Quaternion strangeness

[kavilimone]

I have the following Quaternion(x,y,z,w) = [-0.876204 -0.447583 0.173706 -0.0419935]

If I now do:

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btTransform transform;
transform.setRotation( btQuaternion(-0.0419935,-0.876204,-0.447583,0.173706) ); // because btQuaternion is (w,x,y,z)
std::cout << transform.getRotation().x() << " " << transform.getRotation().y() << " " << transform.getRotation().z() << " " << transform.getRotation().w() << std::endl;

I get:
0.0419935 0.876204 0.447583 -0.173706

Why??

[Dominik]

btQuaternion.h, line 33

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btQuaternion(const btScalar& x, const btScalar& y, const btScalar& z, const btScalar& w)

Why do you think you should pass w first?

[kavilimone]

ok, thanks, this turns the quaternion into:

0.876204 0.447583 -0.173706 0.0419935

So, all sign are altered, which shouldn't be a problem, because of q==-q.

But why does this change the axes of the corresponding rotation matrix....?