Coordinate problem

[AndyQT]

Hello!

I have some problems with coordinates. I have a QGraphicsScene in a QGraphicsView and on the scene there is a QGraphicsItem. I retrieve the center coordinates of the bounding box

QPointF coords= this->mapToScene(this->boundingRect().center());

QPointF coords= this->mapToScene(this->boundingRect().center());

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How can I match the coordinate system for the expression above to one of those for mouse click coordinates?:

void MyClass::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
QPoint clickCoords =  QCursor::pos();
QPointF globalPos = event->screenPos();
...

void MyClass::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
QPoint clickCoords =  QCursor::pos();
QPointF globalPos = event->screenPos();
...

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I'm sorry, this coordinate thing has been discussed so many times, but I can't figure it out.

Andy

[Santosh Reddy]

event->scenePos();

event->scenePos();

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[AndyQT]

Thank you very much, you opened my eyes. Now I realise, that I have focused too much on solving it from the other side, I was looking for a way to get the item boundingbox center coordinates to translate to global coordinates(btw how would I do that?, I didn't find a matching map to global function). But of course, for my needs there is no difference if global or scene corrdinates, they just have to match! Thank you!

[Santosh Reddy]

QGraphcisScene coordinates connot be directly mapped to screen coordinates, as the on screen position of QGraphicsScene is taken care by QGraphicsView. So to get the screen coridinates of a point you have convert the point to QGraphicsSecen coordinates and then to screen codinates using the QGraphicsView which is displaying the QGraphicsScene.

extern QGraphicsView * view;
extern QGraphicsScene * scene;
extern QGraphicsItem * item;
 
view->setScene(scene);
scene->addItem(item);
 
QPointF point;
point = item->boundingRect().center();
point = item->mapToScene(point);
point = view->mapFromScene(point);

extern QGraphicsView * view;
extern QGraphicsScene * scene;
extern QGraphicsItem * item;

view->setScene(scene);
scene->addItem(item);

QPointF point;
point = item->boundingRect().center();
point = item->mapToScene(point);
point = view->mapFromScene(point);

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Note that without having a view there is no sense for screen/viewport/global coordinates.

[AndyQT]

Thank you very much for the detailed explanation. It seems so logical when you see it.
And of course, in my case I should also stick to the scene coordinates, so I don't need to care about the window position. In fact, all my problem started by choosing the wrong source for cursor position - QCursor:os() instead of the cursor scenePos supplied by the mouse event. I simply took what google gave me first :-(.

Now everything is OK!