In a game I’m developing I wanted to have some realistic explosion effects. At first, I thought I would just apply an impulse to all objects within range of the explosion. However, this created unrealistic movement in the affected objects and the objects didn’t rotate. I tried using ray casts to set points to apply the forces to create rotation. This made the explosion look more realistic and worked well enough until the explosions exploded near a static body in which case objects would sometimes be missed as they were shielded by the static body. I decided to dive in fully and use particles to represent the force of the blast.
The first thing I made was an array to store where the explosions would explode and an array to store each particle from the explosion.
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public Array<Vector2> bombsToExplode = new Array<Vector2>(); public Array<ExplosionParticle> partys = new Array<ExplosionParticle>(); |
Next, I created the method that would be run for each explosion.
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private void explodeABomb(Vector2 vector){ int numRays = ExplosionParticle.NUMRAYS; for (int i = 0; i < numRays; i++) { float angle = (i / (float)numRays) * 360 * MathUtils.degreesToRadians; Vector2 rayDir = new Vector2( (float) Math.sin(angle),(float) Math.cos(angle) ); partys. add(new ExplosionParticle(world, vector, rayDir)); // create the particle } this.explosions.add(vector); // add explosion particle effect at vector } |
I needed to create a class for the Explosion particles which would contain a reference to the body. In this class, I create a body definition and use it to create a Body. The body position is then set to the place the bomb exploded. The linear velocity is then set to give the particle a direction and force. A Fixture is then created to give the body a shape, size, weight etc, and added to the body.
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public class ExplosionParticle { public int blastPower = 100; public static final int NUMRAYS = 60; public Body body; public ExplosionParticle(World world, Vector2 vector, Vector2 rayDir){ BodyDef bd = new BodyDef(); bd.type = BodyType.DynamicBody; bd.fixedRotation = true; // rotation not necessary bd.bullet = true; // prevent tunneling at high speed bd.linearDamping = 10; // drag due to moving through air bd.gravityScale = 0; // ignore gravity bd.position.x = vector.getPosition().x; bd.position.y = vector.getPosition().y;// start at blast center rayDir.scl(blastPower); bd.linearVelocity.x = rayDir.x; bd.linearVelocity.y = rayDir.y; body = world.createBody( bd ); //create a reference to this class in the body(this allows us to loop through the world bodies and check if the body is an Explosion particle) body.setUserData(this); CircleShape circleShape = new CircleShape(); circleShape.setRadius(0.05f); // very small FixtureDef fd = new FixtureDef(); fd.shape = circleShape; fd.density = 120 / (float)NUMRAYS; // very high - shared across all particles fd.friction = 0; // friction not necessary fd.restitution = 0.99f; // high restitution to reflect off obstacles fd.filter.groupIndex = -1; // particles should not collide with each other body.createFixture( fd ); } } |
The Box2D world would soon fill up with particles unless they are removed once they’ve finished expelling all their energy.
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for(ExplosionParticle party: partys){ if(Math.abs(party.body.getLinearVelocity().x) < 5f && Math.abs(party.body.getLinearVelocity().y) < 5f){ world.destroyBody(party.body); partys.removeValue(party, true); } } |
In my contact listener I have this code to tell the game model to make an explosion
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parent.createBlast(bomb); |
which runs this method in my Game Model
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public void createBlast(Bomb bomb) { bombsToExplode.add(bomb.body.getPosition()); bomb.isDead = true; // sets the bomb to dead so it can be removed outside of the box2d physics calculations } |
This is how the explosions look when using the box2D debug renderer.
[embedyt] http://www.youtube.com/watch?v=iN6Faut9g0A[/embedyt]
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