personal_username

Let's imagine a guy has a sensor on his forehead that sends constantly (only) quaternion data. No magnetometer etc etc...

He cannot see and he wants to be able to tell when he turns a perfect 180 degree (meaning his head is facing where the back was facing).

As well he wants to know when he is back exactly to the same position he started.

Looks like quaternion are a strange beast, and in my tests there is not an consistent way to have this information, not transforming quaternion to euler, nor axis... What I'm seeing is really some weird, not consistent and unpredictable results.

Let's imagine a guy has a sensor on his forehead that sends constantly (only) quaternion data. No magnetometer etc etc...
He cannot see and he wants to be able to tell when he turns a perfect 180 degree (meaning his head is facing where the back was facing).
As well he wants to know when he is back exactly to the same position he started.

sensor-on-the-forehead-with-quaternion

you can use the jit.quat tools to calculate the angle between two quaternions. you can use this angle to determine if direction is some threshold between angles.

Sensor on the forehead with quaternion