box2d collision groups

Question

does anyone know whether bodies only collide when (body1.categoryBits & body2.maskBits) && (body1.maskBits & body2.categoryBits) ? or do they already collid

Answer 1

I have used walls and many players (our Player and enemies). My player and enemies collides with walls but don't collide with each other. I have done like this and it worked. You have to set the groupIndex to negative value for the objects not to collide with each other. Define the fixtures like this.

    /* The categories. */
public static final short CATEGORYBIT_WALL = 1;
public static final short CATEGORYBIT_PLAYERS = 2;

/* And what should collide with what. */
public static final short MASKBITS_WALL = CATEGORYBIT_WALL + CATEGORYBIT_PLAYERS;

public static final FixtureDef WALL_FIXTURE_DEF = PhysicsFactory.createFixtureDef(0, 0.5f, 0.5f, false, CATEGORYBIT_WALL, MASKBITS_WALL, (short)0);
public static final FixtureDef PLAYERS_FIXTURE_DEF = PhysicsFactory.createFixtureDef(1, 0.5f, 0.5f, false, CATEGORYBIT_PLAYERS, MASKBITS_WALL, (short)-1);

and apply to your bodies.

AnimatedSprite player, enemy;
Body playerBody, enemyBody;

player= new AnimatedSprite(CAMERA_WIDTH/2, CAMERA_HEIGHT/2,     this.playerRegion, this.getVertexBufferObjectManager());
playerBody = PhysicsFactory.createBoxBody(this.mPhysicsWorld, player,     BodyType.DynamicBody, PLAYERS_FIXTURE_DEF);   
this.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(player, playerBody, true, false));

enemy= new AnimatedSprite(CAMERA_WIDTH/4, CAMERA_HEIGHT/2, this.enemyRegion, this.getVertexBufferObjectManager());

enemyBody= PhysicsFactory.createCircleBody(this.mPhysicsWorld, enemy, BodyType.DynamicBody, PLAYERS_FIXTURE_DEF);
this.mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(enemy, enemyBody, true, false));