Google Tango API系列（五）之Frames of Reference

Frames of Reference

参考框架

When describing the position and orientation of something (for example, your Tango device), it is important to indicate the frame of reference you are using to base your description on.

To help understand frames of reference, consider the following: Saying "Mary is standing three feet away" does not really tell you much. If you want to know Mary's position, you must also address the question "three feet from what?" If you say "Mary is standing three feet in front of the entrance to the Statue of Liberty," you can now establish Mary's position because you are using the Statue of Liberty as your frame of reference and you can measure the distance and directon of Mary relative to the Statue.

But Mary isn't simply a point with a position in 3D space—she also has an orientation, which is described in terms of some type of rotation relative to the frame of reference. In other words, Mary, like all 3D objects, faces a certain direction. A full description of Mary's position and orientation (we call this combination a pose) in 3D space would be something like this: "Mary is standing three feet in front of the entrance to the Statue of Liberty, and she is directly facing it." Now you have provided information about her orientation. If Mary turned to her right, you could say "She is now rotated 90 degrees away from the Statue." This would be another description of orientation.

So how does all of this relate to a Tango device? In order to perform motion tracking, a device reports its pose (position and orientation) relative to its chosen frame of reference, which is fixed in 3D space. For example, the device might say "from the place that I first started motion tracking, I am now three feet forward and one foot up, and I have rotated 30 degrees to the right." By doing this, the device has told you its position using meaningful directions: three feet forward and one foot up from its original starting position. It has also told you about a change in its orientation: rotated 30 degrees to the right relative to its starting position.

当描述某物（例如，Tango设备）的位置和方向时，重要的是指示您用来描述的参考框架。

为了帮助理解参考框架，考虑以下：说“玛丽站在三英尺外”并不真正告诉你。如果你想知道玛丽的位置，你还必须解决这个问题“离什么三英尺？如果你说“玛丽站在自由女神像的入口前三英尺”，你现在可以确定玛丽的位置，因为你使用自由女神像作为参考框架，你可以测量玛丽的距离和方向相对于雕像。

但玛丽不仅仅是一个在3D空间中的位置的点 - 她也具有取向，其根据相对于参考系的某种类型的旋转来描述。换句话说，玛丽像所有3D对象一样，面向一定的方向。在3D空间中对玛丽的位置和方向（我们称之为组合姿势）的完整描述将是这样的：“玛丽站在自由女神像的入口前三英尺，她直接面对它。现在你提供了她的方向的信息。如果玛丽转向她的右边，你可以说“她现在旋转了90度远离雕像。这将是对方向的另一种描述。

那么，所有这一切都与Tango设备有关？为了执行运动跟踪，设备相对于其选定的参照系来报告其姿态（位置和取向），其在3D空间中是固定的。例如，设备可能说“从我第一次开始运动跟踪的地方，我现在向前三英尺，一英尺，我已经向右旋转了30度。通过这样做，设备已经告诉你它的位置使用有意义的方向：三英尺前，一英尺从其原始的起始位置。它也告诉你一个方向的变化：相对于它的起始位置向右旋转30度。