Quaternion from world space to app space.

Unity Engine
1

Hello.

When I say world space, I mean real world space. I have the rotation of my android device in the form of a unit quaternion in real world space. I want to mimic the rotations in my unity app on the PC.

Up until now I have come up with a solution to point the device at the screen and press a button - then I get the initial rotation of the mesh (0, 0, 0), I get the rotation of the real world at the time, find the difference between them with the formula

Quaternion relative = Quaternion.Inverse(worldQuaternion) * initialQuaternion

then apply the difference to every world quaternion I receive and set it to my object that’s mimicking the whole thing.

MimickingObject.transform.rotation = differenceBetweenInitialAndWorldAtTimeOfPressingButton * everyWorldQuaternionFromDevice;

This corrects it only a certain case. I have a few images to illustrate.

Here is the unity coordinate system.

This is my real world axis looked at from my PC screen (This is how I orientate my device and its also the initial model rotation - parallel to the screen, with the top pointing at the sky. In this case when I click the button I get the arbitrary rotation that I get from my device, get the difference between it and Quaternion.Euler(0, 0, 0), correct the device quaternion with this difference and I get proper mimicking.

HOWEVER, when I initially orientate the device like this It’s like my axis get scrambled. Rotating around device Y rotates around app X and so on.

Any help will be greatly appreciated, because I can’t seem to be able to ask the right questions and find an answer.

2

It sounds like you’re saying the coordinate space of the ‘real world’ (the rotation as it comes from the phone) is in an x-forward, z-up, y-left system. As opposed to unity’s z-forward, y-up, x-right system.

What you should do is convert the quat, you should be able to do this on a piece-wise basis.

var quat = *origin quat, your real-world rotation*;
var convertedQuat = new Quaternion(-quat.y, quat.z, quat.x, quat.w);

If that doesn’t work, try converting to axis-angle, transpose the axis, and then create the new quat.

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