iphone – 如何创建一个新的CMAttitude参考框架,使重力在Y轴上

>启动设备运动更新
>在后台启动相机预览;)
>将重力存储在本地变量中后，将当前重力读数作为CMAcceleration捕获。
>然后你必须拿2个向量并得到它们之间的角度，在这种情况下，设备重力为(0,0，-1)和真实的重力矢量…
>然后我们将theta转换为thetaPrime …与CoreMotion参考方向匹配的变换
>设置一个计时器来动画….
>在动画获取motionManager的deviceMotion属性的rotationMatrix的逆。
>以正确的顺序应用变换，以反映设备的当前态度(偏航，俯仰，滚动欧拉模式或设备四元数旋转…基本上同样的3种不同的方式)

这是代码：

- (void) initMotionCapture
{
    firstGravityReading = NO;
    referenceAttitude = nil;

    if (motionManager == nil)
    {
        self.motionManager = [CMMotionManager new];
    }
    motionManager.deviceMotionUpdateInterval = 0.01;
    self.gravityTimer = [NSTimer scheduledTimerWithTimeInterval:1/60.0 target:self selector:@selector(getFirstGravityReading) userInfo:nil repeats:YES];
}


- (void) getFirstGravityReading
{
    CMAcceleration currentGravity; 

    CMDeviceMotion *dm = motionManager.deviceMotion;
    referenceAttitude = dm.attitude;
    currentGravity = dm.gravity;

    [motionManager startDeviceMotionUpdates];

    if (currentGravity.x !=0 && currentGravity.y !=0 && currentGravity.z !=0)
    {
        NSLog(@"Gravity = (%f,%f,%f)",currentGravity.x,currentGravity.y,currentGravity.z);

        firstGravityReading = YES;
        [gravityTimer invalidate];
        self.gravityTimer = nil;
        [self setupCompass];
    }
}

- (void) setupCompass
{
    //Draw your cube... I am using a quartz 3D perspective hack!
    CATransform3D initialTransform = perspectiveTransformedLayer.sublayerTransform;
    initialTransform.m34 = 1.0/-10000;


    //HERE IS WHAT YOU GUYS NEED... the vector equations!
    NSLog(@"Gravity = (%f,currentGravity.z);

    //we have current gravity vector and our device gravity vector of (0,-1)
    // get the dot product
    float dotProduct = currentGravity.x*0 + currentGravity.y*0 + currentGravity.z*-1;
    float innerMagnitudeProduct = currentGravity.x*currentGravity.x + currentGravity.y + currentGravity.y + currentGravity.z*currentGravity.z;
    float magnitudeCurrentGravity = sqrt(innerMagnitudeProduct);
    float magnitudeDeviceVector = 1; //since (0,-1) computes to: 0*0 + 0*0 + -1*-1 = 1

    thetaOffset = acos(dotProduct/(magnitudeCurrentGravity*magnitudeDeviceVector));
    NSLog(@"theta(degrees) = %f",thetaOffset*180.0/M_PI);

    //Now we have the device angle to the gravity vector (0,-1)
    //We must transform these coordinates to match our 
    //device's attitude by transforming to theta prime
    float theta_deg = thetaOffset*180.0/M_PI;
    float thetaPrime_deg = -theta_deg + 90; // ThetaPrime = -Theta + 90 <==> y=mx+b

    NSLog(@"thetaPrime(degrees) = %f",thetaOffset*180.0/M_PI);

    deviceOffsetRotation = CATransform3DMakeRotation((thetaPrime_deg) * M_PI / 180.0,1,0);
    initialTransform = CATransform3DConcat(deviceOffsetRotation,initialTransform);

    perspectiveTransformedLayer.sublayerTransform = initialTransform;

    self.animationTimer = [NSTimer scheduledTimerWithTimeInterval:1/60.0 target:self selector:@selector(tick) userInfo:nil repeats:YES];

}

- (void) tick
{
    CMRotationMatrix rotation;

    CMDeviceMotion *deviceMotion = motionManager.deviceMotion;
    CMAttitude *attitude = deviceMotion.attitude;

    if (referenceAttitude != nil)
    {
        [attitude multiplyByInverSEOfAttitude:referenceAttitude];
    }
    rotation = attitude.rotationMatrix;

    CATransform3D rotationalTransform = perspectiveTransformedLayer.sublayerTransform;

    //inverse (or called the transpose) of the attitude.rotationalMatrix
    rotationalTransform.m11 = rotation.m11;
    rotationalTransform.m12 = rotation.m21;
    rotationalTransform.m13 = rotation.m31;

    rotationalTransform.m21 = rotation.m12;
    rotationalTransform.m22 = rotation.m22;
    rotationalTransform.m23 = rotation.m32;

    rotationalTransform.m31 = rotation.m13;
    rotationalTransform.m32 = rotation.m23;
    rotationalTransform.m33 = rotation.m33;

    rotationalTransform = CATransform3DConcat(deviceOffsetRotation,rotationalTransform);
    rotationalTransform = CATransform3DConcat(rotationalTransform,CATransform3DMakeScale(1.0,-1.0,1.0));


    perspectiveTransformedLayer.sublayerTransform = rotationalTransform;
}