Chee Chee - 1 month ago 8x
iOS Question

CoreLocation heading base on back camera (Augmented reality)

I would like to create an augmented reality view that is going to point an object in a direction. However, the CoreLocation heading is not working correctly when you are facing upwards with your camera (say to a top of a 20-storeys building when you are on the ground floor).

It is giving the opposite direction (probably the direction that is pointing by the top of the phone).

I have tried a few methods to get it to work for the direction the camera is pointing, such as:

1, +180 degree when the device orientation is > 45 degree (not accurate enough, suddenly the direction goes off by 10~20 degree)

2, attempted to calculate using CMMotionManager with formula from the below tutorial.

3, attempted to simulate logic from android using ios deviceMotion.magneticField and deviceMotion.gravity.

4, use rotation matrix (some other post in stack overflow, but not accurate)

double heading = M_PI + atan2(self.altitudeData.rotationMatrix.m22, self.altitudeData.rotationMatrix.m12);
heading = heading*180/M_PI;

I am running out of idea what else could I try to get it right. I know there are some apps out there (some app that can see the sun and star) that is doing it correctly.


After much of research and testing. I ended up using GLKit for the calculation, as it saves me lots of trouble as well. Just leave it here for anyone who happen to get to this question.

First, I started the CMMotionManager device motion updates with CMAttitudeReferenceFrameXTrueNorthZVertical.

self.hasMotion = NO;
CMMotionManager *cmmotionManager = [[CMMotionManager alloc] init];
[cmmotionManager startDeviceMotionUpdatesUsingReferenceFrame:CMAttitudeReferenceFrameXTrueNorthZVertical
                                                     toQueue:[[NSOperationQueue alloc] init]
                                                 withHandler:^ (CMDeviceMotion *motion, NSError *error) {
                                                     self.hasMotion = YES;

self.motionManager = cmmotionManager;

From some codes that I found on the web to draw an openGL world using CoreMotion rotation and mix it with getting a point from screen to 3D world:

float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(45.0f), aspect, 0.1f, 100.0f);

CMRotationMatrix r = self.motionManager.deviceMotion.attitude.rotationMatrix;
GLKMatrix4 camFromIMU = GLKMatrix4Make(r.m11, r.m12, r.m13, 0,
                                       r.m21, r.m22, r.m23, 0,
                                       r.m31, r.m32, r.m33, 0,
                                       0,     0,     0,     1);

GLKMatrix4 viewFromCam = GLKMatrix4Translate(GLKMatrix4Identity, 0, 0, 0);
GLKMatrix4 imuFromModel = GLKMatrix4Identity;
GLKMatrix4 viewModel = GLKMatrix4Multiply(imuFromModel, GLKMatrix4Multiply(camFromIMU, viewFromCam));
bool isInvertible;
GLKMatrix4 modelView = GLKMatrix4Invert(viewModel, &isInvertible);

int viewport[4];
viewport[0] = 0.0f;
viewport[1] = 0.0f;
viewport[2] = self.view.frame.size.width;
viewport[3] = self.view.frame.size.height;

bool success;
//assume center of the view
GLKVector3 vector3 = GLKVector3Make(self.view.frame.size.width/2, self.view.frame.size.height/2, 1.0);     
GLKVector3 calculatedPoint = GLKMathUnproject(vector3, modelView, projectionMatrix, viewport, &success);
    //CMAttitudeReferenceFrameXTrueNorthZVertical always point x to true north
    //with that, -y become east in 3D world
    float angleInRadian = atan2f(-calculatedPoint.y, calculatedPoint.x);
    return angleInRadian;