Bombs with Box2D & Cocos2D

Published on June 12th, 2010 in iPhone Code Snippets & Tutorials, iPhone Programming

Here is some example code based off my tutorial on how to implement orbital / radial gravity, but in reverse. This example pushes objects away instead of pulling inward. It makes for a nice explosion effect. Much work would need to be done to make it realistic, but you get the general idea of how to blow things up. You need to have Cocos2D and Box2D installed. Box2D comes packaged with the latest version if Cocos2D.




Download Source Code for this tutorial:
Bomb Demo Box2d & Cocos2d Example Source Code – 799k

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Comments

2 Responses to “Bombs with Box2D & Cocos2D”

  1. afour says:
    June 21, 2010 at 8:54 pm

    Thanks for this… I modified it a little to make a more realistic explosion. I added a ‘bomblife’ variable that turns off the bomb gravity after a set period of ticks. This makes the falling blocks fall without being pushed away from the circles.

    Like this:

    -(void) tick: (ccTime) dt
{
	//It is recommended that a fixed time step is used with Box2D for stability
	//of the simulation, however, we are using a variable time step here.
	//You need to make an informed choice, the following URL is useful
	//http://gafferongames.com/game-physics/fix-your-timestep/
 
	int32 velocityIterations = 8;
	int32 positionIterations = 1;
 
	// Instruct the world to perform a single step of simulation. It is
	// generally best to keep the time step and iterations fixed.
	world->Step(dt, velocityIterations, positionIterations);
 
 
	//Iterate over the bodies in the physics world
	for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
	{
		if(bomb)
		{
			if (bomblife > 0)
			{
				b2Body* ground = planet->GetBody();
				b2CircleShape* circle = (b2CircleShape*)planet->GetShape();
				// Get position of our "Planet" - Nick
				b2Vec2 center = ground->GetWorldPoint(circle->m_p);
				// Get position of our current body in the iteration - Nick
				b2Vec2 position = b->GetPosition();
				// Get the distance between the two objects. - Nick
				b2Vec2 d = center - position;
				// The further away the objects are, the weaker the gravitational force is - Nick
				float force = kRADIAL_GRAVITY_FORCE / d.LengthSquared(); // 150 can be changed to adjust the amount of force - Nick
				d.Normalize();
				b2Vec2 F = force * d;
				// Finally apply a force on the body in the direction of the "Planet" - Nick
				b->ApplyForce(F, position);
				bomblife--;
			}
			else {
				bomb = NO;
			}
 
		}
 
		if (b->GetUserData() != NULL) {
			//Synchronize the AtlasSprites position and rotation with the corresponding body
			CCSprite *myActor = (CCSprite*)b->GetUserData();
			myActor.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
			myActor.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
		}	
	}
}
 
- (void)ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
	//Add a new body/atlas sprite at the touched location
	for( UITouch *touch in touches ) {
		CGPoint location = [touch locationInView: [touch view]];
 
		location = [[CCDirector sharedDirector] convertToGL: location];
 
		if(!bomb) {
			// Create our static "Planet" - Nick
			b2CircleShape shape;
			shape.m_radius = 0.5f;
			shape.m_p.Set(location.x / PTM_RATIO,location.y / PTM_RATIO);
			b2FixtureDef fd;
			fd.shape = &shape;
			planet = groundBody->CreateFixture(&fd);
			// End Create Planet - Nick
			bomb = YES;
			bomblife = 120;
		}
	}
}
  2. Nick Vellios says:
    June 21, 2010 at 9:21 pm

    Funny you posted this. I was just thinking yesterday how I wanted to implement a timer to turn off the force. Thank you! I will be updating this in the next week or so to implement something like this.

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