Box2d object throwing smoother and on same velocity - ios

Question

I have an box2d object which i am throwing from top to bottom and i have set its speed constant but when i run it , that object has different speed sometimes and how can i make this object more smoother.
Following are some methods to show how i have created box2d world and box2d body object.
#pragma -mark Box2D World
-(void)createWorld
{
// Define the gravity vector.
b2Vec2 b_gravity;
b_gravity.Set(0.0f, -9.8f);
// Do we want to let bodies sleep?
// This will speed up the physics simulation
bool doSleep = true;
// Construct a world object, which will hold and simulate the rigid bodies.
world = new b2World(b_gravity);
world->SetAllowSleeping(doSleep);
world->SetContinuousPhysics(true);
}
-(void) createWeb
{
freeBodySprite = [CCSprite spriteWithFile:#"web1.png"];//web_ani_6_1
//freeBodySprite.position = ccp(100, 300);
[self addChild:freeBodySprite z:2 tag:6];
CGPoint startPos = CGPointMake(100, 320/1.25);
bodyDef.type = b2_staticBody;
bodyDef.position = [self toMeters:startPos];
bodyDef.userData = freeBodySprite;
float radiusInMeters = ((freeBodySprite.contentSize.width * freeBodySprite.scale/PTM_RATIO) * 0.5f);
shape.m_radius = radiusInMeters;
fixtureDef.shape = &shape;
fixtureDef.density = 0.07f;
fixtureDef.friction = 0.1f;
fixtureDef.restitution = 0.1f;
circularObstacleBody = world->CreateBody(&bodyDef);
stoneFixture = circularObstacleBody->CreateFixture(&fixtureDef);
freeBody = circularObstacleBody;
}
-(b2Vec2) toMeters:(CGPoint)point
{
return b2Vec2(point.x / PTM_RATIO, point.y / PTM_RATIO);
}
-(b2Body *) getBodyAtLocation:(b2Vec2) aLocation {
for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
{
b2Fixture* bodyFixture = b->GetFixtureList();
if (bodyFixture->TestPoint(aLocation)){
return b;
}
}
return NULL;
}
-(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 = 3;
// 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 (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());
}
}
}
This is my touch Event where i am getting angle and speed to throw .
- (void)ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{
//get the location of the end point of the swipe
UITouch *myTouch = [touches anyObject];
CGPoint location = [myTouch locationInView:[myTouch view]];
location = [[CCDirector sharedDirector] convertToGL:location];
//CCLOG(#"Start -> %0.f || End -> %0.f",startPoint.x,location.x);
if (freeBody) {
//[self calcAngleAndRotateObjectStartingAtPoint:startPoint endingAtPoint:location];
self.isTouchEnabled = NO;
freeBody->SetType(b2_dynamicBody);
//this is the maximum force that can be applied
const CGFloat maxForce = 20;
//get the rotation b/w the start point and the end point
CGFloat rotAngle = atan2f(location.y - startPoint.y,location.x - startPoint.x);
//the distance of the swipe if the force
CGFloat distance = ccpDistance(startPoint, location) * 0.5;
//if (distance>maxForce)
distance = maxForce;
//else
// distance = 10;
//apply force
freeBody->ApplyForce(b2Vec2(cosf(rotAngle) * distance, sinf(rotAngle) * distance), freeBody->GetPosition());
//lose the weak reference to the body for next time usage.
freeBody = nil;
}
}
This is code i am using to throw , but sometimes its speed is faster and some time slower , and i have set maxForce = 20 for constant speed.

Answer 1

As the comment above world->Step() dictates, you should use fixed dt. Verify that dt is fixed and world->Step() is being called at regular interval.

Answer 2

Finnally i have solved this problem. i changed ApplyForce with SetLinearVelocity..
here is the code.
float spd = 10;
b2Vec2 velocity = spd*b2Vec2(cos(rotAngle), sin(rotAngle));
freeBody->SetLinearVelocity(velocity);