Stride: [Math] Quaternion expects left handed coordinate system

[Eideren]

Release Type: Any.

Version: Latest as of the time of writing.

Platform(s): Any.

Describe the bug
Xenko uses a right handed coordinate system(right < 0) while Quaternions expects a left handed one(right > 0), this leads to the issue described below.

What v should logically be equal to, given xenko's coordinate system, is a vector pointing downwards, but it returns a vector pointing upwards instead.

var right = new Vector3(-1f, 0f, 0f);
var forward = new Vector3(0f, 0f, 1f);
var deg90AsRad = 1.570796f;
var cw90Degrees = Quaternion.RotationAxis(right, deg90AsRad);
var v = Vector3.Transform(forward, cw90Degrees);

Well, good luck to whoever decides to fix that one !

[tebjan]

Xenko uses a right handed coordinate system (right < 0) while Quaternions expects a left handed one (right > 0), this leads to the issue described below.

this is technically not the right way to describe handedness. left/right handed is not (only) about the x-axis, it describes how the positive direction of the basis vectors of a coordinate system are related to each other.

the convention in xenko is:
right -> positive x
up -> positive y
forward -> negative z

which is indeed right handed.

so this assumption is incorrect:
> var right = new Vector3(-1f, 0f, 0f);

your camera matrix defines how you perceive the coordinate system on screen. e.g. if you look into the opposite direction, the x axis appears to be flipped.

see also slide 13-16 in this presentation:
https://docs.google.com/presentation/d/14rxW-qZ0ZwDIppdc_4uGGYc_Fdm10NbYLMXOKGZ-BCg/

quaternion math is a well defined set of operations and does not care about handedness at all. the calculations (e.g. multiplying a vector with a quaternion) are always the same. only the usage of it might change depending on the coordinate system convention. for example positive angles might be perceived differently in different coordinate systems.

[Eideren]

Thank you for clarifying this, I should have know. I got too caught up be my own code to think this through !

[tebjan]

always fun to propagate knowledge. this topic is one of the most confusing in 3d engines and i never get tired to help anyone with it. happy coding!