Wednesday, January 23, 2013

Meter and pixel units in a box2d game - LibGDX

I often see posts on the LibGDX forum from users confused about converting game units to pixel units, while coding box2d+scene2d games. I'm writing this short article to present my approach on this matter, which actually doesn't involve any unit conversion. It's all done by different cameras with different viewports.

As you know, box2d wants its bodies sized in meters. "But hey my character is 200 pixels big, I'll make it 200 meters and use a 1:1 conversion" you can't. The box2d manual says that the physics simulation starts to go freaky with dynamic bodies bigger than a truck. So stick with real-size dimensions.

The key is to have 2 different stages with different viewports. In the following example I'll use a stage to represent the box2d world, which will be the "game stage" and will have a viewport of about 3x6 meters; then I'll use another stage on top of the other one, aka "GUI stage", which will be used to render the GUI and will have a viewport of 1024x600 pixels. This way you get a meter-sized game with pixel-exact GUI.

Game Screen - this class is our main game screen. It contains the GUI stage.

 public class GameScreen implements Screen {  
     // this is actually my tablet resolution in landscape mode. I'm using it for making the GUI pixel-exact.  
     public static float SCREEN_WIDTH = 1024;  
     public static float SCREEN_HEIGHT = 600;  
     private GameWorld world; // contains the game world's bodies and actors.  
     private GameRenderer renderer; // our custom game renderer.  
     private Stage stage; // stage that holds the GUI. Pixel-exact size.  
     private OrthographicCamera guiCam; // camera for the GUI. It's the stage default camera.  
     public final void show() {  
         this.stage = new Stage(); // create the GUI stage
         this.stage.setViewport(SCREEN_WIDTH, SCREEN_HEIGHT, false); // set the GUI stage viewport to the pixel size
         world = new GameWorld();  
         renderer = new GameRenderer(world);  
         // add GUI actors to stage, labels, meters, buttons etc.  
         Label labelStatus = new Label("TOUCH TO START",;  
         labelStatus.setPosition(GenericScreen.SCREEN_WIDTH/2-500, GenericScreen.SCREEN_HEIGHT/2);  
         // add other GUI elements here  
     public void render(float delta) {  
         guiCam = (OrthographicCamera) stage.getCamera();  
         guiCam.position.set(SCREEN_WIDTH/2, SCREEN_HEIGHT/2, 0);  
   ;  ;  
         world.update(delta); // update the box2d world          
         stage.act(delta); // update GUI  
         renderer.render(); // draw the box2d world  
         stage.draw(); // draw the GUI  

Game World - our container for the box2d world and other game elements. It also contains the game stage.

 public class GameWorld {  
     // here we set up the actual viewport size of the game in meters.  
     public static float UNIT_WIDTH = GameScreen.SCREEN_WIDTH/160; // 6.4 meters width  
     public static float UNIT_HEIGHT = GameScreen.SCREEN_HEIGHT/160; // 3.75 meters height  
     public static final Vector2 GRAVITY = new Vector2(0, -9.8f);  
     public final Stage stage; // stage containing game actors (not GUI, but actual game elements)  
     public World box2dWorld; // box2d world  
     public Bob bob; // our playing character  
     public GameWorld() {      
         box2dWorld = new World(GRAVITY, true);          
         stage = new Stage(); // create the game stage  
         stage.setViewport(UNIT_WIDTH, UNIT_HEIGHT, false); // set the game stage viewport to the meters size   
     private void createWorld() {  
         // create box2d bodies and the respective actors here.  
         bob = new Bob(this);  
         // add more game elements here  
     public void update(float delta) {  
         // perform game logic here  
         box2dWorld.step(delta, 3, 3); // update box2d world  
         stage.act(delta); // update game stage  

Bob - our character. It extends Image which is a subclass of Actor, and contains a reference to the box2d body (in this case a circle). Its texture is resized to the game meter units, but it will be rendered using the game stage camera, so even if the texture is scaled to 0.8x0.8, it will be quite big since the viewport is 6x3 meters.

 public class Bob extends Image {  
     public static final float RADIUS = 0.4f; // bob is a ball with 0.8m diameter      
     public final Body body; // bob's box2d body  
     public Bob(GameWorld world) {  
         // bob is an Image, so we load the graphics from the assetmanager  
         Texture tex = Assets.manager.get("characters.png", Texture.class);  
         this.setDrawable(new TextureRegionDrawable(new TextureRegion(tex, 0, 256, 128, 128)));  
         // generate bob's box2d body  
         CircleShape circle = new CircleShape();  
         BodyDef bodyDef = new BodyDef();  
         bodyDef.type = BodyType.DynamicBody;  
         bodyDef.position.x = 2f;  
         bodyDef.position.y = 2f;  
         bodyDef.linearDamping = 0.1f;  
         bodyDef.angularDamping = 0.5f;  
         this.body = world.box2dWorld.createBody(bodyDef);  
         Fixture fix = body.createFixture(circle, 50);  
         // generate bob's actor  
         this.setPosition(body.getPosition().x-RADIUS, body.getPosition().y-RADIUS); // set the actor position at the box2d body position  
         this.setSize(RADIUS*2, RADIUS*2); // scale actor to body's size  
         this.setScaling(Scaling.stretch); // stretch the texture  
     public void act(float delta) {  
         // here we override Actor's act() method to make the actor follow the box2d body  
         setOrigin(RADIUS, RADIUS);  
         setRotation(MathUtils.radiansToDegrees * body.getAngle());  
         setPosition(body.getPosition().x-RADIUS, body.getPosition().y-RADIUS);  

Game Renderer - a custom renderer used to draw the game world. It just positions the camera each frame and draws the game stage. Very simple.

 public class GameRenderer  
     GameWorld world;  
     OrthographicCamera camera;  
     Box2DDebugRenderer renderer;  
     public GameRenderer(GameWorld world)  
     {   = world;  
         this.renderer = new Box2DDebugRenderer();  
         // we obtain a reference to the game stage camera. The camera is scaled to box2d meter units   = (OrthographicCamera) world.stage.getCamera();  
         // center the camera on bob (optional)  
         camera.position.x = world.bob.body.getPosition().x;  
         camera.position.y = world.bob.body.getPosition().y;  
     public void render()  
         // have the camera follow bob  
         camera.position.x = world.bob.body.getPosition().x;  
         camera.position.y = world.bob.body.getPosition().y;  
         // box2d debug renderering (optional)  
         renderer.render(world.box2dWorld, camera.combined);  
         // game stage rendering  


  1. Replies
    1. ElementType.BOB is nothing, really, just an integer IIRC. Helps me know that this body is actually BOB when I check collisions etc.
      The filter is the way to tell box2d that a body should only collide with certain other bodies. See more here:

  2. Thank you very much sir. love you

  3. Could you update your code to work with the latest version of libgdx and box2D?
    Right now, at least the line

    this.stage.setViewport(SCREEN_WIDTH, SCREEN_HEIGHT, false);

    doesn't work anymore, as now a whole Viewport object has to be supplied to setViewport().

    I suspect one could use the following code instead:

    this.stage.setViewport(new ScreenViewport());

    and then update the viewport in the resize() methods of the screens.