TBN frame to quaternion

Questions
,
1

I am trying to convert 3 basis vectors to a rotation. My expectation is that:

const rotationMatrix = new THREE.Matrix4().set(
  0.3511234415884646,  0.9363291775690172,  0,  0,
  0.9363291775690172,  -0.3511234415884646,  0,  0,
  0,  0,  1,  0,
  0,  0,  0,  1
);

const quaternion = new THREE.Quaternion().setFromRotationMatrix(rotationMatrix);

console.log(quaternion.toArray());

Should yield me something other than the 90 degree rotation. I have different matrices with different values but setFromRotationMatrix() seems to always return the same quaternion.

2

Those basis vectors in your example there look very close to an identity… i.e.
0,1,0,
1,0,0
0,0,1
which looks very much like a 90 degree rotation of Y onto X, so 90 degrees around Z

So I suspect your data. Perhaps also a row/column swap? but yeah…

3

In your case, try conversion matrix → euler → quaternion:

const euler = new THREE.Euler().setFromRotationMatrix(rotationMatrix);
const quaternion = new THREE.Quaternion().setFromEuler(euler);

However, to be honest, the matrix does not contain pure rotation (or has XY swapped). The matrix looks like rotation around Z axis, but in this case it should be something like:

cos -sin  0  0
sin  cos  0  0
 0    0   1  0
 0    0   0  1

so, the diagonal two values must be the same, like this:

0.3511234415884646,  0.9363291775690172,  0,  0,
-0.9363291775690172,  0.3511234415884646,  0,  0,
0,  0,  1,  0,
0,  0,  0,  1

and the resulting quaternion will be

0, 0, -0.5695948377625694, 0.8219256175556473
2 Likes
4

Yeah i think i swapped some axis. After i swizzlwd i started getting a rotation. I don’t quite understand what’s going on.

5

This may be very basic, but my expectation was that any three orthogonal vector will give me a rotation. It may not be the rotation that I expect but it will be a rotation. However, that’s not true right? I could set them up in such a way that it has a rotation and a scale in say negative x, in which case setFromRotationMatrix fails because, well, it’s not a rotation matrix?

6

How would you rotate a left-handed coordinate system into a right-handed coordinate system?

1 Like
7

Right :slight_smile: I will not, that;s why i got the different diagonal, I flipped it.

1 Like