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qtqgraphicsitemqgraphicsscene

How to get absolute coordinates with QGraphicsScene?

Let's say I have five objects, with the top-left red square and the larger black square both inserted into the scene at 0,0.

A central object with 4 red smaller objects. .

Later, let's say I want to move the bottom right corner of the black square to the bottom right corner of the bottom-right square. The most obvious thing to do would be:

bS.setRect( bS.rect().setBottomRight(redBR.rect().bottomRight) );

That's not exactly correct code, but the idea is simple. I would set the coordinate of the black square's bottom right corner to that of the red square's bottom right corner. Because that's where I want it to be. But in Qt, this seems to be impossible.

Because the call to redBR thinks it's at the origin. Every item thinks it's at the origin. Which means I can never know the coordinates of any item, anywhere, ever.

Is there a way to force QGraphicsScene to tell me the actual coordinates? Alternatively, is there some other graphics framework that uses real coordinates? I'm not going to insist on using Qt if there is no way to make it work.

Any help would be welcome. Please bear in mind my goal is not: "drag corner of box by manipulator". My goal is to be able to put items at coordinates whenever I want an item to be at a specific coordinate.

Edit:

Here's an example of what I mean. The big box hasn't been connected yet, so don't worry about it's coordinates. The problem is that if I don't remap the points coming out of the little box, every box believes it is at the origin. But if I do, then the y values vacillate between 0, 1, and -1.

enter image description here

Solution

  • The base class QGraphicsItem has quite some useful functionality for mapping between scene and item coordinates. One useful function is for mapping a point in item coordinates to scene coordinates:

    QPointF QGraphicsItem::mapToScene(const QPointF &point)

    There is however also a direct mapping available between points in different QGraphicsItems, which is probably the easiest to use here:

    QPointF QGraphicsItem::mapFromItem(const QGraphicsItem *item, const QPointF &point)

    which can be used to map a point in redBR to a point in bS like:

    bS.rect().setBottomRight(bS.mapFromItem( &redBR, redBR.rect().bottomRight() ) )