CMDeviceMotion issues with multiplyByInverseOfAttitude

Question

I am having my first play around with Core Motion and have struck an issue and I can't work out what is going on. I am sure it is going to be a simple issue. Here is some code:

.h file

#import <UIKit/UIKit.h>
#import <CoreMotion/CoreMotion.h>

@interface TempVC : UIViewController {
CMMotionManager *motionManager;
CMAttitude *referenceAttitude;
CMAttitude *attitude;
}
- (void)updateData;
@end

.m file

#import "TempVC.h"
#define DEGREES(radians) (radians / M_PI * 180.0)
@implementation TempVC

- (void)viewDidLoad
{
[super viewDidLoad];
self.view.backgroundColor = [UIColor brownColor];

motionManager = [[CMMotionManager alloc] init]; 
referenceAttitude = nil;

CMDeviceMotion *dm = motionManager.deviceMotion;
attitude = dm.attitude;
referenceAttitude = [attitude retain];

motionManager.deviceMotionUpdateInterval = 0.1;
[motionManager startDeviceMotionUpdates];

[NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(updateData) userInfo:nil repeats:YES];

}

- (void)updateData
{
attitude = motionManager.deviceMotion.attitude;
[attitude multiplyByInverseOfAttitude:referenceAttitude];
NSLog(@"\n Roll: %.0f\n Pitch: %.0f\n Yaw: %.0f",DEGREES(attitude.roll), DEGREES(attitude.pitch), DEGREES(attitude.yaw));
}

This code will crash with no real indication of what goes wrong. If I comment out

[attitude multiplyByInverseOfAttitude:referenceAttitude];

Everything works fine??? Have I implemented the reference wrong?

Answer 1

You access CMMotionManager's deviceMotion property before starting updates. There you store your referenceAttitude which will still be nil at that time. When updateData is called a few millis later [attitude multiplyByInverseOfAttitude:referenceAttitude]; will fail.

Try instead to set the referenceAttitude on the first call of updateData and it should work.