Qt OpenGL - Rotation with mouse drag - negative delta

Question

I have a GLWdiget subclass of QGLWidget where I would like to make rotate a 3D object along Ox and Oy axes.

For this, I have reimplemented mousePressEvent and mouseMoveEvent functions this way :

void GLWidget::mousePressEvent(QMouseEvent *event)
{
 lastPos = event->pos();
}

void GLWidget::mouseMoveEvent(QMouseEvent *event)
{
  float dx = (event->x() - lastPos.x()) / 10.0f;
  float dy = (event->y() - lastPos.y())/ 10.0f;

 if (event->buttons() & Qt::LeftButton) 
   {
   glRotatef(dy*0.1, 1.0f, 0.0f, 0.0f);
   glRotatef(dx*0.1, 0.0f, 1.0f, 0.0f);
   }
}

My problem is that dx and dy are never negative so whatever the direction I do with the mouse, it is always rotating in the same direction.

For example, If I drag horizontally to the right, I want the object to rotate along 0y axes with a positive angle, and If I drag horizontally to the left, with a negative angle.

This would be the same for vertical dragging but the rotation would be along Ox axes.

Is this issue coming from global coordinates ? However, event->x and event->y give positions relative to the GLWidget.

Answer 1

You are not updating the lastPos at the end of your mouseMoveEvent ? From the Qt example:

void GLWidget::mouseMoveEvent(QMouseEvent *event)
{
     int dx = event->x() - lastPos.x();
     int dy = event->y() - lastPos.y();

     if (event->buttons() & Qt::LeftButton) {
         setXRotation(xRot + 8 * dy);
         setYRotation(yRot + 8 * dx);
     } else if (event->buttons() & Qt::RightButton) {
         setXRotation(xRot + 8 * dy);
         setZRotation(zRot + 8 * dx);
     }
    lastPos = event->pos();
}

I suggest you take a look at Qt Hello GL which seems to match your use case.