glRotetef & 4x4 Matrix

[wererabit]

Hi,

Sorry in advance if this is in the wrong place!

I just wonder if anyone know how can I decompose a 4x4 transformation matrix (Rotation & translation) into angles & other thing so I can use with glRotatef, glScalef & glTranslationf.

Thank alot!

[agarny]

That's a pure OpenGL question, so I highly doubt a Qt forum would be the right place for it...

[JohannesMunk]

Hi!

I would suggest to do it the other way around. Use OpenGL to setup your matrix!

// use gl to perform the matrix multiplications. using modelview matrix as temp.. Start fresh
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glScaled(...);
    glTranslated(...);
    if (Angle().x() != 0) glRotatef(Angle().x(),1,0,0);
    if (Angle().y() != 0) glRotatef(Angle().y(),0,1,0);
    if (Angle().z() != 0) glRotatef(Angle().z(),0,0,1);
 
    GLfloat itemtrans[16];
    glGetFloatv(GL_MODELVIEW_MATRIX , itemtrans);
 
    // Restore previous gl matrix
    glPopMatrix();

// use gl to perform the matrix multiplications. using modelview matrix as temp.. Start fresh
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glScaled(...);
    glTranslated(...);
    if (Angle().x() != 0) glRotatef(Angle().x(),1,0,0);
    if (Angle().y() != 0) glRotatef(Angle().y(),0,1,0);
    if (Angle().z() != 0) glRotatef(Angle().z(),0,0,1);

    GLfloat itemtrans[16];
    glGetFloatv(GL_MODELVIEW_MATRIX , itemtrans);

    // Restore previous gl matrix
    glPopMatrix();

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afterwards you can transform this to a QTransform like this:

QTransform GetQTrans(GLfloat* m)
{
    // In QTransform the dx and dy are stored in m31 and m32. which is nuts.
    // In 4x4 GL they are stored in 03 and 13, so that when you multiply
    // with a homogenous (x,y,z,1) vector you get them added automatically.
 
    // Build QTransform of this matrix. QTransform 3x3 recieves the values per row,
    // whereas GL saves the 4x4 matrix per column.
    //        m11 m12 m13        1  2  3            m00 m10 m30         0  1  3
    //    QT: m21 m22 m23        4  5  6        GL: m01 m11 m31         4  5  7
    //         dx  dy  m33       7  8  9            m03 m13 m33        12 13 15
 
    // AAH! QTransform does not use m33, unless the transformation is not affine, m13<>0 and m23 <> 0
    QTransform trans;
    if (qFuzzyCompare(m[3] + 1, 1) && qFuzzyCompare(m[7] + 1, 1))
        trans = QTransform(m[0]/m[15], m[1]/m[15], m[4]/m[15], m[5]/m[15], m[12]/m[15], m[13]/m[15]);
    else
        trans = QTransform(m[0], m[1], m[3], m[4], m[5], m[7], m[12], m[13], m[15]);
    trans.scale(1,-1);
    return trans;
}

QTransform GetQTrans(GLfloat* m)
{
    // In QTransform the dx and dy are stored in m31 and m32. which is nuts.
    // In 4x4 GL they are stored in 03 and 13, so that when you multiply
    // with a homogenous (x,y,z,1) vector you get them added automatically.

    // Build QTransform of this matrix. QTransform 3x3 recieves the values per row,
    // whereas GL saves the 4x4 matrix per column.
    //        m11 m12 m13        1  2  3            m00 m10 m30         0  1  3
    //    QT: m21 m22 m23        4  5  6        GL: m01 m11 m31         4  5  7
    //         dx  dy  m33       7  8  9            m03 m13 m33        12 13 15

    // AAH! QTransform does not use m33, unless the transformation is not affine, m13<>0 and m23 <> 0
    QTransform trans;
    if (qFuzzyCompare(m[3] + 1, 1) && qFuzzyCompare(m[7] + 1, 1))
        trans = QTransform(m[0]/m[15], m[1]/m[15], m[4]/m[15], m[5]/m[15], m[12]/m[15], m[13]/m[15]);
    else
        trans = QTransform(m[0], m[1], m[3], m[4], m[5], m[7], m[12], m[13], m[15]);
    trans.scale(1,-1);
    return trans;
}

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For a project sample have a look at this thread http://www.qtcentre.org/threads/2005...a-tiny-problem

HIH

Johannes