[Solved] QGraphicsIte setPos , moveBy - how do they work?



  • I looked at the "colliding mouse example":http://doc.qt.nokia.com/4.7-snapshot/graphicsview-collidingmice.html that ships with Qt.
    It took me several days to figure out
    @setPos(mapToParent(0,1));@

    moves a QGraphicsItem downwards, everytime 'advance' (QGraphicsView function) is called. I don't understand how this works.

    (This is not part of the code, but I modified the 'setPos' function slightly to see how it works)

    I had to rummage though the code to figure out what was moving the QGraphicsItem. I expected to see something like

    @setPos(mapToParent(0,ypos));@

    to move something downwards, with ypos incremented in 'advance' member function in QGraphicsItem. This seems more intuitive to me. However, it seems this causes the item to move with a constant acceleration.

    I also saw another function somewhere else that works exactly the same way (Correct me if I am wrong)
    @moveBy(0,1);@

    Now this seems intuitive to me. Everytime advance is called, the item is 'moved by' (0,1)

    Could somebody explain how setPos works?
    Thanks!



  • The confusion does not come from your interpretation of "setPos" (which acts as you expect), but from the "mapToParent" function

    THIS is the important point:
    The "mapToParent" function converts a coordinate in item coordinates to coordinate in parent's coordinate system (or scene coordinates if no parent)

    For example, if you have a mouse "myMouse" at (10,10) in your scene:

    • The "myMouse" local coordinates are (0,0), i.e origin of the mouse item
    • The "myMouse" coordinate in scene coordinates are (10,10).

    A call myMouse.mapToParent(0,0) will return (10,10);
    A call myMouse.mapToParent(0,1) will return (10,11);

    and this is the point:
    @myMouse.setPos(10,10); /* Position the mouse in it's parent coordinate system : (10, 10) /
    newPos = myMouse.mapToParent(0,1); /
    newPos = (10,10) + (0,1) = (10, 11); /
    myMouse.setPos(newPos); /
    Position the mouse in it's parent coordinate system : (10, 11) /
    newPos = myMouse.mapToParent(0,1); /
    newPos = (10,11) + (0,1) = (10, 12); /
    myMouse.setPos(newPos); /
    Position the mouse in it's parent coordinate system : (10, 12) */@

    and so on...

    Note: In the "colliding mice" sample, a mouse coordiantes system is yaxis = nose to tail axis: combining with the mouse item rotation, a mouse does not only go down, but in the good direction

    Perhaps, "The Graphics View Coordinate System":http://qt-project.org/doc/qt-4.8/graphicsview.html#the-graphics-view-coordinate-system can help...



  • Thanks sraboisson! Wonderful explanation. Ah so 'increment' part comes from mapToParent as you showed in the code. I think the reason it did not occur to me was I didn't think the origin changed everytime setpos(mapToParent(..)) was called.


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