Hi

I’ve got a quaternion and I need to apply it in QML/Qt to rotate an object to align with the original rotated object. What I have tried so far:

for the reference:

Qt Code: 
  1. quaternion = {0.826351,-0.405726,-0.353097,0.166890}
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and I get Yaw, Pitch, Roll with (java):

Qt Code: 
  1. public static double getPitch(Quat4d q)
  2.      {
  3.       return Math.atan2(2*(q.y*q.z + q.w*q.x), q.w*q.w - q.x*q.x -q.y*q.y +q.z*q.z);
  4.      }
  5.  
  6.      public static double getYaw(Quat4d q)
  7.      {
  8.       return Math.asin(-2*(q.x*q.z - q.w*q.y));
  9.      }
  10.  
  11.      public static double getRoll(Quat4d q)
  12.      {
  13.       return Math.atan2(2*(q.x*q.y + q.w*q.z), q.w*q.w +q.x*q.x - q.y*q.y - q.z*q.z);
  14.      }
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So I try QML:

Qt Code: 
  1. Item {
  2.             id: rectangle1
  3.             width: 300
  4.             height: 300
  5.             scale: 1
  6.             anchors.centerIn:parent;
  7.  
  8.  
  9.             property double angleX: -61.87454134010583
  10.             property double angleY: -26.624468050434572
  11.             property double angleZ: 38.97955600504172
  12.             property int originX: width/2
  13.             property int originY: height/2
  14.  
  15.             }
  16.             transform: Rotation {
  17.                 origin.x: rectangle1.originX;
  18.                 origin.y: rectangle1.originY;
  19.                 axis { x: 0; y: 0; z: -1 }
  20.                 angle: rectangle1.angleZ }
  21.  
  22.             Item {
  23.                   id: rectangle2
  24.                   width: 300
  25.                   height: 300
  26.                   scale: 1
  27.                   anchors.centerIn:parent;
  28.  
  29.                   transform: Rotation {
  30.                       origin.x: rectangle1.originX;
  31.                       origin.y: rectangle1.originY;
  32.                       axis { x: 0; y: -1; z: 0 }
  33.                       angle: rectangle1.angleY }
  34.  
  35.                   Rectangle {
  36.                         id: rectangle3
  37.                         width: 300
  38.                         height: 300
  39.                         color: "#ffffff"
  40.                         scale: 1
  41.                         opacity: 0.70
  42.                         anchors.centerIn:parent;
  43.  
  44.                         focus: true
  45.  
  46.                         transform: Rotation {
  47.                             origin.x: rectangle1.originX;
  48.                             origin.y: rectangle1.originY;
  49.                             axis { x: -1; y: 0; z: 0 }
  50.                             angle: rectangle1.angleX }
  51.                   }
  52.             }
  53.         }
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I used 2 Items and 1 Rectangle to get the rotation right on the canvases, but the data doesn’t seem to fit.
So I thought maybe I just don’t know jack about Quaternions and I’m getting the wrong data out of it, so I have tried Qt and here I am right now – I found the beginNativePainting() in QPainter, but I don’t fully understand it, and the example in the documentation doesn’t even work.

So I have a transformation matrix that is 4×4 from OpenGL and I try:

Qt Code: 
  1. void Dialog::paintEvent(QPaintEvent *e)
  2.     {
  3.         QPainter painter(this);
  4.         painter.fillRect(50,50,100,100,Qt::red);
  5.         painter.beginNativePainting();
  6.  
  7.         double modelview_matrix[16]= {-0.874483, 1.527267, -2.199979, -0.760204, -0.576585, -0.299399, 0.000000, -0.078945, -0.375442, 0.923478, 0.000000, 0.644870, -0.725668, -0.239894, 0.000000, -0.095929};
  8.  
  9.         glMatrixMode(GL_MODELVIEW);
  10.         glLoadIdentity();
  11.         glLoadMatrixd(modelview_matrix);
  12.  
  13.         glColor3f(1,0.4,0.4);
  14.         glPushMatrix();
  15.         glPopMatrix();
  16.  
  17.         painter.endNativePainting();
  18.     }
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And I tried using the quaternion itself (which is very desired, small data etc.)

Qt Code: 
  1. void Dialog::paintEvent(QPaintEvent *e)
  2.     {
  3.         QPainter painter(this);
  4.         painter.fillRect(50,50,100,100,Qt::red);
  5.         painter.beginNativePainting();
  6.  
  7.         QMatrix4x4 mat;
  8.         mat.setToIdentity();
  9.         const QQuaternion *q = new QQuaternion(0.826351,-0.405726,-0.353097,0.166890);
  10.         mat.rotate(q->normalized());
  11.  
  12.         painter.endNativePainting();
  13.     }
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So I guess my question would be how does the QPainter::beginNativePainting() work, how do I make it work, because as I said even the example from http://harmattan-dev.nokia.com/docs/...NativePainting doesn’t seem to work, and in my case nothing happens in both examples. Do I have to draw something inside the begin – end native painting and it takes an effect on that or should it work with the

Qt Code: 
  1. painter.fillRect(50,50,100,100,Qt::red);
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declared earlier?

Another question I have is about the QMatrix4×4, I don’t fully understand how it Identifies itself with setToIdentity, what does it identify itself with in my example even? Does it identify itself with the QPainter object?