How to get the roll angle for a spacecraft/plane

[adihodos]

I'm using this code to get the roll angle for a plane-like vehicle const float roll_angle = sd->rotation.GetEulerAngles().GetZ(); but it does not work as I expected it as applying pitch and yaw to the object results in non zero values for the roll angle. Most likely I'm doing something wrong here, so what would be the proper way to get the roll angle ? In the screenshot I only applied pitch then yaw and it resulted in a value of -2.94 radians for the roll

[jrouwe]

You cannot go from Euler angles to quaternion/matrix and back to the same Euler angles. The 3 Euler angles are not independent and multiple Euler angles represent the same rotation.

I would avoid using Euler angles altogether and e.g. rotate the ship by creating a rotation matrix around the forward axis and applying that to the ship's rotation:

new_rotation = old_rotation * Quat::sRotation(ship_forward, delta_angle)

[adihodos]

I don't rotate the ship directly, I just apply force and torque to it. So there's no way I can get the roll angle using this approach ?

[adihodos]

How should a spacecraft be implemented ? Should I use a SixDOFConstraint and use SetTargetOrientationCS ?

[jrouwe]

Applying forces and torques should work just fine.

To calculate the roll angle you would take the normalized up vector of the ship (U) and the normalized forward vector of the ship (F), project the world up vector (W = Vec3(0, 1, 0)?) onto the right/up plane of the ship:

Vec3 w_projected = W - W.Dot(F) * F

Normalize the result:

w_projected= w_projected.NormalizedOr(Vec3::sZero())

And then use the dot product to get the roll angle:

float angle = ACos(w_projected.Dot(U))

This angle is always positive, so you must negate it depending on what you consider to be positive/negative roll.

This approach breaks down when the ship is flying straight up as roll is undefined in this case.