CMMotionManager changes from cached reference frame not working as expected - quaternions

Question

When I call startDeviceMotionUpdatesUsingReferenceFrame, then cache a reference to my first reference frame and call multiplyByInverseOfAttitude on all my motion updates after that, I don't get the change from the reference frame that I am expecting. Here is a really simple demonstration of what I'm not understanding.
self.motionQueue = [[NSOperationQueue alloc] init];
self.motionManager = [[CMMotionManager alloc] init];
self.motionManager.deviceMotionUpdateInterval = 1.0/20.0;
[self.motionManager startDeviceMotionUpdatesUsingReferenceFrame: CMAttitudeReferenceFrameXArbitraryZVertical toQueue:self.motionQueue withHandler:^(CMDeviceMotion *motion, NSError *error){
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
CMAttitude *att = motion.attitude;
if(self.motionManagerAttitudeRef == nil){
self.motionManagerAttitudeRef = att;
return;
}
[att multiplyByInverseOfAttitude:self.motionManagerAttitudeRef];
NSLog(#"yaw:%+0.1f, pitch:%+0.1f, roll:%+0.1f, att.yaw, att.pitch, att.roll);
}];
}];
First off, in my application I only really care about pitch and roll. But yaw is in there too to demonstrate my confusion.
Everything works as expected if I put the phone laying on my flat desk, launch the app and look at the logs. All of the yaw, pitch roll values are 0.0, then if I spin the phone 90 degrees without lifting it off the surface only the yaw changes. So all good there.
To demonstrate what I think is the problem... Now put the phone inside of (for example) an empty coffee mug, so that all of the angles are slightly tilted and the direction of gravity would have some fractional value in all axis. Now launch the app and with the code above you would think everything is working because there is again a 0.0 value for yaw, pitch and roll. But now spin the coffee mug 90 degrees without lifting it from the table surface. Why do I see significant change in attitude on all of the yaw, pitch and roll?? Since I cached my initial attitude (which is now my new reference attitude), and called muptiplyByInverseOfAttitude shouldn't I just be getting a change in the yaw only?

Answer 1

I don't really understand why using the attitude multiplied by a cached reference attitude doesn't work... And I don't think it is a gimbal lock problem. But here is what gets me exactly what I need. And if you tried the experiment with the coffee mug I described above, this provides exactly the expected results (i.e. spinning the coffee mug on a flat surface doesn't affect pitch and roll values, and tilting the coffee mug in all other directions now only affects one axis at a time too). Plus instead of saving a reference frame, I just save the reference pitch and roll, then when the app starts, everything is zero'ed out until there is some movement.
So all good now. But still wish I understood why the other method did not work as expected.
self.motionQueue = [[NSOperationQueue alloc] init];
self.motionManager = [[CMMotionManager alloc] init];
self.motionManager.deviceMotionUpdateInterval = 1.0/20.0;
[self.motionManager startDeviceMotionUpdatesUsingReferenceFrame: CMAttitudeReferenceFrameXArbitraryZVertical toQueue:self.motionQueue withHandler:^(CMDeviceMotion *motion, NSError *error)
{
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
if(self.motionManagerAttitude == nil){
CGFloat x = motion.gravity.x;
CGFloat y = motion.gravity.y;
CGFloat z = motion.gravity.z;
refRollF = atan2(y, x) + M_PI_2;
CGFloat r = sqrtf(x*x + y*y + z*z);
refPitchF = acosf(z/r);
self.motionManagerAttitude = motion.attitude;
return;
}
CGFloat x = motion.gravity.x;
CGFloat y = motion.gravity.y;
CGFloat z = motion.gravity.z;
CGFloat rollF = refRollF - (atan2(y, x) + M_PI_2);
CGFloat r = sqrtf(x*x + y*y + z*z);
CGFloat pitchF = refPitchF - acosf(z/r);
//I don't care about yaw, so just printing out whatever the value is in the attitude
NSLog(#"yaw: %+0.1f, pitch: %+0.1f, roll: %+0.1f", (180.0f/M_PI)*motion.attitude.yaw, (180.0f/M_PI)*pitchF, (180.0f/M_PI)*rollF);
}];
}];