Add Radial Gravity to Box2D

Published on June 6th, 2010 in iPhone Code Snippets & Tutorials, iPhone Programming

Radial gravity, orbital gravity, orbital mechanics, whatever you want to call it there is no denying it can be useful for many kinds of games. Unfortunately, neither Box2D nor Chipmunk Physics Engines for the iPhone had support for it. I had a hell of a time trying to figure this out but in the end it was so simple. Here is how to do it.

First, create a new Cocos2D / Box2D Hello World project from the supplied Cocos2D templates. Add the following code to HellowWorldScene.mm and HelloWorldScene.h:

HelloWorldScene.mm

-(id) init {
//... Add at end of init after body->CreateFixture(&fixtureDef);
	// Create our static "Planet"
	b2CircleShape shape;
	shape.m_radius = 1.0f;
	shape.m_p.Set(8.0f, 8.0f);
	b2FixtureDef fd;
	fd.shape = &shape;
	planet = groundBody->CreateFixture(&fd);
}
 
-(void) tick: (ccTime) dt
{
	int32 velocityIterations = 8;
	int32 positionIterations = 1;
 
	world->Step(dt, velocityIterations, positionIterations);
 
	for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
	{
		b2Body* ground = planet->GetBody();
		b2CircleShape* circle = (b2CircleShape*)planet->GetShape();
		// Get position of our "Planet"
		b2Vec2 center = ground->GetWorldPoint(circle->m_p);
		// Get position of our current body in the iteration
		b2Vec2 position = b->GetPosition();
		// Get the distance between the two objects.
		b2Vec2 d = center - position;
		// The further away the objects are, the weaker the gravitational force is
		float force = 250.0f / d.LengthSquared(); // 150 can be changed to adjust the amount of force
		d.Normalize();
		b2Vec2 F = force * d;
		// Finally apply a force on the body in the direction of the "Planet"
		b->ApplyForce(F, position);
 
		if (b->GetUserData() != NULL) {
			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());
		}	
	}
}

HelloWorldScene.h

@interface HelloWorld : CCLayer
{
	b2World* world;
	GLESDebugDraw *m_debugDraw;
	b2Fixture* planet; // This will be a static body
}

And here is the result!



Enjoy!

Download Source Code for this tutorial:
Radial Gravity Box2d & Cocos2d Example Source Code – 692k

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Comments

10 Responses to “Add Radial Gravity to Box2D”

  1. Radial Gravity w/ Box2D SOURCE « Vellios.com - Nick Vellios says:
    June 11, 2010 at 10:43 pm

    [...] other day I wrote a tutorial on how to implement orbital / radial gravity in with the use of Box2D and Cocos2D. I have decided [...]

  2. Joseph Neuman says:
    June 12, 2010 at 1:48 am

    interesting.

    i did something similar to implement “suns” in my space pack

    here is my code: (just the physics part, i obviously added a sprite/particle effect over it)

    THIS IS ALL WITHIN A UPDATE LOOP THAT RUNS AS FOLLOWS: [self schedule:@selector(tick:)];

    //In Box2D the bodies are a linked list, so keep getting the next one until it doesn't exist.
	CGPoint pos = CGPointMake([[CCDirector sharedDirector] winSize].width/2, [[CCDirector sharedDirector] winSize].height/2);
	for (b2Body* b = _world->GetBodyList(); b; b = b->GetNext())
	{
		//Box2D uses meters, there's 32 pixels in one meter. PTM_RATIO is defined somewhere in the class.
		b2Vec2 b2TouchPosition = b2Vec2(pos.x/PTM_RATIO, pos.y/PTM_RATIO);
		b2Vec2 b2BodyPosition = b2Vec2(b->GetPosition().x, b->GetPosition().y);
 
		//Don't forget any measurements always need to take PTM_RATIO into account
		//Remove maxDistance or set it to something really high if you want it to have a global effect
		float maxDistance = 6; // In your head don't forget this number is low because we're multiplying it by 32 pixels;
		float maxForce = -0.66; // Lower this number, it's pretty extreme
		CGFloat distance;
		CGFloat strength;
		float force;
		CGFloat angle;
 
		distance = b2Distance(b2BodyPosition, b2TouchPosition);
		if(distance > maxDistance) distance = maxDistance - 0.01;
 
		// Normally if distance is max distance, it'll have the most strength, this makes it so the opposite is true - closer = stronger
		strength = (maxDistance - distance) / maxDistance; // This makes it so that the closer something is - the stronger, instead of further
		force = strength * maxForce;
		angle = atan2f(b2BodyPosition.y - b2TouchPosition.y, b2BodyPosition.x - b2TouchPosition.x);
		//NSLog(@" distance:%0.2f,force:%0.2f,angle:%0.2f", distance, force, angle);
		// Apply an impulse to the body, using the angle
		b->ApplyImpulse(b2Vec2(cosf(angle) * force, sinf(angle) * force), b->GetPosition());
	}
  3. Bombs with Box2D & Cocos2D « Vellios.com - Nick Vellios says:
    June 12, 2010 at 3:02 pm

    [...] is some example code based off my tutorial on how to implement orbital / radial gravity, but in reverse. This example pushes objects away [...]

  4. Nick Vellios says:
    June 12, 2010 at 3:33 pm

    Thanks Joseph, check out my recent post about using this code to build a bomb-like effect.

  5. Required says:
    June 15, 2010 at 1:23 pm

    Is there anyway you can port this to Action script so I may use it in flash.

  6. Nick Vellios says:
    June 15, 2010 at 1:31 pm

    I don’t use Flash, sorry. If you are familiar with Flash and Actionscript, you should be able to port this yourself.

  7. iPhone Game Physics Tutorial: Radial Gravity | iPhone iOS 4 iPad SDK Development Tutorials, Programming Tips, News says:
    July 26, 2010 at 1:49 pm

    [...] How To Add Radial Gravity To Box 2D Radial Gravity w/ Box2D SOURCE [...]

  8. forex robot says:
    July 30, 2010 at 10:00 am

    Keep posting stuff like this i really like it

  9. MOOGAB MYUNG says:
    August 14, 2010 at 12:25 am

    Hi ~
    This is very simple and useful post.

    I find this danyeoteotda.

  10. Nick Vellios says:
    August 14, 2010 at 1:54 am

    Thanks for reading!

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