Cocos2D - How to move sprite on the X axis, using the accelerometer

Question

I'm trying to make a vertical shooter, using Cocos2D and a few tutorials. Unfortunately, most tutorials only show how to make an horizontal shooter.

Still, I tried to adapt the code from a horizontal shooter to a vertical one, but am having a few problems. Mainly, my ship keeps moving right, even when the iPhone is sitting perfectly still.

Seeing as I'm a beginner, I'm having trouble pinpointing the exact issue and I'd love if anyone could help me.

Here's my accelerometer method:

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

    define kFilteringFactor 0.1

    define kRestAccelX -0.6

    define kShipMaxPointsPerSec (winSize.width*0.5)  

    define kMaxDiffX 0.2

    UIAccelerationValue rollingX;

    rollingX = (acceleration.x * kFilteringFactor) + (rollingX * (1.0 - kFilteringFactor));

    float accelX = acceleration.x - rollingX;

    CGSize winSize = [CCDirector sharedDirector].winSize;

    float accelDiff = accelX - kRestAccelX;
    float accelFraction = accelDiff / kMaxDiffX;
    float pointsPerSec = kShipMaxPointsPerSec * accelFraction;

    _shipPointsPerSecX = pointsPerSec;


}

And my update method:

-(void)update:(ccTime)dt {

    CGSize winSize = [CCDirector sharedDirector].winSize;
    float maxX = winSize.width - _ship.contentSize.width/2;
    float minX = _ship.contentSize.width/2;

    float newX = _ship.position.x + (_shipPointsPerSecX * dt);
    newX = MIN(MAX(newX, minX), maxX);
    _ship.position = ccp(newX, _ship.position.y);

}

What could be causing this?

Answer 1

You don't really need this much code to do sliding across the ground.

Try simply:

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
     _shipPointsPerSecX = acceleration.x * 10;
}

To get the ship to accelerate and decelerate you could try

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
     float targetSpeed = acceleration.x * 10;
     _shipPointsPerSecX = (_shipPointsPerSecX * .8f) + (targetSpeed * .2f);
}

But this isn't really accurate, a more realistic response might involve sliding towards the target value:

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
    float targetSpeed = acceleration.x * 10;
    if(_shipPointsPerSecX < targetSpeed) {
        _shipPointsPerSecX = MIN(_shipPointsPerSecX + 2, targetSpeed);
    } else {
        _shipPointsPerSecX = MAX(_shipPointsPerSecX - 2, targetSpeed);
    }
}

Answer 2

Move the sprite from top to bottom and left to right using Accelerometer:

CCScene* PhysicsGame::scene()
{
    // 'scene' is an autorelease object
    CCScene *scene = CCScene::create();

    // 'layer' is an autorelease object
    PhysicsGame *layer = PhysicsGame::create();

    // add layer as a child to scene
    scene->addChild(layer);

    // return the scene
    return scene;
}

// on "init" you need to initialize your instance
bool PhysicsGame::init()
{

    // 1. super init first
    if ( !CCLayer::init())
    {
        return false;
    }

    winSize=CCDirector::sharedDirector()->getWinSize();

    BG=CCSprite::create("Yellowbg.png");
    BG->setPosition(ccp(winSize.width/2,winSize.height/2));
    this->addChild(BG,1);

//    _backgroundNode = CCParallaxNodeExtras::node();
//    this->addChild(_backgroundNode,-1);

    asteroid = CCSprite::create("Ball_image.png");
    asteroid->setPosition(ccp(winSize.width*0.1,winSize.height/2));
    this->addChild(asteroid,1);

    this->scheduleUpdate();

    this->setAccelerometerEnabled(true);

    return true;

}

void PhysicsGame::update(float dt)
{

    winSize = CCDirector::sharedDirector()->getWinSize();

    float maxY = winSize.height - asteroid->getContentSize().height/2;
    float minY = asteroid->getContentSize().height/2;

    float diff = (_shipPointsPerSecY * dt);
    float newY = asteroid->getPosition().y + diff;
    newY = MIN(MAX(newY, minY), maxY);

    asteroid->setPosition(ccp(asteroid->getPosition().x, newY));


    float minX=asteroid->getContentSize().width/2;

    float maxX = winSize.width - asteroid->getContentSize().width/2;

    float diff1 = (_shipPointsPerSecX * dt);

    float newX=asteroid->getPosition().x + diff1;

    newX=MIN(MAX(newX, minX), maxX);

    asteroid->setPosition(ccp(newX, asteroid->getPosition().y));

}


void PhysicsGame::didAccelerate(CCAcceleration *pAccelerationValue)
{


#define KFILTERINGFACTOR 0.1
#define KRESTACCELX -0.6
#define KSHIPMAXPOINTSPERSEC (winSize.height*0.5)
#define KMAXDIFFX 0.2


    double rollingX;

    // Cocos2DX inverts X and Y accelerometer depending on device orientation
    // in landscape mode right x=-y and y=x !!! (Strange and confusing choice)

    rollingX = (pAccelerationValue->x * KFILTERINGFACTOR) + (rollingX * (1.0 - KFILTERINGFACTOR));

    float accelX = pAccelerationValue->x - rollingX;

    float accelDiff = accelX - KRESTACCELX;

    float accelFraction = accelDiff / KMAXDIFFX;

    float pointsPerSec = KSHIPMAXPOINTSPERSEC * accelFraction;

    _shipPointsPerSecX = pointsPerSec;


#define KFILTERINGFACTOR1 0.1
#define KRESTACCELY -0.6
#define KSHIPMAXPOINTSPERSEC1 (winSize.width*0.5)
#define KMAXDIFFX1 0.2

    double rollingY;

    rollingY = (pAccelerationValue->y * KFILTERINGFACTOR) + (rollingY * (1.0 - KFILTERINGFACTOR1));

    float accelY = pAccelerationValue->y - rollingY;

    float accelDiff1= accelY - KRESTACCELY;

    float accelFraction1 = accelDiff1 / KMAXDIFFX1;

    float pointsPerSec1 = KSHIPMAXPOINTSPERSEC1 * accelFraction1;

    _shipPointsPerSecY = pointsPerSec1;

}