 

Click to open in new Window (It's probably already run by the time you scrolled down)


 

In this example we’ve added another dynamic physics body.  Both bodies fall thanks to gravity, however when the second physics body makes contact with the “ground”, an upward force is applied to both bodies.

 

The heart of this example is the ContactListener we added to our physics world.  This handler is called for each collision that occurred in the world and allows you to handle the event before and after contact, then before and after the physics engine does the calculations on what to do.  In this example we used the beginContact callback, although truth is, all 4 would have behaved identically.  In each case a Contact object is passed in, which has details on the collision such as the friction, restitution and speed of the collision.  In this case we actually want to check what types of objects collided, which we do using getFixtureA() and getFixtureB() which returns the FixtureDef of the colliding objects.  Since we don’t actually know which collides with which, we test for both directions of collisions.  If the bodies colliding are Body2 and bodyEdgeScreen, we apply our upward force.  Obviously you could handle whatever kind of calculation you wanted at this point.

 

One very common activity, and something I didn’t cover in this example is tying a physics body back to the object in the real world that this body represents.  In this particular example we’ve simply hard coded things, but in a real life application this isn’t an option.  Fortunately there is an easy elegant solution.  Each physics Body has a property called UserData, which can be basically whatever you want it to be.  Let’s take a quick look at storing the game sprite using UserData of the physics Body.

        //Create the body
        body = world.createBody(bodyDef);
        //Store the sprite the body represents in UserData
        body.setUserData(sprite);
        //Access the sprite
        ((Sprite)body.getUserData()).setPosition(body.getPosition().x,body.getPosition().y);

 

This provides an easy way to link the object represented with the physics entity itself.

 

So far we covered dynamic objects, which can be moved and positioned and participate in the physics simulation, and static bodies, that do not move but can participate in the simulation, what what happens if you want a body to be aware of the physics simulation, but not effect it.  For example, what if you wanted to trigger some code when a physics body went off screen, but you didn’t want this to cause a collision.  There is a very simple solution to this, sensors.

        fixtureDef.isSensor = true;

 

This is all you need to do an now your physics object will no longer cause a reaction when collided with, except whatever you provide in code.  For example, in the earlier example, if you changed fixtureDef3 to be a sensor, the first body will simply fall through it.  However when the second body hits it, it will trigger the code in our ContactListener.

 

If you thought to yourself, wow, that’s an ugly way to handle tests for collisions between different objects… that whole test in both directions thing will get awfully ugly quickly.  You are 100% correct, fortunately there is a clean solution I will cover shortly.

Programming


22. September 2014

 

 

Today we are going to look at adding game pad support into LibGDX, specifically the XBox 360 controller.  Why the Xbox 360 controller?  Well, it’s the controller that I ( and 90% of other PC gamers it seems ) own.  You should be able to modify the following to work with any gamepad, but it’s left as an exercise for the viewer.

 

There are two things we need to be aware of right away.  First, Controller support is via an extension, so when you are creating your project, you need to select Controller like so:

libgdxController

 

Next, LibGDX doesn’t actually support the 360 controller out of the box, it only ships with mappings for the Ouya controller.  Fortunately, thanks to the power of Google, I found someone else to do the work for us!  There is a code sample midway through this thread.  Download the included source and save it to a file named XBox360Pad.java.  Yes, case is important.  So it should look like this:

 

package com.gamefromscratch;

import com.badlogic.gdx.controllers.PovDirection;

// This code was taken from http://www.java-gaming.org/index.php?topic=29223.0
// With thanks that is!

public class XBox360Pad
{
    /*
     * It seems there are different versions of gamepads with different ID 
     Strings.
     * Therefore its IMO a better bet to check for:
     * if (controller.getName().toLowerCase().contains("xbox") &&
                   controller.getName().contains("360"))
     *
     * Controller (Gamepad for Xbox 360)
       Controller (XBOX 360 For Windows)
       Controller (Xbox 360 Wireless Receiver for Windows)
       Controller (Xbox wireless receiver for windows)
       XBOX 360 For Windows (Controller)
       Xbox 360 Wireless Receiver
       Xbox Receiver for Windows (Wireless Controller)
       Xbox wireless receiver for windows (Controller)
     */
    //public static final String ID = "XBOX 360 For Windows (Controller)";
    public static final int BUTTON_X = 2;
    public static final int BUTTON_Y = 3;
    public static final int BUTTON_A = 0;
    public static final int BUTTON_B = 1;
    public static final int BUTTON_BACK = 6;
    public static final int BUTTON_START = 7;
    public static final PovDirection BUTTON_DPAD_UP = PovDirection.north;
    public static final PovDirection BUTTON_DPAD_DOWN = PovDirection.south;
    public static final PovDirection BUTTON_DPAD_RIGHT = PovDirection.east;
    public static final PovDirection BUTTON_DPAD_LEFT = PovDirection.west;
    public static final int BUTTON_LB = 4;
    public static final int BUTTON_L3 = 8;
    public static final int BUTTON_RB = 5;
    public static final int BUTTON_R3 = 9;
    public static final int AXIS_LEFT_X = 1; //-1 is left | +1 is right
    public static final int AXIS_LEFT_Y = 0; //-1 is up | +1 is down
    public static final int AXIS_LEFT_TRIGGER = 4; //value 0 to 1f
    public static final int AXIS_RIGHT_X = 3; //-1 is left | +1 is right
    public static final int AXIS_RIGHT_Y = 2; //-1 is up | +1 is down
    public static final int AXIS_RIGHT_TRIGGER = 4; //value 0 to -1f
}

 

Now let's jump right in with a sample:

 

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.controllers.Controller;
import com.badlogic.gdx.controllers.ControllerListener;
import com.badlogic.gdx.controllers.Controllers;
import com.badlogic.gdx.controllers.PovDirection;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.BitmapFont;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.Vector3;

public class Gamepad extends ApplicationAdapter implements ControllerListener {
   SpriteBatch batch;
   Sprite sprite;
   BitmapFont font;
   boolean hasControllers = true;
   String message = "Please install a controller";

   @Override
   public void create () {
      batch = new SpriteBatch();
      sprite = new Sprite(new Texture("badlogic.jpg"));
        sprite.setPosition(Gdx.graphics.getWidth()/2 -sprite.getWidth()/2,
                           Gdx.graphics.getHeight()/2-sprite.getHeight()/2);

        // Listen to all controllers, not just one
        Controllers.addListener(this);

        font = new BitmapFont();
        font.setColor(Color.WHITE);


        if(Controllers.getControllers().size == 0)
        {
            hasControllers = false;
        }
    }

   @Override
   public void render () {
      Gdx.gl.glClearColor(0, 0, 0, 0);
      Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
      batch.begin();
        if(!hasControllers)
            font.draw(batch,message,
                    Gdx.graphics.getWidth()/2 - font.getBounds(message).width/2,
                    Gdx.graphics.getHeight()/2 - font.getBounds(message).height/2);
        else
          batch.draw(sprite, sprite.getX(), sprite.getY(),sprite.getOriginX(),sprite.getOriginY(),
                    sprite.getWidth(),sprite.getHeight(),
                    sprite.getScaleX(),sprite.getScaleY(),sprite.getRotation());
      batch.end();
   }

    // connected and disconnect dont actually appear to work for XBox 360 controllers.
    @Override
    public void connected(Controller controller) {
        hasControllers = true;
    }

    @Override
    public void disconnected(Controller controller) {
        hasControllers = false;
    }

    @Override
    public boolean buttonDown(Controller controller, int buttonCode) {
        if(buttonCode == XBox360Pad.BUTTON_Y)
            sprite.setY(sprite.getY() + 1);
        if(buttonCode == XBox360Pad.BUTTON_A)
            sprite.setY(sprite.getY()-1);
        if(buttonCode == XBox360Pad.BUTTON_X)
            sprite.setX(sprite.getX() - 1);
        if(buttonCode == XBox360Pad.BUTTON_B)
            sprite.setX(sprite.getX() + 1);

        if(buttonCode == XBox360Pad.BUTTON_LB)
            sprite.scale(-0.1f);
        if(buttonCode == XBox360Pad.BUTTON_RB)
            sprite.scale(0.1f);
        return false;
    }

    @Override
    public boolean buttonUp(Controller controller, int buttonCode) {
        return false;
    }

    @Override
    public boolean axisMoved(Controller controller, int axisCode, float value) {
        // This is your analog stick
        // Value will be from -1 to 1 depending how far left/right, up/down the stick is
        // For the Y translation, I use a negative because I like inverted analog stick
        // Like all normal people do! ;)

        // Left Stick
        if(axisCode == XBox360Pad.AXIS_LEFT_X)
            sprite.translateX(10f * value);
        if(axisCode == XBox360Pad.AXIS_LEFT_Y)
            sprite.translateY(-10f * value);

        // Right stick
        if(axisCode == XBox360Pad.AXIS_RIGHT_X)
            sprite.rotate(10f * value);
        return false;
    }

    @Override
    public boolean povMoved(Controller controller, int povCode, PovDirection value) {
        // This is the dpad
        if(value == XBox360Pad.BUTTON_DPAD_LEFT)
            sprite.translateX(-10f);
        if(value == XBox360Pad.BUTTON_DPAD_RIGHT)
            sprite.translateX(10f);
        if(value == XBox360Pad.BUTTON_DPAD_UP)
            sprite.translateY(10f);
        if(value == XBox360Pad.BUTTON_DPAD_DOWN)
            sprite.translateY(-10f);
        return false;
    }

    @Override
    public boolean xSliderMoved(Controller controller, int sliderCode, boolean value) {
        return false;
    }

    @Override
    public boolean ySliderMoved(Controller controller, int sliderCode, boolean value) {
        return false;
    }

    @Override
    public boolean accelerometerMoved(Controller controller, int accelerometerCode, Vector3 value) {
        return false;
    }
}

 

Amazingly enough, LibGDX actually manages to support controllers in their HTML target, so below is the following code running.  Maybe.

 

(Open in new window)

 

Of course, this is gamepad support in HTML5 we are talking about here, so it will only work in a very small subset of browsers and certainly may not work as expected.  However, the fact it runs at all is rather astonishing.  You may need to open the examples in a separate window using the link above to get it to work correctly.  No worries though, on a Desktop target, the above code works perfectly.

 

In this sample we are taking an event driven approach.  That is, as the controller is updated, it pushes a variety of events to our class.  This is done via the ControllerListener interface.  As you can see from the override’s, there is a great deal of functionality ( motion, sliders, etc ) that is OUYA specific.  Of interest to us are buttonDown, axisMoved and povMoved.  buttonDown is called predictably enough when one of the controller buttons is pressed, this includes face buttons, select, start and the bumpers, but not the triggers.  axisMoved is called for either of the analog sticks, and perhaps confusingly at first, the triggers are moved.  The reason triggers are supported this way is do the the fact there is a range of values instead of just a binary option like when dealing with buttons.  The amount the trigger is depressed is the range along the trigger axis.  Finally there is povMoved, this is your DPad, which really is just a set of 4 buttons.  One last thing to note here… the disconnect and connect events simply never fired for me, the logic may be OUYA specific.

 

You may notice however that movement using the analog stick is jerky as hell.  This is because event driven approach isn’t really ideal for analog controls.  You have two options here.  Instead of updating the controls each time an event is fired, you update a flag and apply some smoothing logic, to keep movement between events smooth.  Or, much easier, you use polling instead.  Let’s take a look at how you can poll controls in LibGDX.

 

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.controllers.Controller;
import com.badlogic.gdx.controllers.Controllers;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.BitmapFont;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;

public class Gamepad2 extends ApplicationAdapter {
    SpriteBatch batch;
    Sprite sprite;
    Controller controller;
    BitmapFont font;
    boolean hasControllers = true;
    String message = "Please install a controller";

    @Override
    public void create () {
        batch = new SpriteBatch();
        sprite = new Sprite(new Texture("badlogic.jpg"));
        sprite.setPosition(Gdx.graphics.getWidth()/2 -sprite.getWidth()/2,
                Gdx.graphics.getHeight()/2-sprite.getHeight()/2);

        font = new BitmapFont();
        font.setColor(Color.WHITE);

        if(Controllers.getControllers().size == 0)
        {
            hasControllers = false;
        }
        else
            controller = Controllers.getControllers().first();
    }

    @Override
    public void render () {
        Gdx.gl.glClearColor(0, 0, 0, 0);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        batch.begin();
        if(!hasControllers)
            font.draw(batch,message,
                    Gdx.graphics.getWidth()/2 - font.getBounds(message).width/2,
                    Gdx.graphics.getHeight()/2 - font.getBounds(message).height/2);
        else {
            // Update movement based on movement of left stick
            // Give a "deadzone" of 0.2 - -0.2, meaning the first 20% in either direction will be ignored.
            // This keeps controls from being too twitchy
            // Move by up to 10 pixels per frame if full left or right.
            // Once again I flipped the sign on the Y axis because I prefer inverted Y axis controls.
            if(controller.getAxis(XBox360Pad.AXIS_LEFT_X) > 0.2f  || 
                    controller.getAxis(XBox360Pad.AXIS_LEFT_X) < -0.2f)
                sprite.translateX(controller.getAxis(XBox360Pad.AXIS_LEFT_X) * 10f);
            if(controller.getAxis(XBox360Pad.AXIS_LEFT_Y) > 0.2f  || 
                    controller.getAxis(XBox360Pad.AXIS_LEFT_Y) < -0.2f)
                sprite.translateY(controller.getAxis(XBox360Pad.AXIS_LEFT_Y) * -10f);

            // Poll if user hits start button, if they do, reset position of sprite
            if(controller.getButton(XBox360Pad.BUTTON_START))
                sprite.setPosition(Gdx.graphics.getWidth() / 2 - sprite.getWidth() / 2,
                        Gdx.graphics.getHeight() / 2 - sprite.getHeight() / 2);
            batch.draw(sprite, sprite.getX(), sprite.getY(), sprite.getOriginX(), sprite.getOriginY(), 
                    sprite.getWidth(),sprite.getHeight(), 
                    sprite.getScaleX(), sprite.getScaleY(), sprite.getRotation());
        }
        batch.end();
    }
}

 

Now here is this code running ( sorta, maybe )

(Open in new window)

 

Here you can see we simply poll the controller for it’s status each frame.  getAxis returns the amount the controller is depressed in either direction along that axis as a value from –1 to 1.  getButton on the other hand returns a boolean representing if the button is currently pressed or not.  One important thing to keep in mind here, this code is incredibly fragile.  LibGDX controller supports multiple controllers, but in this case I simply check to see if any exist and use the first one I can find.  This means if you have multiple controllers plugged into your PC, all but the first one will be ignored.  Second, this code simply assumes the controller is an XBox 360 controller, no idea what will happen if another controller is used instead.  Most likely the worst case scenarios is buttons might be mismapped or non-existent.

 

The only other thing of note ( and mentioned in the comments ) is I applied a dead zone value of –0.2 to 0.2 for each analog stick.  This keeps the controller from being overly twitchy and from moving when the user would think the control should be still.  Generally this dead zone value is what you would configure via a sensitivity setting in your game’s settings.  I also flipped the Y axis value because, well, that’s the way it should be! :)

 

Programming


10. September 2014

 

In the previous tutorial part we look at creating about the simplest physics simulation possible in LibGDX and Box2D, a single object affected by gravity.  In this tutorial we are going to take things one step further and apply force, impulses and torque ( and no gravity ) to our physics body.  This part is going to consist of a single large code example.

 

Speaking of which, here it is!

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input;
import com.badlogic.gdx.InputProcessor;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.Matrix4;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.*;

public class Physics2 extends ApplicationAdapter implements InputProcessor {
    SpriteBatch batch;
    Sprite sprite;
    Texture img;
    World world;
    Body body;
    Box2DDebugRenderer debugRenderer;
    Matrix4 debugMatrix;
    OrthographicCamera camera;
    
    
    float torque = 0.0f;
    boolean drawSprite = true;
    
    final float PIXELS_TO_METERS = 100f;

    @Override
    public void create() {

        batch = new SpriteBatch();
        img = new Texture("badlogic.jpg");
        sprite = new Sprite(img);

        sprite.setPosition(-sprite.getWidth()/2,-sprite.getHeight()/2);

        world = new World(new Vector2(0, 0f),true);
        
        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        bodyDef.position.set((sprite.getX() + sprite.getWidth()/2) / 
                             PIXELS_TO_METERS, 
                (sprite.getY() + sprite.getHeight()/2) / PIXELS_TO_METERS);

        body = world.createBody(bodyDef);

        PolygonShape shape = new PolygonShape();
        shape.setAsBox(sprite.getWidth()/2 / PIXELS_TO_METERS, sprite.getHeight()
                       /2 / PIXELS_TO_METERS);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = shape;
        fixtureDef.density = 0.1f;

        body.createFixture(fixtureDef);
        shape.dispose();
        
        Gdx.input.setInputProcessor(this);
        
        // Create a Box2DDebugRenderer, this allows us to see the physics 
        simulation controlling the scene
        debugRenderer = new Box2DDebugRenderer();
        camera = new OrthographicCamera(Gdx.graphics.getWidth(),Gdx.graphics.
                 getHeight());
    }

    private float elapsed = 0;
    @Override
    public void render() {
        camera.update();
        // Step the physics simulation forward at a rate of 60hz
        world.step(1f/60f, 6, 2);
        
        // Apply torque to the physics body.  At start this is 0 and will do 
        nothing.  Controlled with [] keys
        // Torque is applied per frame instead of just once
        body.applyTorque(torque,true);
        
        // Set the sprite's position from the updated physics body location
        sprite.setPosition((body.getPosition().x * PIXELS_TO_METERS) - sprite.
                           getWidth()/2 , 
                (body.getPosition().y * PIXELS_TO_METERS) -sprite.getHeight()/2 )
                 ;
        // Ditto for rotation
        sprite.setRotation((float)Math.toDegrees(body.getAngle()));

        Gdx.gl.glClearColor(1, 1, 1, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        
        batch.setProjectionMatrix(camera.combined);
        
        // Scale down the sprite batches projection matrix to box2D size
        debugMatrix = batch.getProjectionMatrix().cpy().scale(PIXELS_TO_METERS, 
                      PIXELS_TO_METERS, 0);
        
        batch.begin();
        
        if(drawSprite)
            batch.draw(sprite, sprite.getX(), sprite.getY(),sprite.getOriginX(),
                       sprite.getOriginY(),
                sprite.getWidth(),sprite.getHeight(),sprite.getScaleX(),sprite.
                                getScaleY(),sprite.getRotation());
                        
        batch.end();
        
        // Now render the physics world using our scaled down matrix
        // Note, this is strictly optional and is, as the name suggests, just 
        for debugging purposes
        debugRenderer.render(world, debugMatrix);
    }

    @Override
    public void dispose() {
        img.dispose();
        world.dispose();
    }

    @Override
    public boolean keyDown(int keycode) {
        return false;
    }

    @Override
    public boolean keyUp(int keycode) {
        
        // On right or left arrow set the velocity at a fixed rate in that 
        direction
        if(keycode == Input.Keys.RIGHT) 
            body.setLinearVelocity(1f, 0f);
        if(keycode == Input.Keys.LEFT)
            body.setLinearVelocity(-1f,0f);

        if(keycode == Input.Keys.UP)
            body.applyForceToCenter(0f,10f,true);
        if(keycode == Input.Keys.DOWN)
            body.applyForceToCenter(0f, -10f, true);
        
        // On brackets ( [ ] ) apply torque, either clock or counterclockwise
        if(keycode == Input.Keys.RIGHT_BRACKET)
            torque += 0.1f;
        if(keycode == Input.Keys.LEFT_BRACKET)
            torque -= 0.1f;
        
        // Remove the torque using backslash /
        if(keycode == Input.Keys.BACKSLASH)
            torque = 0.0f;
        
        // If user hits spacebar, reset everything back to normal
        if(keycode == Input.Keys.SPACE) {
            body.setLinearVelocity(0f, 0f);
            body.setAngularVelocity(0f);
            torque = 0f;
            sprite.setPosition(0f,0f);
            body.setTransform(0f,0f,0f);
        }
        
        // The ESC key toggles the visibility of the sprite allow user to see 
        physics debug info
        if(keycode == Input.Keys.ESCAPE)
            drawSprite = !drawSprite;

        return true;
    }

    @Override
    public boolean keyTyped(char character) {
        return false;
    }

    
    // On touch we apply force from the direction of the users touch.
    // This could result in the object "spinning"
    @Override
    public boolean touchDown(int screenX, int screenY, int pointer, int button) {
        body.applyForce(1f,1f,screenX,screenY,true);
        //body.applyTorque(0.4f,true);
        return true;
    }

    @Override
    public boolean touchUp(int screenX, int screenY, int pointer, int button) {
        return false;
    }

    @Override
    public boolean touchDragged(int screenX, int screenY, int pointer) {
        return false;
    }

    @Override
    public boolean mouseMoved(int screenX, int screenY) {
        return false;
    }

    @Override
    public boolean scrolled(int amount) {
        return false;
    }
}

 

Here it is running in your browser.  You may have to click the application to give it focus.

 

 

You can control it using the following controls:

  • Left and Right Arrow:  Apply impulse along X axis
  • Up and Down Arrow: Apply force along Y axis.
  • [ and ]: Apply torque
  • \: Set torque to 0
  • SPACEBAR or '2': Reset all
  • Click, apply force from point of click
  • ESC or '1' toggles display of the sprite graphic on and off

EDIT: OK, several keyboard combo’s do not work correctly in an iframe.  To fully interact with the above application click here to open it in it’s own window:

 

 

You may notice applying force will cause your object to not just move, but also rotate, depending on the angle you click.  You can also offset this force by clicking in the mirrored opposite side.  Also notice that applying impulse moves the object along at a constant rate, and applying the opposite impulse will bring the object to a standstill.  Hitting left or right multiple times however will not increase the speed.  However, applying force, by hitting up or down will cause it to get faster for each time you add force.  This is one of the major differences between impulse and force.  Torque works very similar,  to stop it from spinning you need to apply the appropriate amount of torque from the opposite direction.  Even setting torque to 0 will have little effect due to momentum.

 

Now let’s take a look at a few key concepts here.  First thing you should notice is:

final float PIXELS_TO_METERS = 100f;

 

Well, remember when I said in the last part that the units you use don’t really matter as long as you are consistent?  Well, let’s reclassify that as “kinda true”.  Or, completely true, but not really!  Clear now?

 

Truth of the matter is, Box2d is “tuned” to work in MKS units, which to you non-Canadian/European readers translates to Meters, Kilograms and Seconds.  Don’t get me wrong, you can still work in lbs, miles or parsecs, but you are going to fight with Box2D a bit.  Additionally, this blurb from Box2D documentation is also pretty important:

 

Box2D is tuned for MKS units. Keep the size of moving objects roughly between 0.1 and 10 meters. You'll need to use some scaling system when you render your environment and actors. The Box2D testbed does this by using an OpenGL viewport transform. DO NOT USE PIXELS.

 

So, what did I use in the prior example?  Yeah… pixels.  In reality though, when dealing with gravity only it really doesn’t matter all that much.  Once terminal velocity kicks in, all things fall at the same rate anyways.

 

But… now that we are going to working with forces and such, suddenly it becomes very important not to use pixels, and let me quickly explain why.  Consider when we create our physics object, we define it’s shape and density, like so:

 

shape.setAsBox(sprite.getWidth()/2, sprite.getHeight()/2);
FixtureDef fixtureDef = new FixtureDef();
fixtureDef.shape = shape;
fixtureDef.density = 1f;

 

Our sprite is an image 256x256 in size.  Our density, which by the way is by default kg/ms2 is 1.  As a direct result, in our simulation or sprite has a default mass of…  65,536kg.  Hmmm, that’s kind of heavy and is going to require a hell of a lot of force to move!  If you are trying out box2d and you are finding that force isn’t working as you’d expect, take a look at your units, that’s the most likely culprit.  A rather easy work around is to use one set of coordinates in Box2D and translate to and from pixels to meters and back again.  For that we use:

 

When translating to box coordinates, divide by this value, when translating back, multiply by it.  Basically what we are saying is 1 meter in box2d represents 100 pixels in our game.  So for our 256x256 image, in box it is roughly 2.5m x 2.5m instead of 250x250!  You will notice however that this isn’t the only translation we do:

 

To box2d:

        bodyDef.position.set((sprite.getX() + sprite.getWidth()/2) / 
                             PIXELS_TO_METERS, 
                (sprite.getY() + sprite.getHeight()/2) / PIXELS_TO_METERS);

From box2d:

        sprite.setPosition((body.getPosition().x * PIXELS_TO_METERS) - sprite.
                           getWidth()/2 , 
                (body.getPosition().y * PIXELS_TO_METERS) -sprite.getHeight()/2 )
                 ;

 

There is another calculation in there, each way we are either adding or subtracting half the sprites width and height.  This is due to box2d and LibGDX using two different coordinate systems.  In LibGDX, the sprite’s origin is it’s bottom left corner, while in box2d, it’s the middle.  You may be thinking, hey, I’ll just set the origin.  That wont do you any good actually, as in LibGDX this is only used for scaling and rotation.  So, when moving between the two coordinate systems be mindful of the different origins.

 

Otherwise the code is fairly straight forward.  From playing around with the application the difference between force and impulse is easy to see.  You can think of Force like the action of pushing someone.  It will require a higher value as you have to overcome their inertia, however pushing someone again will cause them to accelerate faster and faster.  Impulse on the other hand can be though of like the acceleration in a car.  A car with an impulse of 50km, moves along steadily at a rate of 50km.  The only way to change speeds would be to change impulse, the values would not be cumulative.  However, if another car rear ended you ( FORCE! ), you can certainly see an increase in velocity! ;)

 

Torque on the other hand is a measure of rotational force.  Unlike force and impulse, in Box2D, torque needs to be applied each frame.  Stop applying torque each frame and things stop, um, torqueing.  Finally, in terms of motion, the touch handler applies force from a given direction.  When the user presses up or down, the force was applied to the center of the object equally.  However, when the user touches or clicks the screen, the force is applied from the direction of contact.  Think just like real life, if you push a glass of beer at it’s center point, it will slide across the table.  However, push it at the top and it will probably tip.  You do have the option of turning a bodies rotation off if so required.

 

You can also set values direction, like I did when the user hits the space bar.  I set the position, torque, rotation and speed of the object back to zero.  This kind of stuff REALLY screws with Box2D though, so if you don’t have to, don’t directly manipulate values, instead work through forces.  In a simple simulation like this, its not really a big deal.

 

Finally you may notice I enabled the Box2DDebugRenderer.  In the running example, hit the ESC key ( you may have to click to give keyboard focus ) and you will see a rectangle in place of the sprite.  This is an internal representation of the physics body in box2d.  When debugging physics problems, this renderer can be invaluable.  Not however that we had to modify it’s projection matrix to correspond with our 100pixel per meter scaling factor.

 

Now that we’ve got things moving, in the next part we will look at what happens when they collide.

Programming


8. September 2014

 

The following is a guest tutorial by Avetis showing how to use LibGDX and Overlap2D to create an Angry Birds like game.  Overlap2D is a level and UI editor handy for separating game coding from content creation.  Keep in mind, Overlap2D is fairly young, currently at 0.0.4 version. Now, on with the tutorial!

 



Making Flappy Bird with Overlap2D – Overlappy Tutorial

 

 

Agenda


In this tutorial we are going to create a FlappyBird clone game (for learning purposes only) using Java, LibGDX framework and accelerate it all by moving our UI, Level, and Asset management into Overlap2D Editor. One thing I figured while coding is that you can’t make a quick tutorial about a flappy bird clone, ever. It’s going to be long if we want details. It is easier though to use the explain approach instead of step by step guide.  So I am going to share all the sources and resources, and then go with you through setting this all up, show how things work, and hopefully this all will make sense.

 

Prerequisites


Java knowledge, basic LibGDX experience, computer, time. Make sure you use latest Overlap2D editor and runtimes (v 0.0.4 or higher)

Get latest LibGDX project setup here: http://bitly.com/1i3C7i3 ( Details here )
Get latest Overlap2D editor here: http://overlap2d.com

 

First - Get Game Assets (Resources)


First let’s download the assets. I made a small pack of images I found on the internet with Flappy Bird images, animations and font. You can get it here: http://overlap2d.com/?p=236

 

 

Setting up Your Overlap2D Project

 

Unzip your Overlap2D zip file, and run the Overlap2D.jar.


First things first, you are currently viewing demo project, so go to File and Create New Project. Call it OverlappyBird and then create the scene size, let it be 480 width, 800 in height. It’s a standard small mobile device resolution, and since Flappy Bird is a portrait game, it should be just right. Click Create Project.
You have now created an empty project and it’s ready for setup, now you need to import the assets. Here we have 3 types of assets, first is just image textures, and second is a sprite animation for bird itself, and lastly the TTF font for score label. I have separated them in folders so you can import easily. Click on File, import to library.


First click on “…” button near the import images text field to select your images; locate them, select all. When done click on import and they all will get imported to your assets area on the right middle panel. Then open same the dialog again, and this time find Import Sprite Animations (note: not spine, but sprite) Click on “…” button there and locate bird.atlas file. Click Import, and it will get added to your assets list as well.


Lastly open import dialog again, and import font, by locating ttf file.


Your project setup is ready.

 

Making The Main Scene

 

First let’s see how main scene looks in Overlap2D when it is finished:

 

image


 
I decided to make several layers to have things organized, bg layer for background image that is not moving, default layer for pipes and bird itself, ground layer for moving the ground that is covering pipes, ui layer for “get ready” popups and score labels, and finally dlg layer for end game dialog. The reason I am not putting dialog inside ui layer is that I can position it in its layer correctly, and later hide that layer so it will not intervene with my work.  So, go ahead and create this layers in the order I described from bottom to top. (bg layer goes below them all, and you can rearrange them by drag-and-dropping)


I also recommend you to open my example project (provided with resources). Open it with Overlap2D and see how things are positioned and grouped. (To open a project, click file, open, and then locate .pit file inside project.)


Compose your own project like I did,. Put background on bg layer, and add bird on default layer. Make sure it looks like the picture above.


The ground is going to be a moving part, so we need to have 2 of this near each other for seamless movement. Put two grounds near each other and then group them into composites, like on this picture, do not forget to give identifier to it for later use.


image

 

For the pipes we are going to do the following trick. First of all we are going to put pipes on top of each other with a gap, and convert them into composite, and then add to library to have kind of a prefab for a pipe column. Later in code we are going to pre-load 3 of this because there is never more than 3 columns of pipes visible on screen, so we can re-use the ones that are left of the screen for next pipe iteration.


image


Now some of the pipes are going to vary in position on Y axis. So there are a minimum and maximum values that it should not be below or above. To avoid having this values in code, I just put 2 of pipe columns on screen: one at the lowest point and the other at the highest, and give them identifiers in order to read those values from code later.

 

image

 

Next click on bird in properties box and add 2 custom variables, by clicking on custom variables button. First is jump_speed=400, and second is gravity=1000 (so you can later tweak game from editor not from code)


image

 

Next up, put a “hint” and “get ready” assets convert them into composite as they always go together and give them id to hide show from game. They should go to ui layer.

 

Making Menu Scene

 

This one is easier: click on File, Scenes, Create New Scene, and choose MenuScene as name.


Make sure to put up all things just like on this picture. And note that you should make a composite for ground agai n (as we do not yet share prefabs between the scenes).


Put a Play button as image and give it an id to add listeners later in your code.


Here is how it should look:

 

image

 

Overlap2D Part DONE!


Looks pretty good! Both scenes are set up and ready for some coding. Most importantly your scene and UI are very flexible and code-independent.

 

 

 

Setting up LibGDX Project and Getting Ready to Code:

 

Download the LibGDX setup application and setup an empty project (make sure to have checked box2d and freetype fonts), import it to your eclipse or other IDE, run the desktop app to make sure it works, then strip it from LibGDX picture  and make it just a black screen.


Make sure your desktop launcher has code to specify screen size of 480x800 (config variables in launcher).


We are going to render everything using a simple LibGDX Stage, so the code structure will be the following: GameStage extending Stage that will load all the Overlap2D data and display it; Main class that will be rendering GameStage in its render method.


For all the rest we are going to use iScript functionality of Overlap2D. Let me go a bit in depth for that. Overlap2D runtime provides an interface that you can implement to create a “brain” of any composite item. Basically it is your way of attaching logic to an object. In this example we are going to create 3 iScripts, one is going to get attached to entire mainScreen and be a GameScreenScript with all game logic, other will be MenuScreenScript for menu, and the last one will be BirdScript that we will attach to the bird. It is important to mention that you cannot attach script to an image, or sprite animation, so I have converted bird animation to composite item in order to do it (right click, convert into composite).

 

You can find the full project on my github:  https://github.com/azakhary/OverlappyBird

Here are the well commented sources:

 

OverlappyBird.java

package com.underwater.demo.overflappy;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;

/*
 * This is our ApplicationListener the main thing that things start in libGDX
 */
public class OverflappyBird extends ApplicationAdapter {
   
   /*
    * GameStage will be holding both menu and main game
    */
   private GameStage stage;
   
   @Override
   public void create () {
      stage = new GameStage();
   }

   @Override
   public void render () {
      // Clearing the screen before each render
      Gdx.gl.glClearColor(0, 0, 0, 1);
      Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
      
      // calling stage act method and passing delta time passed since last call
      stage.act(Gdx.graphics.getDeltaTime());
      // drawing all actors
      stage.draw();
      
   }
}

GameStage.java

package com.underwater.demo.overflappy;

import java.io.File;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.scenes.scene2d.Stage;
import com.uwsoft.editor.renderer.DefaultAssetManager;
import com.uwsoft.editor.renderer.SceneLoader;

/*
 * GameStage 
 */
public class GameStage extends Stage {
   
   // Speed of pixels per second of how fast things move left (required both for 
                                                               menu and the game,
                                                               thus put here)
   public float gameSpeed = 200;

   // Overlap2D  provides this easy asset manager that loads things as they are 
   provided by default when exporting from overlap
   private DefaultAssetManager assetManager;
   
   public GameStage() {
      super();
      
      // Set this is input processor so all actors would be able to listen to 
      touch events
      Gdx.input.setInputProcessor(this);
      
      // Initializing asset manager
      assetManager = new DefaultAssetManager();
      
      // providing the list of sprite animations which is one in this case, to 
      avoid directory listing coding
      assetManager.spriteAnimationNames = new String[1]; assetManager.
                                          spriteAnimationNames[0] = "bird";
      
      // loading assets into memory
      assetManager.loadData();
      
      // Menu goes first
      initMenu();
   }
   
   public void initMenu() {
      clear();
      
      // Creating Scene loader which can load an Overlap2D scene
      SceneLoader menuLoader = new SceneLoader(assetManager);
      
      // loading MenuScene.dt from assets folder
      menuLoader.loadScene(Gdx.files.internal("scenes" + File.separator + 
                           "MenuScene.dt"));
      
      // Initializing iScript MenuSceneScript that will be holding all menu 
      logic, and passing this stage for later use
      MenuScreenScript menuScript = new MenuScreenScript(this);
      
      // adding this script to the root scene of menu which is hold in 
      menuLoader.sceneActor
      menuLoader.sceneActor.addScript(menuScript);
      
      // Adding root actor to stage
      addActor(menuLoader.sceneActor);
      
      
   }
   
   public void initGame() {
      clear();
      
      // Creating Scene loader which can load an Overlap2D scene
      SceneLoader mainLoader = new SceneLoader(assetManager);
      
      // loading MainScene.dt from assets folder
      mainLoader.loadScene(Gdx.files.internal("scenes" + File.separator + 
                           "MainScene.dt"));
      
      // Initializing iScript GameSceneScript that will be holding all game, and 
      passing this stage for later use
      GameScreenScript gameScript = new GameScreenScript(this, mainLoader);
      
      // adding this script to the root scene of game which is hold in 
      mainLoader.sceneActor
      mainLoader.sceneActor.addScript(gameScript);
      
      // Adding root actor to stage
      addActor(mainLoader.sceneActor);
   }

}

GameScreenScript.java

package com.underwater.demo.overflappy;

import java.util.ArrayList;

import com.badlogic.gdx.math.MathUtils;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.scenes.scene2d.InputEvent;
import com.badlogic.gdx.scenes.scene2d.Touchable;
import com.badlogic.gdx.scenes.scene2d.actions.Actions;
import com.badlogic.gdx.scenes.scene2d.actions.AddAction;
import com.badlogic.gdx.scenes.scene2d.utils.ClickListener;
import com.uwsoft.editor.renderer.SceneLoader;
import com.uwsoft.editor.renderer.actor.CompositeItem;
import com.uwsoft.editor.renderer.actor.LabelItem;
import com.uwsoft.editor.renderer.script.IScript;

/*
 * iScript for entire game logic
 */
public class GameScreenScript implements IScript {
   
   /*
    * reference to GameStage
    */
   private GameStage stage;
   
   /*
    * Main actor that holds root of game screen
    */
   private CompositeItem game;
   
   // Screen loader reference to be later used to retrieve prefabs from library
   private SceneLoader loader;
   
   // Game over Dialog actor
   private CompositeItem gameOverDlg;
   
   /*
    * this will be holding 2-ground system composite item 
    */
   private CompositeItem groundRotator;
   
   /*
    * Instead of holding bird actor we are going to hold birdscript that will 
    provide all bird logic and methods.
    * Also it will have bird actor inside
    */
   private BirdScript bird;
   
   // Hint Box actor the one that shown in begining of game
   private CompositeItem hintBox;
   
   // some helping booleans
   private boolean gameStarted = false;
   private boolean groundStop = false;
   
   // going to hold what is the possible low and high position for a pipe column
   private float minPipe;
   private float maxPipe;
   
   private int gameScore = 0;
   private LabelItem scoreLbl;
   
   // Going to hold 3 pipes here to reuse as pipe pool
   private ArrayList<CompositeItem> pipes = new ArrayList<CompositeItem>();
   
   public GameScreenScript(GameStage stage, SceneLoader loader) {
      this.stage = stage;
      this.loader = loader;
   }
   
   @Override
   public void init(CompositeItem gameItem) {
      game = gameItem;
      
      gameScore = 0;
      
      // Creating and holding BirdScript that will hold entire bird logic.
      bird = new BirdScript(stage);
      game.getCompositeById("bird").addScript(bird);
      
      groundRotator = game.getCompositeById("groundRotator");
      hintBox = game.getCompositeById("hintBox");
      scoreLbl = game.getLabelById("scoreLbl");
      
      // Adding listeners to listen for taps to make bird jump
      game.addListener(new ClickListener() {
         public boolean touchDown (InputEvent event, float x, float y, int 
                                   pointer, int button) {
            return true;
         }
         public void touchUp (InputEvent event, float x, float y, int pointer, 
                              int button) {
            // screen tap was done
            screenTap();
         }
      });
      
      // Loading min/max positions from level editor
      minPipe = game.getCompositeById("minPipe").getY();
      maxPipe = game.getCompositeById("maxPipe").getY();

      // Retrieving 3 pipe columns from library putting them into array,
      // and adding on screen in minus coordinates so the will becom "availible"
      for(int i = 0;  i < 3; i++) {
         CompositeItem pipe = loader.getLibraryAsActor("pipeGroup");             
         pipe.setX(-pipe.getWidth());        
         game.addItem(pipe);
         
         pipes.add(pipe);
      }
      
      
      // Making sure first pipe will be added not sooner then 3 seconds from now
      game.addAction(Actions.sequence(Actions.delay(3.0f), Actions.run(new 
                     Runnable() {
         
         @Override
         public void run() {
            putPipe();
         }
      })));
      
      // hiding game over dialog
      gameOverDlg = game.getCompositeById("gameOverDlg");
      // it should not listen for taps
      gameOverDlg.setTouchable(Touchable.disabled);
      gameOverDlg.setVisible(false);
   }

   @Override
   public void act(float delta) {
      
      // if game is not yet started or started (but most importantly not ended, 
                                                ground is moving) 
      if(!groundStop) {
         groundRotator.setX(groundRotator.getX() - delta * stage.gameSpeed);     
         if(groundRotator.getX() < -groundRotator.getWidth()/2) groundRotator.
            setX(0);
      }
      
      // if game is started, so first tap fone, then we dhould check for 
      collisions and move pipes
      if(gameStarted) {
         for(int i = 0; i < pipes.size(); i++) {
            // get pipe
            CompositeItem pipe = pipes.get(i);
            
            // move it if it has positive coordinate
            if(pipe.getX() > -pipe.getWidth()) {
               // if pipe was right of thebird, and will now become left of the 
               bird, add to score
               if(pipe.getX() >= bird.getBirdCenter().x && pipe.getX() - delta * 
                  stage.gameSpeed < bird.getBirdCenter().x) {
                  gameScore++;
               }
               pipe.setX(pipe.getX() - delta * stage.gameSpeed);
            }
            
            //check for collision with bird
            collisionCheck();
         }
      }
      
      // update scorel label
      scoreLbl.setText(gameScore+"");
   }
   
   /*
    * Check for bird versus pipe row collision
    */
   private void collisionCheck() {
      // iterate through all 3 pipes
      for(int i = 0; i < pipes.size(); i++) {
         CompositeItem pipe = pipes.get(i);
         
         // to make it easy going to think about bird as circle with 5 radius 
         Vector2 birdPoint = bird.getBirdCenter();
         
         // Is there collision? if yes stop the game and allow bird to fall
         if(birdPoint.x+5 > pipe.getX() && birdPoint.x - 5 < pipe.getX() + pipe.
            getWidth() && (pipe.getY() + 532 > birdPoint.y - 5 || pipe.getY()+
            701 < birdPoint.y + 5)) {
            stopGame(); 
         }
         
         // Did bird hit the ground? not only stop the game but also 
         // disable gravity to keep from further falling, and consider bird dead 
         ( animations stop )
         if(birdPoint.y-5 < groundRotator.getY()+groundRotator.getHeight()) {
            if(!groundStop) {
               stopGame();
            }
            bird.disableGravity();
            bird.getBird().setY(groundRotator.getY()+groundRotator.getHeight()+5)
                         ;
            // killitwithfire
            bird.die();
         }
      }
   }
   
   /*
    * Stops the game
    */
   private void stopGame() {
      gameStarted = false;
      groundStop = true;
      game.clearActions();
      
      // show end game dialog
      showEndDialog();
   }
   
   /*
    * showing end game dialog
    */
   public void showEndDialog() {
      // enabling touch back, showing
      gameOverDlg.setTouchable(Touchable.enabled);
      gameOverDlg.setVisible(true);
      // setting transparency to full
      gameOverDlg.getColor().a = 0;
      // and fading it in
      gameOverDlg.addAction(Actions.fadeIn(0.4f));
      
      // setting play button listener to replay the game
      gameOverDlg.getImageById("playBtn").addListener(new ClickListener() {
         public boolean touchDown (InputEvent event, float x, float y, int 
                                   pointer, int button) {
            return true;
         }
         public void touchUp (InputEvent event, float x, float y, int pointer, 
                              int button) {
            stage.initGame();
         }
      });
   }
   
   /*
    * Called when screen is tapped
    */
   private void screenTap() {
      // if ground is not moving then bird is dead no actin required on tapp
      if(groundStop) return;
      
      // if game started just jump the bird
      if(gameStarted) {
         bird.jump();
      } else {
         // if game is not yet started, start the game and jump the bird
         gameStarted = true;
         hintBox.addAction(Actions.fadeOut(0.3f));
         // and also enable gravity from now on
         bird.enableGravity();
         bird.jump();
      }
   }
   
   /*
    * get's availible pipe
    * availible pipe is any pipe that is left of screen and not visible
    */
   public CompositeItem getAvailablePipe() {
      for(int i = 0; i < pipes.size(); i++) {
         if(pipes.get(i).getX() <= -pipes.get(i).getWidth()) {
            return pipes.get(i);
         }
      }
      
      return null;
   }
   
   /*
    * this is called every X time to put a new pipe on the right
    */
   public void putPipe() { 
      // getting availible pipe
      CompositeItem pipe = getAvailablePipe();
      
      // when you die at bad moment, it can be null sometimes
      if(pipe == null) return;
      
      // put pipe column on the random hight from min to max range
      pipe.setX(stage.getWidth());
      pipe.setY(MathUtils.random(minPipe, maxPipe));
      
      // schedule next pipe to be put in 1.3 seconds
      game.addAction(Actions.sequence(Actions.delay(1.3f), Actions.run(new 
                     Runnable() {
         
         @Override
         public void run() {
            // call itself
            putPipe();
         }
      })));
   }

}

 

MenuScreenScript.java

 

package com.underwater.demo.overflappy;

import com.badlogic.gdx.math.MathUtils;
import com.badlogic.gdx.scenes.scene2d.InputEvent;
import com.badlogic.gdx.scenes.scene2d.utils.ClickListener;
import com.uwsoft.editor.renderer.actor.CompositeItem;
import com.uwsoft.editor.renderer.actor.ImageItem;
import com.uwsoft.editor.renderer.actor.SpriteAnimation;
import com.uwsoft.editor.renderer.script.IScript;

/*
 * iScript for menu logic
 */
public class MenuScreenScript implements IScript {
   
   /*
    * reference to GameStage
    */
   private GameStage stage;
   
   /*
    * this is the main root menu actor to work with
    */
   private CompositeItem menu;
   
   /*
    * this will be holding 2-ground system composite item 
    */
   private CompositeItem groundRotator;
   
   /*
    * this will be the bird sprite animation displayed in center of screen
    */
   private SpriteAnimation bird;
   
   // this variables are used to wiggle bird up and down with sin function
   private float iterator = 0;
   private float birdInitialPos;
   
   public MenuScreenScript(GameStage stage) {
      this.stage = stage;
   }

   public void init(CompositeItem menuItem) {
      menu = menuItem;
      
      // Finding playButton by id and storing in variable
      ImageItem playBtn = menuItem.getImageById("playBtn");
      
      // Finding ground composite and storing in variable 
      groundRotator = menuItem.getCompositeById("groundRotator");
      
      // Finding bird and storing in variable
      bird = menuItem.getSpriteAnimationById("bird");
      
      // let's remember where bird was initially
      birdInitialPos = bird.getY();
      
      // Adding a Click listener to playButton so we can start game when clicked
      playBtn.addListener(new ClickListener() {
         // Need to keep touch down in order for touch up to work normal (libGDX 
                                                                          awkward
                                                                          ness)
         public boolean touchDown (InputEvent event, float x, float y, int 
                                   pointer, int button) {
            return true;
         }
         public void touchUp (InputEvent event, float x, float y, int pointer, 
                              int button) {
            // when finger is up, ask stage to load the game
            stage.initGame();
         }
      });
   }
   
   /*
    * This is called every frame
    */
   public void act(float delta) {
      // moving ground left with game speed multiplied by delta as delta shows 
      what part of second was passed since last call
      groundRotator.setX(groundRotator.getX() - delta * stage.gameSpeed);     
      
      // if ground rotator got half way left, we can just put it back to 0, and 
      to eye it will look like it endlessly moves
      if(groundRotator.getX() < -groundRotator.getWidth()/2) groundRotator.setX(
         0);
      
      // Now this part is to wiggle bird up and down, we are going change 
      iterator based on time passed
      iterator += delta*400;
      
      // Then figure out the bird offset from it's original position based on 
      iterator which is based on time passed, and do it with sinus function
      float birdOffset = MathUtils.sinDeg(iterator)*5;
      
      // put bird on it's original pos + offset
      bird.setY(birdInitialPos + birdOffset); 
   }
   
}

 

BirdScript.java

 

package com.underwater.demo.overflappy;

import com.badlogic.gdx.math.MathUtils;
import com.badlogic.gdx.math.Vector2;
import com.uwsoft.editor.renderer.actor.CompositeItem;
import com.uwsoft.editor.renderer.actor.SpriteAnimation;
import com.uwsoft.editor.renderer.script.IScript;

/**
 * Bird Script
 * @author azakhary
 * This is brain of the bird, it's physics and everything
 */
public class BirdScript implements IScript {

   /*
    * reference to GameStage
    */
   private GameStage stage;
   
   // Bird composite item actor
   private CompositeItem bird;
   
   // Inside bird composite actor there is the bird sprite animation actor
   private SpriteAnimation birdAnimation;
   
   // used to wiggle the bird in the air using Sine function
   private float iterator = 0;
   
   // current vertical velocity of the bird
   private float currVerticalVelocity = 0;
   
   // boolean to know if gravity is enabled or not
   private boolean isGravityEnabled = false;
   
   // to avoid jumping to rotation bird will try to always rotate a bit towards 
   desired rotation
   private float desiredRotation;
   
   // is it alive?
   private boolean isAlive = true;
   
   
   public BirdScript(GameStage stage) {
      this.stage = stage;
   }
   
   @Override
   public void init(CompositeItem item) {
      bird = item;
      
      // find animation from the composite
      birdAnimation = bird.getSpriteAnimationById("birdAnimation");
      
      // set origin of the bird in it's center, so it will rotate normally
      bird.setOrigin(bird.getWidth()/2, bird.getHeight()/2);
      
      // set desired rotation to current rotation which is 0
      desiredRotation = bird.getRotation();
   }

   @Override
   public void act(float delta) {
      
      if(!isGravityEnabled && isAlive) {
         // Wiggling when no gravity only
         iterator += delta*400;
         float birdOffset = MathUtils.sinDeg(iterator)*5;
         birdAnimation.setY(birdOffset);
      }
      
      // aplying gravity every frame
      gravity(delta);
      
      // moving to new position based on current vertical velocity
      bird.setY(bird.getY() + delta*currVerticalVelocity);
      
      // manage bird rotation based on it's vertical speed
      manageRotation(delta);
      
   }
   
   /*
    * manage bird rotation based on it's vertical speed
    * this is a part of code that is not interesting boring and whatever..
    */
   private void manageRotation(float delta) {
      if(isGravityEnabled) {
         if(currVerticalVelocity > -200) {
            float rotation = currVerticalVelocity+200;
            desiredRotation = rotation/15f;
         }
         if(currVerticalVelocity <= -200) {
            float rotation = currVerticalVelocity+200;
            if(rotation < -400) rotation = -400;
            desiredRotation = rotation/4.4f;
         }
         
         if(desiredRotation != bird.getRotation()) {
            if(desiredRotation > bird.getRotation()) {
               bird.setRotation(bird.getRotation() + 900*delta);
               if(desiredRotation < bird.getRotation()) bird.setRotation(
                  desiredRotation);
            }
            if(desiredRotation < bird.getRotation()) {
               bird.setRotation(bird.getRotation() - 900*delta);
               if(desiredRotation > bird.getRotation()) bird.setRotation(
                  desiredRotation);
            }
         }
      }
   }
   
   public void enableGravity() {
      isGravityEnabled = true;
   }
   
   public void disableGravity() {
      isGravityEnabled = false;
   }
   
   public void jump() {
      // if bird is dead do not jump (I think I checked it somewhere already)
      if(!isAlive) return;
      
      // if bird is higher then screen then do not jump
      if(bird.getY() > stage.getHeight()) return;
      
      // if jumped get the custom variable jump_speed from bird actor and set it 
      as current vertical velocity
      currVerticalVelocity = bird.getCustomVariables().getFloatVariable(
                             "jump_speed");
   }
   
   /*
    * Apply gravity each frame (get's delta time since last frame)
    */
   private void gravity(float delta) {
      if(isGravityEnabled) {
         // change curernt velocity based on gravity (gravity changes velocity 
                                                      every second by gravity 
                                                      amount)
         currVerticalVelocity -= delta*bird.getCustomVariables().
                                 getFloatVariable("gravity");
      }
   }
   
   public CompositeItem getBird() {
      return bird;
   }
   
   // get's the bird center coordinates as vector 2 needed for collision 
   detection in GameScreenScript
   public Vector2 getBirdCenter() {
      Vector2 vec = new Vector2(bird.getX() + bird.getWidth()/2, bird.getY() + 
                    bird.getHeight()/2);
      return vec;
   }
   
   // Kills the bird, for reals
   public void die() {
      currVerticalVelocity = 0;  
      isAlive = false;
      desiredRotation = 0; 
      bird.setRotation(0);
      birdAnimation.pause();
   }

}

Programming Design


27. August 2014

 

Today we are going to look at implementing physics in LibGDX.  This technically isn’t part of LibGDX itself, but instead is implemented as an extension.  The physics engine used in LibGDX is the popular Box2D physics system a library that has been ported to basically every single platform and language ever invented, or so it seems. We are going to cover how to implement Box2D physics in your 2D LibGDX game.  This is a complex subject so will require multiple parts.

 

If you’ve never used a Physics Engine before, we should start with a basic overview of what they do and how they work.  Essentially a physics engine takes scene information that you provide, then calculates “realistic” movement using physics calculations.  It goes something like this:

  • you describe all of the physics entities in your world to the physics engine, including bounding volumes, mass, velocity, etc
  • you tell the engine to update, either per frame or on some other interval
  • the physics engine calculates how the world has changed, what’s collided with what, how much gravity effects each item, current speed and position, etc
  • you take the results of the physics simulation and update your world accordingly.

 

Don’t worry, we will look at exactly how in a moment.

 

First we need to talk for a moment about creating your project.  Since Box2D is now implemented as an extension ( an optional LibGDX component ), you need to add it either manually or when you create your initial project.  Adding a library to an existing project is IDE dependent, so I am instead going to look at adding it during project creation… and totally not just because it’s really easy that way.

 

When you create your LibGDX project using the Project Generator, you simply specify which extensions you wish to include and Gradle does the rest.  In this case you simply check the box next to Box2d when generating your project like normal:

 

image

 

… and you are done.  You may be asking, hey what about Box2dlights?  Nope, you don’t currently need that one.  Box2dlights is a project for simulating lighting and shadows based off the Box2d physics engine.  You may notice in that list another entity named Bullet.  Bullet is another physic engine, although more commonly geared towards 3D games, possibly more on that at a later date.  Just be aware if you are working in 3D, Box2d isn’t of much use to you, but there are alternatives.

 

Ok, now that we have a properly configured project, let’s take a look at a very basic physics simulation.  We are simply going to take the default LibGDX graphic and apply gravity to it, about the simplest simulation you can make that actually does something.  Code time!

 

package com.gamefromscratch;

import com.badlogic.gdx.ApplicationAdapter;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Sprite;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.*;

public class Physics1 extends ApplicationAdapter {
    SpriteBatch batch;
    Sprite sprite;
    Texture img;
    World world;
    Body body;

    @Override
    public void create() {

        batch = new SpriteBatch();
        // We will use the default LibGdx logo for this example, but we need a 
        sprite since it's going to move
        img = new Texture("badlogic.jpg");
        sprite = new Sprite(img);

        // Center the sprite in the top/middle of the screen
        sprite.setPosition(Gdx.graphics.getWidth() / 2 - sprite.getWidth() / 2,
                Gdx.graphics.getHeight() / 2);

        // Create a physics world, the heart of the simulation.  The Vector 
        passed in is gravity
        world = new World(new Vector2(0, -98f), true);

        // Now create a BodyDefinition.  This defines the physics objects type 
        and position in the simulation
        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        // We are going to use 1 to 1 dimensions.  Meaning 1 in physics engine 
        is 1 pixel
        // Set our body to the same position as our sprite
        bodyDef.position.set(sprite.getX(), sprite.getY());

        // Create a body in the world using our definition
        body = world.createBody(bodyDef);

        // Now define the dimensions of the physics shape
        PolygonShape shape = new PolygonShape();
        // We are a box, so this makes sense, no?
        // Basically set the physics polygon to a box with the same dimensions 
        as our sprite
        shape.setAsBox(sprite.getWidth()/2, sprite.getHeight()/2);

        // FixtureDef is a confusing expression for physical properties
        // Basically this is where you, in addition to defining the shape of the 
        body
        // you also define it's properties like density, restitution and others 
        we will see shortly
        // If you are wondering, density and area are used to calculate over all 
        mass
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = shape;
        fixtureDef.density = 1f;

        Fixture fixture = body.createFixture(fixtureDef);

        // Shape is the only disposable of the lot, so get rid of it
        shape.dispose();
    }

    @Override
    public void render() {

        // Advance the world, by the amount of time that has elapsed since the 
        last frame
        // Generally in a real game, dont do this in the render loop, as you are 
        tying the physics
        // update rate to the frame rate, and vice versa
        world.step(Gdx.graphics.getDeltaTime(), 6, 2);

        // Now update the spritee position accordingly to it's now updated 
        Physics body
        sprite.setPosition(body.getPosition().x, body.getPosition().y);

        // You know the rest...
        Gdx.gl.glClearColor(1, 1, 1, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        batch.begin();
        batch.draw(sprite, sprite.getX(), sprite.getY());
        batch.end();
    }

    @Override
    public void dispose() {
        // Hey, I actually did some clean up in a code sample!
        img.dispose();
        world.dispose();
    }
}

 

The program running:

 

test

 

What's  going on here is mostly defined in the comments, but I will give a simpler overview in English.  Basically when using a Physics Engine, you create a physical representation for each corresponding object in your game.  In this case we created a physics object ( Body ) that went along with our sprite.  It’s important to realize, there is no actual relationship between these two objects.  There are a couple of components that go into a physics body, BodyDef which defines what type of body it is ( more on this later, for now realize DynamicBody means a body that is updated and capable of movement ) and FixtureDef, which defines the shape and physical properties of the Body.  Of course, there is also the World, which is the actual physics simulation.

 

So, basically we created a Body which is the physical representation of our Sprite in the physics simulation.  Then in render() we call the incredibly important step() method.  Step is what advances the physics simulation… basically think of it as the play button.  The physics engine then calculations all the various mathematics that have changes since the last call to step.  The first value we pass in is the amount of time that has elapsed since the last update.  The next two values control the amount of accuracy in contact/joint calculations for velocity and position. Basically the higher the values the more accurate your physics simulation will be, but the more CPU intensive as well.  Why 6 and 2?  ‘cause that’s what the LibGDX site recommend and that works for me.  At the end of the day these are values you can tweak to your individual game.  The one other critical take away here is we update the sprites position to match the newly updated body’s position.  Once again, in this example, there is no actual link between a physics body and a sprite, so you have to do it yourself.

 

There you go, the worlds simplest physics simulation.  There are a few quick topics to discuss before we move on.  First, units.

 

This is an important and sometimes tricky concept to get your head around with physics systems.  What does 1 mean?  One what?  The answer is, whatever the hell you want it to be, just be consistent about it!  In this particular case I used pixels.  Therefore 1 unit in the physics engine represents 1 pixel on the screen.  So when I said gravity is (0,-98) that means gravity is applied at a rate of –98 pixels along the y axis per second.  Just as commonly, 1 in the physics engine could be meters, feet, kilometer, etc… then you use a custom ratio for translating to and from screen coordinates.  Most physics systems, Box2d included, really don’t like you mixing your scales however.  For example, if you have a universe simulation where 1 == 100 miles, then you want to calculate the movement of an Ant at 0.0000001 x 100miles per hour, you will break the simulation, hard.  Find a scale that works well with the majority of your game and stick with it.  Extremely large and extremely small values within that simulation will cause problems.

 

Finally, a bit of a warning about how I implemented this demo and hopefully something I will cover properly at a later date.  In this case I updated the physics system in the render loop.  This is a possibility but generally wasteful.  It’s fairly common to run your physics simulation at a fixed rate ( 30hz and 60hz being two of the most common, but lower is also a possibility if processing restrained ) and your render loop as fast as possible.

 

In the next part we will give our object something to collide with, stay tuned.

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