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My question is similar to Changing sensor coordinate system in android

I want to be able to compare a user's movements with each other regardless of device orientation. So that when the users holds out the phone in portrait orientation and bends his arm, acceleration readings are the same as when he holds out his phone in landscape and then bends his arm in the same direction.

This is what I call the "user" coordinate system. It is different from the world coordinate system since it should not matter what wind direction the user is facing. It is different from device coordinates since it should not matter how the user holds his device.

It is acceptable in my application to do a calibration step before each movement so the base/resting orientation matrices can be determined. Is it perhaps just a matter of multiplying the matrix of the first movement with the inverse of the second (and then with the new values?)

The answer in the question mentioned seems about right, but I need a more concrete explanation, actual code samples would be ideal.

Note remapCoordinateSystem won't suffice, it only accepts right angles. I need to be able to work with small deviations since the device is strapped to a wrist, which might not always be at right angles with the arm.

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Nino van Hooff
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1 Answers1

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I'm currently working on this issue, and I think this might help: convert device coordinate system to world corrdinate system

We can assume that for the most of the time, people use the phone standing or walking or sitting, which means the user coordinate system share the same z-axis(gravity)， and a fixed difference in degree between y-axis (user coordinate, front of user's face) and y-axis(world coordinate, north direction). The difference of degree can be obtained via TYPE_MAGNETIC_FIELD sensor. Thus we can transform from world coordinate to user coordinate.

What's about the user using the phone lying on the bed? I think for that kind of case, a pre-calibration is need by define y-axis of user coordinate, like a movement to tell the phone which direction is the front of user's face.

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daoyuan jia
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