Moving and rotating a sprite with Accelerometer in SpriteKit

Question

I'm trying to make my first game using Spritekit, so i have a sprite that i need to move around using my accelerometer. Well, no problem doing that; movement are really smooth and responsive, the problem is that when i try to rotate my sprite in order to get it facing its own movement often i got it "shaking" like he has parkinson. (:D)

i did realize that this happens when accelerometer data are too close to 0 on one of x, y axes.

So the question: Is there a fix for my pet parkinson?? :D

Here is some code:

-(void) update:(NSTimeInterval)currentTime{
    static CGPoint oldVelocity;
    //static CGFloat oldAngle;

    if(_lastUpdatedTime) {
        _dt = currentTime - _lastUpdatedTime;
    } else {
        _dt = 0;
    }

    _lastUpdatedTime = currentTime;

    CGFloat updatedAccelX = self.motionManager.accelerometerData.acceleration.y;
    CGFloat updatedAccelY = -self.motionManager.accelerometerData.acceleration.x+sinf(M_PI/4.0);


    CGFloat angle = vectorAngle(CGPointMake(updatedAccelX, updatedAccelY));

    _velocity = cartesianFromPolarCoordinate(MAX_MOVE_PER_SEC, angle);


    if(oldVelocity.x != _velocity.x || oldVelocity.y != _velocity.y){
        _sprite.physicsBody.velocity = CGVectorMake(0, 0);
        [_sprite.physicsBody applyImpulse:CGVectorMake(_velocity.x*_sprite.physicsBody.mass, _velocity.y*_sprite.physicsBody.mass)];

        _sprite.zRotation = vectorAngle(_velocity);
        oldVelocity = _velocity;

    }
}

static inline CGFloat vectorAngle(CGPoint v){
    return atan2f(v.y, v.x);
}

i did try to launch the update of the _velocity vector only when updatedAccelX or updatedAccelY are, in absolute value >= of some values, but the result was that i got the movement not smooth, when changing direction if the value is between 0.1 and 0.2, and the problem wasn't disappearing when the value was under 0.1.

i would like to maintain direction responsive, but i also would like to fix this "shake" of the sprite rotation.

I'm sorry for my bad english, and thanks in advance for any advice.

Answer 1

You can try a low pass filter (cf. to isolate effect of gravity) or high pass filter (to isolate effects of user acceleration).

    #define filteringFactor 0.1

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

        //low pass
        accelerX = (acceleration.x * filteringFactor) + (accelerX * (1.0 - filteringFactor));
        //idem …  accelerY
        //idem … accelerZ 

        //or high pass
        accelerX = acceleration.x - ( (acceleration.x * filteringFactor) + (accelerX * (1.0 - filteringFactor)) );
        //idem …  accelerY
        //idem … accelerZ
    }