coordinate transformation of graphics item

[sajis997]

Hello forum,

There are three functions i confused to use, when to use, they are:

1. mapFromItem(....)

2. mapToItem(...)

3. mapFromParent(....)

4. mapFromScene(...)


Please take a look at the attached image. Screenshot-3.jpg


You can see that a small ellipse is positioned inside the large ellipse.


While defining the bounding rectangle for each of them, it of its center point is defined to be at (0,0) as follows:


Bounding rectangle for the large ellipse

QRectF RootGraphicsItem::boundingRect() const
{
 
    //!get the bounding rectangle of the enclosing text
    QRectF rect = m_textGraphicsItem->boundingRect();
 
//    qDebug() << "************** the bounding rectangle of the text graphics item: *************" << endl;
//    qDebug() << "Top left: " << rect.topLeft() << endl;
//    qDebug() << "Center: " << rect.center() << endl;
//    qDebug() << "Bottom right: " << rect.bottomRight() << endl;
 
    //the following condition decides if the boundary needs
    //to be changed
    if( rect.width() < m_drawRectMinimumWidth )
        rect.setWidth(m_drawRectMinimumWidth);
 
    if(rect.height() < m_drawRectMinimumHeight)
        rect.setHeight(m_drawRectMinimumHeight);
 
    //set the rectangle width with the text item spacing
    rect.setWidth(rect.width() + getTextItemSpacing() );
 
    //the bounding rectangle will have its center at (0,0)
    rect.translate(-rect.center());
 
 
//    qDebug() << "After the adjustments" << endl;
//    qDebug() << "Center: " << rect.center() << endl;
//    qDebug() << "Top Left: " << rect.topLeft() << endl;
//    qDebug() << "Bottom Right: " << rect.bottomRight() << endl;
 
 
 
    //and return the rectangle
    return rect;
}

QRectF RootGraphicsItem::boundingRect() const
{

    //!get the bounding rectangle of the enclosing text
    QRectF rect = m_textGraphicsItem->boundingRect();

//    qDebug() << "************** the bounding rectangle of the text graphics item: *************" << endl;
//    qDebug() << "Top left: " << rect.topLeft() << endl;
//    qDebug() << "Center: " << rect.center() << endl;
//    qDebug() << "Bottom right: " << rect.bottomRight() << endl;

    //the following condition decides if the boundary needs
    //to be changed
    if( rect.width() < m_drawRectMinimumWidth )
        rect.setWidth(m_drawRectMinimumWidth);

    if(rect.height() < m_drawRectMinimumHeight)
        rect.setHeight(m_drawRectMinimumHeight);

    //set the rectangle width with the text item spacing
    rect.setWidth(rect.width() + getTextItemSpacing() );

    //the bounding rectangle will have its center at (0,0)
    rect.translate(-rect.center());


//    qDebug() << "After the adjustments" << endl;
//    qDebug() << "Center: " << rect.center() << endl;
//    qDebug() << "Top Left: " << rect.topLeft() << endl;
//    qDebug() << "Bottom Right: " << rect.bottomRight() << endl;



    //and return the rectangle
    return rect;
}

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and Bounding rectangle for the small ellipse

QRectF H3DHierarchyArrowDockGraphicsItem::boundingRect() const
{
    QRectF rect(0,0,sizeNormal.width(),sizeNormal.height());
 
    //center will be (0,0)
    rect.translate(-rect.center());
 
    return rect;
}

QRectF H3DHierarchyArrowDockGraphicsItem::boundingRect() const
{
    QRectF rect(0,0,sizeNormal.width(),sizeNormal.height());

    //center will be (0,0)
    rect.translate(-rect.center());

    return rect;
}

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Now when i layout the smaller one in the function of large one's function definition, which of the mapping function i should be using ?


Right now i am doing it as follows, seems like a hard-coding to me, What you guys think ?

void RootGraphicsItem::layoutChildItems()
{
   ...................................
 
   ...................................
    x = 0.0;
 
    //the first bounding rectangle is large ellipse's bounding rectangle 
   // and the second one is the small ellipses bounding rectangle
    y = (boundingRect().height()/2.0) - (m_dockOutGraphicsItem->boundingRect().height()/2.0);
 
 
    m_dockOutGraphicsItem->setPos(x,-y);
 
   ...................................
 
   ...................................
}

void RootGraphicsItem::layoutChildItems()
{
   ...................................

   ...................................
    x = 0.0;

    //the first bounding rectangle is large ellipse's bounding rectangle 
   // and the second one is the small ellipses bounding rectangle
    y = (boundingRect().height()/2.0) - (m_dockOutGraphicsItem->boundingRect().height()/2.0);


    m_dockOutGraphicsItem->setPos(x,-y);

   ...................................

   ...................................
}

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The arrangement is functional, but might be more accurate if i do the mapping before setting the position. since i am rendering in big ellipses coordinate system now, do i have to call mapToItem(...). I called it as follows:

m_dockOutGraphicsItem->setPos(mapFromItem(m_dockOutGraphicsItem,x,-y));

m_dockOutGraphicsItem->setPos(mapFromItem(m_dockOutGraphicsItem,x,-y));

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Bu the effect is the same, so what is the use of this mapping and when to use it.

A more explanation will be very helpful, any reference ?


Regards
Sajjad

[Spitfire]

It's a complex subject.
If you haven't read The Graphics View Coordinate System I would suggest "Coordinate Mapping" there as a starting point.