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I have an app that records angles as user is walking around an object, while pointing device (preferably) at the center of the object. Angle gets reset on user's command - so reference attitude gets reset.

Using Euler angles produces Gimbal lock, so I am currently using quaternions and calculating angles this way:

double angleFromLastPosition = acos(fromLastPositionAttitude.quaternion.w) * 2.0f;

This gives off good precision and it works perfectly IF device's pitch and yaw does not change. In other words, as the angle shows 360 degrees I end up in the same place as the start of the circle.

Problem 1: if device's yaw and pitch change slightly (user not pointing directly at center of the object), so does the angleFromLastPosition. I understand this part, as my angle formula just shows the angle in between two device attitudes in 3D space.

Scenario:

  • I mark the start of rotation attitude and start moving in a circle around the object while pointing at the center
  • I stop at, say, 45 degrees and change pitch of the device by pointing it higher or lower. Angle changes accordingly.
  • What I would love to see is: angle stays at 45 degrees, even if pitch or yaw changes.

Question 1 is, how can I calculate only the Roll of the device using quaternions, and disregard changes in other two axes (at least within some reasonable number of degrees).

Problem 2: if I rotate for a bit and then freeze the device completely (on tripod so there's no shaking at all), the angleFromLastPosition drifts at a rate of 1 degree per about 10-20 seconds, and it appears not to be linear. In other words, it drifts fast at first, then slows down considerably. Sometimes I get no drift at all - angle is rock-solid if device is stationary. And this makes me lost in understanding what's going on.

Question 2, what is going on here, and how can I take care of the drift?

I went through quite a few articles and tutorials, and quaternion math is beyond me at the moment, hope someone will be able to help with a tip, link, or few lines of code.

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1 回答 1

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我已经对此进行了测试，它似乎可以根据您在问题 1 中寻找的内容工作，Andrei。

我最初将 homeangle 设置为 0，并在第一次通过后立即将 walkaroundAngleFromAttitude:fromHomeAngle: 返回的角度存储在 homeangle 中，以备将来使用。

我的测试包括使用参考框架启动设备更新：

[_motionManager 
startDeviceMotionUpdatesUsingReferenceFrame:CMAttitudeReferenceFrameXArbitraryZVertical 
toQueue:operationQueue 
withHandler:handler];

并使用在处理程序中调用的以下方法：

- (CMQuaternion) multiplyQuanternion:(CMQuaternion)left withRight:(CMQuaternion)right {

    CMQuaternion newQ;
    newQ.w = left.w*right.w - left.x*right.x - left.y*right.y - left.z*right.z;
    newQ.x = left.w*right.x + left.x*right.w + left.y*right.z - left.z*right.y;
    newQ.y = left.w*right.y + left.y*right.w + left.z*right.x - left.x*right.z;
    newQ.z = left.w*right.z + left.z*right.w + left.x*right.y - left.y*right.x;

    return newQ;
}

-(float)walkaroundRawAngleFromAttitude:(CMAttitude*)attitude {

    CMQuaternion e =  (CMQuaternion){0,0,1,1};
    CMQuaternion quatConj = attitude.quaternion;
    quatConj.x *= -1; quatConj.y *= -1; quatConj.z *= -1;
    CMQuaternion quat1 = attitude.quaternion;
    CMQuaternion quat2 = [self multiplyQuanternion:quat1 withRight:e];
    CMQuaternion quat3 = [self multiplyQuanternion:quat2 withRight:quatConj];

    return atan2f(quat3.y, quat3.x);
}
-(float)walkaroundAngleFromAttitude:(CMAttitude*)attitude fromHomeAngle:(float)homeangle {

    float rawangle = [self walkaroundRawAngleFromAttitude:attitude];
    if (rawangle <0) rawangle += M_PI *2;
    if (homeangle < 0) homeangle += M_PI *2;
    float finalangle = rawangle - homeangle;
    if (finalangle < 0) finalangle += M_PI *2;

    return finalangle;
}

这是使用从查找法线向量到 iOS 设备的一些修改和扩展代码

编辑以处理问题 2 和问题 2：

这可能无法解决。我已经在其他应用程序（例如 360 pano）中看到它，并且已经阅读了有关陀螺仪等错误读数的信息。如果您尝试对其进行补偿，那么当补偿代码抛出一些真实的旋转运动时，您当然会感到紧张不安。就我过去几年的解释而言，这是一个基于硬件的问题。

于 2013-05-27T08:33:32.233 回答