QMatrix4x4 rotate

beecksche

Hi,

i want to use the rotate function from the QMatrix4x4 class. But i don't get the right QVector3D:

QMatrix4x4 rot;
rot.setToIdentity();
rot.rotate(45, QVector3D(0, 0, 1)); // create rotation matrix with 45° and rotation vector in positive z-direction

QVector3D test = QVector3D(1, 0, 0) * rot; //calculate the rotated vector 
qDebug() << test; // QVector3D(0.70710678118, -0.70710678118, 0)

The calculated vector should be QVector(0.70710678118, +0.70710678118, 0). Like here: http://www.nh.cas.cz/people/lazar/celler/online_tools.php?start_vec=1,0,0&rot_ax=0,0,1&rot_ang=45

Is this a bug or do i forget something?

BjornW

I'm not very familiar with this class, but usually the matrix is multiplied to the leftof the vector. (Mv is not the same as vM)

Try:

QVector3D test = rot * QVector3D(1, 0, 0); //calculate the rotated vector

beecksche

@BjornW

Oh thanks! Yeah that was my fault!

BjornW

You should mark this as solved :-)

kshegunov

Just for completeness, you can multiply the vector from the right side too, but you have to transpose the matrix (it's a unitary one) beforehand to obtain a correct result. And as a general side note, I'd advise just to use QTransform instead.

BjornW

Good comment.

Further clarification; the inverse of a rotational matrix is always its transposition. This is useful :-)