Document why accelerometer values need to be negated

[dylanmckay]

No description provided.

[m-7761]

They don't. The only interesting thing, is the X & Y components of the gyroscope are swapped, according to orthodoxy. The Madgwick algorithm available online uses the same convention, so it may not be unorthodox in the field, but it is for 3D video games, where X is normally for rotation about the X axis (pitch) and Y is about the Y axis (Yaw.)

[m-7761]

ACTUALLY there is one reason to negate things... which is (I believe) constructing a quaternion, etc. without negating the values, yields something like an inverse matrix. I don't know if that is by design or not. Of course, an inverse matrix is a "model view" matrix. To get a picture of headset's movements (the object matrix) you invert it. To get what the eyes are meant to see (the view matrix) you leave it alone.