Accelerometer in Box2D - iphone

Question

I am new in Box2D....
I have ball image in CCSprite. I want to move ball in whole screen using accelerometer...
tell me
How to use accelerometer in box2d??
Thanks...in advance

Answer 1

The standard cocos2d-box2d template file moves boxes using the accelerometer by applying gravity relative to the accelerometer value.
- (void)accelerometer:(UIAccelerometer*)accelerometer didAccelerate:(UIAcceleration*)acceleration
{
static float prevX=0, prevY=0;
//#define kFilterFactor 0.05f
#define kFilterFactor 1.0f // don't use filter. the code is here just as an example
float accelX = (float) acceleration.x * kFilterFactor + (1- kFilterFactor)*prevX;
float accelY = (float) acceleration.y * kFilterFactor + (1- kFilterFactor)*prevY;
prevX = accelX;
prevY = accelY;
// accelerometer values are in "Portrait" mode. Change them to Landscape left
// multiply the gravity by 10
b2Vec2 gravity( -accelY * 10, accelX * 10);
world->SetGravity( gravity );
}
You need to be more specific on what you want the ball to do dependent on how you move the phone. Your question is difficult to answer in its current form.

Answer 2

Get the accelerometer measurements and say Force = coefficient*measurements. The apply this force to your b2Body

Answer 3

Let your Ball Sprite having tag is 1.
Replace this code with your Accelerometer delegate,
I test it on device, its working.
and your ball will move with accelerometer.
-(void)accelerometer:(UIAccelerometer*)accelerometer didAccelerate:(UIAcceleration*)acceleration
{
#define kFilterFactor 0.75
accelerometer.updateInterval = 1.0f/60.0f;
static UIAccelerationValue rollingX = 0, rollingY = 0;
for (b2Body *b = world->GetBodyList(); b; b = b->GetNext())
{
if (b->GetUserData() != NULL)
{
CCSprite *sprite = (CCSprite*)b->GetUserData();
if (sprite.tag == 1) {
rollingX = (acceleration.x * kFilterFactor) + (rollingX * 0.25);
rollingY = (acceleration.y * kFilterFactor) + (rollingY * 0.25);
float accelX = rollingX;
float accelY = rollingY;
CGPoint moveNewPosition = sprite.position;
if (accelX > 0.1) {
moveNewPosition.y += 2;
} if (accelX < 0.1) {
moveNewPosition.y -= 2;
}
if (accelY > 0.1) {
moveNewPosition.x -= 2;
} if (accelY < -0.1) {
moveNewPosition.x += 2;
}
b->SetLinearVelocity(b2Vec2(2,2));
sprite.position = ccp(moveNewPosition.x , moveNewPosition.y );
sprite.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
}
}
}
}
I hope it'll work.