iPhone Dev

In this tutorial, we will utilize Accelerometer in the Bouncing Ball example. The ball will move when you rotate or tilt your iPhone. To complete this tutorial, you need to get the certificate to put your app on a device. It is because when your application asks for updates from Accelerometer, the simulator will not response.

Generally, you need three steps to make your Cocos2d app to have accelerometer capability. Before that, we need to make walls and roof to prevent the ball moving out of the screen.  Add the following codes at the end of setChipmunk function:

floorShape = cpSegmentShapeNew(floorBody, cpv(0,0), cpv(0,480), 0);
floorShape->e = 0.5;
floorShape->u = 0.1;
floorShape->collision_type = 0;
cpSpaceAddStaticShape(space, floorShape);

floorShape = cpSegmentShapeNew(floorBody, cpv(0,480), cpv(320,480), 0);
floorShape->e = 0.5;
floorShape->u = 0.1;
floorShape->collision_type = 0;
cpSpaceAddStaticShape(space, floorShape);

floorShape = cpSegmentShapeNew(floorBody, cpv(320,0), cpv(320,480), 0);
floorShape->e = 0.5;
floorShape->u = 0.1;
floorShape->collision_type = 0;
cpSpaceAddStaticShape(space, floorShape);
Step 1: Enable accelerometer in Cocos2d app. Add the following code at the end of init method of BallLayer class.

isAccelerometerEnabled = YES;

Step 2: Set update interval. This interval determines how frequently we get the data from accelerometer. It is also how frequently the updating function (implemented in Step 3) will be called. In this example, we choose 0.033. You should choose appropriate interval based on the types of your application. Add the following code at the end of init method of BallLayer class.

[[UIAccelerometer sharedAccelerometer] setUpdateInterval:(1.0 / 30)];

Step 3: Implement updating function. You should acquire the data from accelerometer in this function. Add the following function to the implementation of class BallLayer.

#define kFilterFactor 0.05
- (void)accelerometer:(UIAccelerometer*)accelerometer didAccelerate:(UIAcceleration*)acceleration{
static float prevX=0, prevY=0;

float accelX = acceleration.x * kFilterFactor + (1- kFilterFactor)*prevX;
float accelY = acceleration.y * kFilterFactor + (1- kFilterFactor)*prevY;

prevX = accelX;
prevY = accelY;

cpVect v = cpv( accelX, accelY);
space->gravity = cpvmult(v, 2000);
}

Line 2 and 3 of the codes should be noted. Here we use low-pass filter (0.05) to reduce the influence of sudden movement (i.e., less sensitive). For more information about low-pass and high-pass filter, please refer to iPhone Application Programming Guide or related books. In this function, Z axis is ignored. According to the new values of X and Y, the gravity of ChipMunk space is calculated. Note: The value of gravity determines the direction which the ball moves to.

Now, the Bouncing Ball app owns accelerometer capability.