Likely bug in Quaternion.copyRotationInto

[robertmuth]

For this code:

import 'package:vector_math/vector_math.dart' as VM;

void TestMe(VM.Quaternion dir) {
  print("quad: ${dir}");
  VM.Matrix3 rot = VM.Matrix3.identity();
  VM.Vector3 x = VM.Vector3.zero();
  x.setValues(0.0, 1.0, 0.0);
  dir.rotate(x);
  print("Direction from quad: ${x}");
  //
  dir.copyRotationInto(rot);
  x.setValues(0.0, 1.0, 0.0);
  rot.transform(x);
  print("Direction from rot: ${x}");
}

I would expect the last two print statments to mostly agree but they do not, e.g.

quad: -0.3830223083496094, -0.3213938772678375, -0.6634140610694885 @ -0.5566705465316772
Direction from quad: [0.9848082065582275,-0.1736481785774231,-4.0803353584806246e-8]
Direction from rot: [-0.49240386486053467,-0.17364810407161713,0.8528685569763184]

There also seems to be no test for this in the test suite

[robertmuth]

It could be that I am merely misunderstanding the API instead of

dir.rotate(x);

using

x.applyQuaternion(dir);

seems to work.

If that is indeed the case, maybe the documentation could be updated.

I also have a related feature request: quaternion multiplication always forces a allocation.
It would be nice tio have a version that multiplies into a given (empty) quaternion.

[notakshitgoyal]

void TestMe(VM.Quaternion dir) {
  print("quad: ${dir}");
  VM.Matrix3 rot = VM.Matrix3.identity();

  VM.Vector3 x = VM.Vector3.zero();
  x.setValues(0.0, 1.0, 0.0);
  dir.rotate(x);
  x.normalize();  // Normalize after quaternion rotation
  print("Direction from quad: ${x}");

  dir.copyRotationInto(rot);
  x.setValues(0.0, 1.0, 0.0);
  rot.transform(x);
  x.normalize();  // Normalize after matrix transformation
  print("Direction from rot: ${x}");
}

[robertmuth]

Could you add a little more "prose" to the answer - people that look at this in a couple of years will be thankful.

E.g.:

Why do you have to normalize? Is this a well known math fact? If so, provide a link.
Or maybe this is only necessary for a certain class of Quaternions. If so, how can one tell?

Also, what is the recommended course of action: do nothing, add the normalization step to the library, add a warning to the library documentation.

[stuartmorgan-g]

This package has moved to https://github.com/flutter/core-packages, so if this issue is still relevant please re-file it in https://github.com/flutter/flutter. Thanks!