Help me out with some sample code for getting the position of a sprite in Box2d in iPhone.
To get the position of a sprite from a box2d bodyDef you would first want to save your sprite in the userData property of the bodyDef.
For example, in a factory method that creates a ball within the physics environment:
//create the body
b2BodyDef initBodyDef;
initBodyDef.type = b2_dynamicBody;
initBodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
//Save the sprite in the userData property of the bodyDef, to access later
initBodyDef.userData = ballSprite;
b2Body *body = world->CreateBody(&initBodyDef);
//Rest of the factory method ............ (i.e. create shape, create fixture)
Then when you want to access the position of the sprite, for example when doing collision detection, you would get the pointer to the sprite within the userData property of the bodyDef:
This would be in the tick method (where collision detection takes place), or whereever you need to get the position of the sprite.
CCSprite *mySprite = (CCSprite *) bodyDef->GetUserData();
CGPoint spritePosition = mySprite.position;
In the first line of code above we create a sprite object and call the GetUserData method on our bodyDef, which returns the sprite we saved earlier. Note we have to cast the returned userData or it will return an error. Once we have the sprite saved all we have to do is call the position property on our mySprite pointer. Thats it.
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I've spent too long on this. Someone has solved this already, please help.
Enemy fires on my ship. SKPhysicsContact takes over and I get contactPoint. I explode the fired missile at the contactPoint, in world coordinates. All good. Now I would like to use the point, not in the scene coordinates returned by contactPoint, but in my ship coordinates to start emitting smoke from the ship, where I've been hit. I've used convertPoint function before, with success, but I can't seem to get it right here...Docs say the contactPoint is expressed in Scene coordinates, while myShip has its coordinates, living as a child of a world node, which is a child of the scene node. They are in the same Node hierarchy. I think...I have a Scene->World->Camera, where myShip is a child of World. My code, I think, is saying convert contactPoint from Scene coordinates to coordinates in myShip. But this does not work. Nor does any other combination. What am I missing? I suspect the Camera hierarchy, but unsure. The numbers being returned into smoke.position are way out of whack...
- (SKEmitterNode *) newSmokeEmitter: (SKPhysicsContact *) contact
{
NSString *smokePath = [[NSBundle mainBundle] pathForResource:#"ShipSmoke" ofType:#"sks"];
SKEmitterNode *smoke = [NSKeyedUnarchiver unarchiveObjectWithFile:smokePath];
smoke.targetNode = myShip;
smoke.name = #"shipSmoke";
smoke.position = [self convertPoint:contact.contactPoint toNode:myShip];
//my temporary kludge that places the smoke on the ship, randomly
//smoke.position = CGPointMake(skRand(-25,25), skRand(-25,+25));
NSLog(#"Ship at world pos: %f,%f", myShip.position.x, myShip.position.y);
NSLog(#"Contact at scene pos: %f,%f", contact.contactPoint.x, contact.contactPoint.y);
NSLog(#"Smoke position at ship pos: %f,%f", smoke.position.x, smoke.position.y);
[myShip addChild:smoke];
return smoke;
}
If you haven't solved it already, the answer is to convert from the node's scene:
smoke.position = [self convertPoint:contact.contactPoint fromNode:self.scene];
Or, in Swift:
smoke.position = convertPoint(contact.contactPoint, fromNode: scene)
If I understand your node hierarchy correctly this could fix the issue.
CGPoint contact = [self convertPoint:contact.contactPoint toNode:self.world];
smoke.position = [self.world convertPoint:contact toNode:myShip];
I don't think you can convert the point directly to ship. You have to convert it down the hierarchy until you get to the node you want.
Hopefully that helps.
Edit
To verify that the actual contact point is on the scene where I would expect you could try this.
SKSpriteNode *sprite = [SKSpriteNode spriteNodeWithColor:[SKColor greenColor] size:CGSizeMake(50, 50)];
sprite.position = contact.contactPoint;
[self addChild:sprite];
This should add a sprite exactly where the contact point is if the point returned is in the scene coordinate system. If it it does show up at the right spot than it truly does come down to converting points correctly.
I am having a physicsSprite of kinematics body type and I want to move the body and sprite bit down and back to its position. I tried it like this Inside update method:
for (NSValue *bodyValue in [self getGoalPostBodiesList])
{
b2Body *gPBody = (b2Body *)[bodyValue pointerValue];
PhysicsSprite *pSprite =(PhysicsSprite *) gPBody->GetUserData();
NSLog(#"\n tag value of Sprite = %d",pSprite.tag);
if(pSprite == goal1)
{
pSprite.position = CGPointMake((gPBody->GetPosition().x)*32.0,(gPBody->GetPosition().y)*32.0);
float angle = gPBody->GetAngle();
pSprite.rotation = -(CC_RADIANS_TO_DEGREES(angle));
id moveDownAction = [CCMoveTo actionWithDuration:0.5 position:CGPointMake(pSprite.position.x,(pSprite.position.y )- 40)];
id moveUpAction = [CCMoveTo actionWithDuration:0.5 position:CGPointMake(pSprite.position.x,(pSprite.position.y )+ 40)];
CCSequence *seqAction = [CCSequence actions:moveDownAction,moveUpAction, nil];
[pSprite runAction:seqAction];
b2Vec2 pos = b2Vec2(pSprite.position.x/32.0, pSprite.position.y/32.0);
gPBody->SetTransform(pos, CC_DEGREES_TO_RADIANS(pSprite.rotation));
gPBody->SetLinearVelocity(b2Vec2(0.0f, 0.0f));
gPBody->SetAngularVelocity(0.0f);
}
}
Still the sprite is not changing its position.
Anyone's help will be deeply appreciated.
Thanks all,
MONISH
To summarize your code, you update the position of your sprite to reflect that of the body, start an animation, and then update the position of the body to correspond to the position of the sprite. So naturally, nothing should move here, since your CCMoveTo actions have not exerted any effect on your sprite yet.
Second, your update method may be called very often, like dozens of times per second, so the animation gets reset continously and will not make any visible progress.
To follow a consistent pattern, how about you set the velocity of your kinematic bodies. Also, update the position of your sprites to correspond to these bodies as you would do for dynamic bodies, but don't set the transformation of your bodies to correspond to their sprites.
I'm trying to follow some sprite by some instance of CCParticleSystem using CCFollow. I don't want to make instance of CCParticleSystem a child of sprite, because I want it to be displayed some time after sprite removed.
When moving sprite from bottom-left to top-right corner my ParticleSystem moves from center to bottom-left corner. I can't understand why does it happen.
Here is sample code:
-(id) init
{
if( (self=[super init]) ) {
CCSprite *someSprite = [CCSprite spriteWithFile:#"Icon.png"];
[self addChild:someSprite];
id action = [CCMoveTo actionWithDuration:5 position:ccp(480,320)];
[someSprite runAction:action];
CCParticleSystemQuad *effect = [CCParticleMeteor node];
// effect.positionType = kCCPositionTypeFree;
// effect.positionType = kCCPositionTypeRelative;
// effect.positionType = kCCPositionTypeGrouped; changing of positionType to any of this options does not make any sense
[effect runAction:[CCFollow actionWithTarget:someSprite]];
[self addChild:effect];
}
return self;
}
It's a case of bad documentation.
I tried this myself and got the same result. By looking at the code and the fragment of documentation that exists, the CCFollow action is intended to be used only on fullscreen nodes like CCLayer, CCScene and perhaps CCNode. I would document it as follows:
CCFollow is an action for scrolling the entire screen by allowing the
layer or scene follow another node, such as a player object. It's an
alternative for moving the Camera object or updating the position
manually.
I have a standard cocos2d startup layer( HelloWorldLayer). I created another class of type CCNode named "Terrain" for my terrain. Then i add it to my layer in the layer's init:
terrain = [[Terrain alloc] initWithWorld:world AndLevel:0];
[self addChild:terrain z:1];
i add a 'CarObject' class (a CCSprite class), and add a car object to my terrain
car = [[CarObject alloc] initWithWorld:world];
[terrain addChild:car];
-i.e. in both the initWithWorld for terrain and car, i initialize some Box2d code
I then try to center my car object to my screen when i move it, i do this in my update method:
float offsetX = car.position.x;
float offsetY = car.position.y;
[terrain setOffsetX:(int)offsetX andOffsetY:(int)offsetY];
where the setOffsetX.. method is:
- (void) setOffsetX:(int)newOffsetX andOffsetY:(int)newOffsetY {
_offsetX = newOffsetX;
_offsetY = newOffsetY;
CGSize winSize = [CCDirector sharedDirector].winSize;
self.position = CGPointMake(-(_offsetX - winSize.width/2), -(_offsetY - winSize.height/2));
}
When i use a NSLog to see if the terrain position changes, i can see that the position actually chages, but the view does not. What am i doing wrong? am sure it's a dumb mistake!
btw, if i try this in my HelloWorldLayer's update method (instead of [terrain setOffsetX..])
self.position = CGPointMake(self.position.x-1, self.position.y);
the terrain is moving.
Car is a child of Terrain. Car's position is therefore relative to Terrain's position. Since you base Terrain's position on Car's position, which is actually relative to Terrain's position, you may be simply running into the effect that your position updates simply cancel each other out.
If you want to move the Terrain while keeping the Car centered, you shouldn't add the Car as a child of Terrain. Instead add it to the same node as the Terrain (HelloWorldLayer). Then you can move the Car and Terrain independently of each other.
I have 2 questions with b2Body:
What is the difference between b2Body and b2BodyDef?
How would I add a b2Body to a CCScene with coordinates from a CGRect which I already have coded? Also how would I add userData to it so I can keep a reference to this?
Thanks!
A b2BodyDef is used to define information about a body as a whole, such as position and rotation. Compared to the other information you require for a b2Body, such as friction and resititution, that is defined on a per fixture basis using b2Fixtures. The b2Body is an amalgamation of a body definition and at least one fixture.
With regard to creating the body from a predefined rect, I'd advise using setAsBox: assuming you are using a b2PolygonShape.
The way I usually accomplish the joining of the two is to create a class called BodyNode which has ivars of a b2Body and a CCSprite. Assign either the BodyNode, i.e. self or the sprite as the userData and update them as follows:
-(void) onEnter
{
[self scheduleUpdate];
[super onEnter];
}
-(void) update:(ccTime) dt
{
//Update the position of the sprite to the position of the body
//Update the rotation of the body to the rotation of the sprite. Take care to note that the rotation of the sprite is in degrees whereas the rotation of the body is in radians.
}