Problems with QQuaternion

basil_fawlty

Hello everyboy,

i want to rotate my object via gui-arows and quaternions. With up and down arrow it should be possible to rotate round the x-axis. with left and right round y and with two other buttons round the z axis.

I am using opengl in qt. I have achived to rotate the object round every axis via these slot-functions:
@
void OpenGLScene::rotate_plus_x()
{
OpenGLScene::anglex += 5;
test->rotation = QQuaternion::fromAxisAndAngle(QVector3D(1,0,0),OpenGLScene::anglex);
update();

}

void OpenGLScene::rotate_minus_x()
{
OpenGLScene::anglex -= 5;
test->rotation = QQuaternion::fromAxisAndAngle(QVector3D(1,0,0),OpenGLScene::anglex);
update();
}

void OpenGLScene::rotate_plus_y()
{
OpenGLScene::angley += 5;
test->rotation = QQuaternion::fromAxisAndAngle(QVector3D(0,1,0),OpenGLScene::angley);
update();
}
@

In this class my actual rotation is happening.(Where the rotation is multiplied with the modelview matrix)

@
QMatrix4x4 matrix;
matrix.rotate(rotation);
glMatrixMode (GL_MODELVIEW);
glLoadIdentity ();

            glPushMatrix();
            light->drawLight(light->light_rotate_x,light->light_rotate_y,light->light_rotate_z);

            gluLookAt(   0,   0, -400,  
                         0,   0,    0,   
                         0,   1,    0);  
            qMultMatrix(matrix);
            object->drawObject(Red,Green,Blue);
            draw->drawAxes();
            glPopMatrix();

@

(The qmultmatrix function is from the pbuffers-example from the qt-examples.)

My problem is, that the object is always jumping back to the initial position, when i change the rotation-axis. For instance, when i rotate round the x-axis and afterwards round the y-axis the object starts to rotate from the initial position and not from the newest one.

I think there might be a problem with how i multiply my matrix with the modelview-matrix. I think i should store the last changed matrix and multiply it with the newest one. How to do that is a mystery to me.
I have tried several things but nothing is working.

Maybe you could help me.

Thanks in Advance

Franzk

You need to do the math. You create a unit matrix and then rotate it according to a certain quaternion -- that is, you're throwing away the previous rotation by assigning them in your slots.

IIRC the following should provide better results:
@
void OpenGLScene::rotate_plus_x()
{
OpenGLScene::anglex += 5;
test->rotation *= QQuaternion::fromAxisAndAngle(QVector3D(1,0,0),OpenGLScene::anglex);
update();

}

void OpenGLScene::rotate_minus_x()
{
OpenGLScene::anglex -= 5;
test->rotation *= QQuaternion::fromAxisAndAngle(QVector3D(1,0,0),OpenGLScene::anglex);
update();
}

void OpenGLScene::rotate_plus_y()
{
OpenGLScene::angley += 5;
test->rotation *= QQuaternion::fromAxisAndAngle(QVector3D(0,1,0),OpenGLScene::angley);
update();
}
@

ZapB

As FranzK says you are resetting your rotation matrices in the rotate_*() functions. Instead of replacing the matrix, calculate another matrix from the quarternion and pre-multiply the pre-existing transformation matrix with it.

I think the order of the matrix multiplication should be opposite to what Franzk said though (but to be fair I've not tried it myself). For e.g.:

@
void OpenGLScene::rotate_plus_x()
{
OpenGLScene::anglex += 5;
QMatrix4x4 newRotation = QQuaternion::fromAxisAndAngle(QVector3D(1,0,0),OpenGLScene::anglex);
test->rotation = newRotation * test->rotation;
update();
}
@

ZapB

Gah! Sorry ignore my last post I was thinking of matrices not quaternions. Sorry for the confusion. Long day ;-)

basil_fawlty

Thanks to both of you. Especially to FranzK. It was exactly the problem.
Now its working. One thing was wrong as well.
It should be:

@
OpenGLScene::anglex = 5;
@
or
@
OpenGLScene::anglex = -5;
@

Otherwise the rotationangle gets bigger and bigger with every click.
For those who are interested.