box2d concave body - iphone

Question

I am creating a box2d body for my iOS game, that is built from 4 shapes that are convex.
The problem is that it fails when calling the init method.
Here's my code:
#implementation Banan
-(void)createBodyAtLocation:(CGPoint)location
{
int num;
float density = 1.0f;
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position = b2Vec2(location.x/PTM_RATIO/RETSIZE, location.y/PTM_RATIO/RETSIZE);
body = world->CreateBody(&bodyDef);
body -> SetUserData(self);
//first shape
b2FixtureDef fixtureDef1;
b2PolygonShape shape1;
num = 4;
b2Vec2 verts1[4];
verts1[0].Set(58.9f / PTM_RATIO, 46.3f / PTM_RATIO);
verts1[1].Set(63.3f / PTM_RATIO, 41.5f / PTM_RATIO);
verts1[2].Set(47.4f / PTM_RATIO, 15.6f / PTM_RATIO);
verts1[3].Set(43.6f / PTM_RATIO, 24.3f / PTM_RATIO);
shape1.Set(verts1, num);
fixtureDef1.shape = &shape1;
fixtureDef1.density = density;
//second
b2FixtureDef fixtureDef2;
b2PolygonShape shape2;
num = 5;
b2Vec2 verts2[5];
verts2[0].Set(42.1f / PTM_RATIO, 21.7f / PTM_RATIO);
verts2[1].Set(46.6f / PTM_RATIO, -0.1f / PTM_RATIO);
verts2[2].Set(29.1f / PTM_RATIO, -32.2f / PTM_RATIO);
verts2[3].Set(2.5f / PTM_RATIO, -45.2f / PTM_RATIO);
verts2[4].Set(6.8f / PTM_RATIO, -10.4f / PTM_RATIO);
shape2.Set(verts2, num);
fixtureDef2.shape = &shape2;
fixtureDef2.density = density;
//third
b2FixtureDef fixtureDef3;
b2PolygonShape shape3;
num = 4;
b2Vec2 verts3[4];
verts3[0].Set(5.6f / PTM_RATIO, -9.7f / PTM_RATIO);
verts3[1].Set(-0.3f / PTM_RATIO, -45.7f / PTM_RATIO);
verts3[2].Set(-32.7f / PTM_RATIO, -41.2f / PTM_RATIO);
verts3[3].Set(-28.2f / PTM_RATIO, -15.7f / PTM_RATIO);
shape3.Set(verts3, num);
fixtureDef3.shape = &shape3;
fixtureDef3.density = density;
//fourth
b2FixtureDef fixtureDef4;
b2PolygonShape shape4;
num = 4;
b2Vec2 verts4[4];
verts4[0].Set(-28.7f / PTM_RATIO, -14.8f / PTM_RATIO);
verts4[1].Set(-40.8f / PTM_RATIO, -36.0f / PTM_RATIO);
verts4[2].Set(-60.5f / PTM_RATIO, -2.4f / PTM_RATIO);
verts4[3].Set(-58.0f / PTM_RATIO, 2.9f / PTM_RATIO);
shape4.Set(verts3, num);
fixtureDef4.shape = &shape4;
fixtureDef4.density = density;
//attach to shape
body->CreateFixture(&fixtureDef1);
body->CreateFixture(&fixtureDef2);
body->CreateFixture(&fixtureDef3);
body->CreateFixture(&fixtureDef4);
}
- (id)initWithWorld:(b2World *)tworld atLocation:(CGPoint)location {
if ((self = [super init])) {
world = tworld;
[self initWithFile:#"Banana.png"];
[self createBodyAtLocation:location];
}
return self;
}
-(void)dealloc
{
[super dealloc];
}
#end
The '-initWithWorld: atLocation:' method is invoked by ccTouchBegan method and then kills my app.
The errors are the following:
Assertion failed: (area > 1.19209290e-7F), function ComputeCentroid, ...
-(void)createBananaAtLocation:(CGPoint)location
{
Banan *ban = [[[Banan alloc] initWithWorld:world atLocation:location] autorelease];//fails at this line when is invoked
[self addChild:ban];
}
-(BOOL)ccTouchBegan:(UITouch *)touch withEvent:(UIEvent *)event
{
CGPoint touchLocation = [touch locationInView:[touch view]];
touchLocation = [[CCDirector sharedDirector]
convertToGL:touchLocation];
touchLocation = [self convertToNodeSpace:touchLocation];
b2Vec2 locationWorld =
b2Vec2(touchLocation.x/PTM_RATIO, touchLocation.y/PTM_RATIO);
[self createBananaAtLocation:touchLocation]; //error at this line
return YES;
}

Answer 1

In your case vertices are oriented clockwise, while b2Polygon requirement is counterclockwise.
It leads to calculation of negative area of the polygon. You have to change order of your vertices.