如何在 iOS 5 中使用 CMDeviceMotion 获取设备的标题 [英] how can i get the heading of the device with CMDeviceMotion in iOS 5

问题描述

我正在使用陀螺仪开发 AR 应用.我使用了一个苹果代码示例 pARk.它使用旋转矩阵来计算坐标的位置并且它做得非常好，但现在我正在尝试实现一个雷达"，我需要根据设备航向旋转它.我正在使用 CLLocationManager 标题，但它不正确.

I'm developing an AR app using the gyro. I have use an apple code example pARk. It use the rotation matrix to calculate the position of the coordinate and it do really well, but now I'm trying to implement a "radar" and I need to rotate this in function of the device heading. I'm using the CLLocationManager heading but it's not correct.

问题是，如何使用 CMAttitude 获取设备的标题以准确反映我在屏幕上看到的内容?

The question is, how can I get the heading of the device using the CMAttitude to reflect exactly what I get in the screen??

我是旋转矩阵之类的新手.

I'm new with rotation matrix and that kind of things.

这是用于计算 AR 坐标的代码的一部分.用姿态更新 cameraTransform:

This is part of the code used to calculate the AR coordinates. Update the cameraTransform with the attitude:

CMDeviceMotion *d = motionManager.deviceMotion;
if (d != nil) {
    CMRotationMatrix r = d.attitude.rotationMatrix;
    transformFromCMRotationMatrix(cameraTransform, &r);
[self setNeedsDisplay];
}

然后在 drawRect 代码中:

and then in the drawRect code:

mat4f_t projectionCameraTransform;
multiplyMatrixAndMatrix(projectionCameraTransform, projectionTransform, cameraTransform);

int i = 0;
for (PlaceOfInterest *poi in [placesOfInterest objectEnumerator]) {
    vec4f_t v;
    multiplyMatrixAndVector(v, projectionCameraTransform, placesOfInterestCoordinates[i]);

    float x = (v[0] / v[3] + 1.0f) * 0.5f;
    float y = (v[1] / v[3] + 1.0f) * 0.5f;

我还使用俯仰角旋转视图.运动更新开始使用北:

I also rotate the view with the pitch angle. The motions updates are started using the north:

[motionManager startDeviceMotionUpdatesUsingReferenceFrame:CMAttitudeReferenceFrameXTrueNorthZVertical];

所以我认为必须可以在任何位置(任何俯仰和偏航......)获得设备的滚动"/航向，但我不知道如何.

So I think that must be possible to get the "roll"/heading of the device in any position (with any pitch and yaw...) but I don't know how.

推荐答案

有几种方法可以根据 CMDeviceMotion 返回的旋转矩阵计算航向.这假设您使用与 Apple 指南针相同的定义，其中 +y 方向(iPhone 的顶部)指向正北返回 0 的航向，向右旋转 iPhone 会增加航向，因此东为 90，南为 180，等等.

There are a few ways to calculate heading from the rotation matrix returned by CMDeviceMotion. This assumes you use the same definition of Apple's compass, where the +y direction (top of the iPhone) pointing due north returns a heading of 0, and rotating the iPhone to the right increases the heading, so East is 90, South is 180, and so forth.

首先，当您开始更新时，请务必检查以确保标题可用:

First, when you start updates, be sure to check to make sure headings are available:

if (([CMMotionManager availableAttitudeReferenceFrames] & CMAttitudeReferenceFrameXTrueNorthZVertical) != 0) {
   ...
}

接下来，当您启动运动管理器时，请求姿态作为从 X 指向真北的旋转(或磁北，如果您出于某种原因需要):

Next, when you start the motion manager, ask for attitude as a rotation from X pointing true North (or Magnetic North if you need that for some reason):

[motionManager startDeviceMotionUpdatesUsingReferenceFrame: CMAttitudeReferenceFrameXTrueNorthZVertical
                                                   toQueue: self.motionQueue
                                               withHandler: dmHandler];

当运动管理器报告运动更新时，您想知道设备在 X-Y 平面中旋转了多少.由于我们对 iPhone 的顶部感兴趣，我们将在那个方向选取一个点并使用返回的旋转矩阵旋转它以获得旋转后的点:

When the motion manager reports a motion update, you want to find out how much the device has rotated in the X-Y plane. Since we are interested in the top of the iPhone, we'll pick a point in that direction and rotate it using the returned rotation matrix to get the point after rotation:

   [m11 m12 m13] [0]   [m12]
   [m21 m22 m23] [1] = [m22]
   [m31 m32 m33] [0]   [m32]

时髦的括号是矩阵；这是我能用 ASCII 做的最好的事情.:)

The funky brackets are matrices; it's the best I can do using ASCII. :)

航向是旋转点与真北之间的角度.我们可以使用旋转点的 X 和 Y 坐标来提取反正切，它给出了该点与 X 轴之间的角度.这实际上与我们想要的相差 180 度，因此我们必须相应地进行调整.生成的代码如下所示:

The heading is the angle between the rotated point and true North. We can use the X and Y coordinates of the rotated point to extract the arc tangent, which gives the angle between the point and the X axis. This is actually 180 degrees off from what we want, so we have to adjust accordingly. The resulting code looks like this:

CMDeviceMotionHandler dmHandler = ^(CMDeviceMotion *aMotion, NSError *error) {
    // Check for an error.
    if (error) {
        // Add error handling here.
    } else {
        // Get the rotation matrix.
        CMAttitude *attitude = self.motionManager.deviceMotion.attitude;
        CMRotationMatrix rm = attitude.rotationMatrix;

        // Get the heading.
        double heading = PI + atan2(rm.m22, rm.m12);
        heading = heading*180/PI;
        printf("Heading: %5.0f
", heading);
    }
};

有一个问题:如果 iPhone 的顶部笔直向上或笔直向下，则方向是不确定的.结果是 m21 和 m22 为零，或非常接近于零.您需要决定这对您的应用意味着什么，并相应地处理这种情况.例如，当 m12*m12 + m22*m22 接近于零时，您可能会切换到基于 -Z 轴的航向(在 iPhone 后面).

There is one gotcha: If the top of the iPhone is pointed straight up or straight down, the direction is undefined. The result is m21 and m22 are zero, or very close to it. You need to decide what this means for your app and handle the condition accordingly. You might, for example, switch to a heading based on the -Z axis (behind the iPhone) when m12*m12 + m22*m22 is close to zero.

这一切都假设您想围绕 X-Y 平面旋转，就像 Apple 通常为他们的指南针所做的那样.它起作用是因为您正在使用运动管理器返回的旋转矩阵来旋转沿 Y 轴指向的向量，即这个矩阵:

This all assumes you want to rotate about the X-Y plane, as Apple usually does for their compass. It works because you are using the rotation matrix returned by the motion manager to rotate a vector pointed along the Y axis, which is this matrix:

[0]
[1]
[0]

要旋转一个不同的向量——比如说，一个指向 -Z 的向量——使用不同的矩阵，比如

To rotate a different vector--say, one pointed along -Z--use a different matrix, like

[0]
[0]
[-1]

当然，你还得取不同平面的反正切，所以不是

Of course, you also have to take the arc tangent in a different plane, so instead of

double heading = PI + atan2(rm.m22, rm.m12);

你会使用

double heading = PI + atan2(-rm.m33, -rm.m13);

得到X-Z平面的旋转.

to get the rotation in the X-Z plane.

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