real world coordinates in graphics view / scene

[robertson1]

Hello,

I'm trying to figure out the best way to retain the real world coordinates of items in a graphics scene. i.e so that items can be added at any stage as required in there correct world coordinates (like in a cad program).

I have a subclassed QGraphics view with some mouse events for panning / zooming round and display the cursor position (in scene coordinates) in a QLabel, but am struggling to get my head around the scene coordinates which are displayed when I am simply scrolling (panning) around the view.

They Y axis scene coordinates behave as expected but I cant figure out whats going on with the X axis coordinates (why do they change when I am just scrolling around? (drag with middle mouse button).

For example:

#include <QApplication>
#include <QMainWindow>
#include <QStatusBar>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsTextItem>
#include <QMouseEvent>
#include <QScrollBar>
#include <QLabel>
#include <math.h>
 
class MapView : public QGraphicsView
{
 
public:
	MapView(QWidget *parent = 0);
	QPoint sceneCoordinates;
	QLabel *coordinateDisplayLabel;
 
protected:
	void scaleView(qreal scaleFactor);
	void wheelEvent(QWheelEvent *event);
	void mousePressEvent(QMouseEvent *event);
	void mouseMoveEvent(QMouseEvent *event);
    	void mouseReleaseEvent(QMouseEvent *event);
 
private:
	QString cursorXString;
	QString cursorYString;
	void storeMouseEvent(QMouseEvent *event);
    	bool scrolling;
    	QMouseEvent lastMouseEvent;
 
};
 
MapView::MapView(QWidget *parent)
    : QGraphicsView(parent), lastMouseEvent(QEvent::None,QPoint(),
    	Qt::NoButton,Qt::NoButton,Qt::NoModifier)
 
{	
	setAttribute (Qt::WA_SetCursor);
	setCursor(Qt::CrossCursor); 
	scale(1,-1);
	coordinateDisplayLabel = new QLabel(0);
	setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    	setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    	scrolling = false;
}
 
void MapView::wheelEvent(QWheelEvent *event)
{
    	scaleView(pow((double)2, event->delta() / 240.0));
}
 
void MapView::scaleView(qreal scaleFactor)
{
    	qreal factor = matrix().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width();
    	if (factor < 0.07 || factor > 100)
        	return;
    	scale(scaleFactor, scaleFactor);
} 
 
void MapView::storeMouseEvent(QMouseEvent *event)
{
   	lastMouseEvent = QMouseEvent(event->type(), event->pos(), event->globalPos(),
    	event->button(),event->buttons(),event->modifiers());
}
 
void MapView::mousePressEvent(QMouseEvent *event)
{
        if(event->button() == Qt::MidButton) 
            {
        	setCursor(Qt::ClosedHandCursor);
            	scrolling = true;
            	storeMouseEvent(event);
            	event->accept();
            	return;
            }
}
 
void MapView::mouseMoveEvent(QMouseEvent *event)
{
	if(scrolling) 
	    {
        	QPoint delta = event->globalPos() - lastMouseEvent.globalPos();
        	QScrollBar *hBar = horizontalScrollBar();
        	QScrollBar *vBar = verticalScrollBar();
        	hBar->setValue(hBar->value() - delta.x());
        	vBar->setValue(vBar->value() - delta.y());
        	storeMouseEvent(event);
        	event->accept();
        	return;
    	}
 
	sceneCoordinates = mapFromScene(event->pos());
	cursorXString.setNum(sceneCoordinates.x());
	cursorYString.setNum(sceneCoordinates.y());
	coordinateDisplayLabel->setText("x = " + cursorXString + "    " + "y = " + cursorYString);	
}
 
void MapView::mouseReleaseEvent(QMouseEvent *event)
{
    if((event->button() == Qt::MidButton) && (scrolling == true))
    	{
    		setCursor(Qt::CrossCursor);
        	scrolling = false;
        	storeMouseEvent(event);
        	event->accept();
        	return;
    	}
    event->ignore();
}
 
 
int main( int argc, char **argv )
{
        QApplication a( argc, argv );
    	QMainWindow *mw = new QMainWindow();
  	QGraphicsScene *scene = new QGraphicsScene;
  	MapView *view = new MapView;
  	QStatusBar *statusbar = new QStatusBar;
 
  	scene->setSceneRect(-30000, -30000, 60000, 60000);
  	scene->setBackgroundBrush(Qt::black);
 
	scene->addLine ( -50, 0, 50, 0, (QPen(Qt::white, 0)));
	scene->addLine ( 0, -50, 0, 50, (QPen(Qt::white, 0)));
	QGraphicsTextItem item1("0,0");
	item1.setDefaultTextColor(Qt::white);
	item1.setPos(0,0);
	item1.scale(1,-1);
	scene->addItem(&item1);
 
	scene->addLine ( 200, 250, 300, 250, (QPen(Qt::white, 0)));
	scene->addLine ( 250, 300, 250, 200, (QPen(Qt::white, 0)));
	QGraphicsTextItem item2("250,250");
	item2.setDefaultTextColor(Qt::white);
	item2.setPos(250,250);
	item2.scale(1,-1);
	scene->addItem(&item2);
 
	view->setScene(scene);
	mw->setCentralWidget(view);
	statusbar->addPermanentWidget(view->coordinateDisplayLabel, 0);
	mw->setStatusBar(statusbar);
 
  	mw->show();
 
  	return a.exec();
}

#include <QApplication>
#include <QMainWindow>
#include <QStatusBar>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsTextItem>
#include <QMouseEvent>
#include <QScrollBar>
#include <QLabel>
#include <math.h>

class MapView : public QGraphicsView
{

public:
	MapView(QWidget *parent = 0);
	QPoint sceneCoordinates;
	QLabel *coordinateDisplayLabel;
	
protected:
	void scaleView(qreal scaleFactor);
	void wheelEvent(QWheelEvent *event);
	void mousePressEvent(QMouseEvent *event);
	void mouseMoveEvent(QMouseEvent *event);
    	void mouseReleaseEvent(QMouseEvent *event);
 		
private:
	QString cursorXString;
	QString cursorYString;
	void storeMouseEvent(QMouseEvent *event);
    	bool scrolling;
    	QMouseEvent lastMouseEvent;
	
};

MapView::MapView(QWidget *parent)
    : QGraphicsView(parent), lastMouseEvent(QEvent::None,QPoint(),
    	Qt::NoButton,Qt::NoButton,Qt::NoModifier)
    
{	
	setAttribute (Qt::WA_SetCursor);
	setCursor(Qt::CrossCursor); 
	scale(1,-1);
	coordinateDisplayLabel = new QLabel(0);
	setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    	setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
	setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    	scrolling = false;
}

void MapView::wheelEvent(QWheelEvent *event)
{
    	scaleView(pow((double)2, event->delta() / 240.0));
}
 
void MapView::scaleView(qreal scaleFactor)
{
    	qreal factor = matrix().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width();
    	if (factor < 0.07 || factor > 100)
        	return;
    	scale(scaleFactor, scaleFactor);
} 
 
void MapView::storeMouseEvent(QMouseEvent *event)
{
   	lastMouseEvent = QMouseEvent(event->type(), event->pos(), event->globalPos(),
    	event->button(),event->buttons(),event->modifiers());
}

void MapView::mousePressEvent(QMouseEvent *event)
{
        if(event->button() == Qt::MidButton) 
            {
        	setCursor(Qt::ClosedHandCursor);
            	scrolling = true;
            	storeMouseEvent(event);
            	event->accept();
            	return;
            }
}

void MapView::mouseMoveEvent(QMouseEvent *event)
{
	if(scrolling) 
	    {
        	QPoint delta = event->globalPos() - lastMouseEvent.globalPos();
        	QScrollBar *hBar = horizontalScrollBar();
        	QScrollBar *vBar = verticalScrollBar();
        	hBar->setValue(hBar->value() - delta.x());
        	vBar->setValue(vBar->value() - delta.y());
        	storeMouseEvent(event);
        	event->accept();
        	return;
    	}

	sceneCoordinates = mapFromScene(event->pos());
	cursorXString.setNum(sceneCoordinates.x());
	cursorYString.setNum(sceneCoordinates.y());
	coordinateDisplayLabel->setText("x = " + cursorXString + "    " + "y = " + cursorYString);	
}

void MapView::mouseReleaseEvent(QMouseEvent *event)
{
    if((event->button() == Qt::MidButton) && (scrolling == true))
    	{
    		setCursor(Qt::CrossCursor);
        	scrolling = false;
        	storeMouseEvent(event);
        	event->accept();
        	return;
    	}
    event->ignore();
}


int main( int argc, char **argv )
{
        QApplication a( argc, argv );
    	QMainWindow *mw = new QMainWindow();
  	QGraphicsScene *scene = new QGraphicsScene;
  	MapView *view = new MapView;
  	QStatusBar *statusbar = new QStatusBar;
  
  	scene->setSceneRect(-30000, -30000, 60000, 60000);
  	scene->setBackgroundBrush(Qt::black);
  	
	scene->addLine ( -50, 0, 50, 0, (QPen(Qt::white, 0)));
	scene->addLine ( 0, -50, 0, 50, (QPen(Qt::white, 0)));
	QGraphicsTextItem item1("0,0");
	item1.setDefaultTextColor(Qt::white);
	item1.setPos(0,0);
	item1.scale(1,-1);
	scene->addItem(&item1);
	
	scene->addLine ( 200, 250, 300, 250, (QPen(Qt::white, 0)));
	scene->addLine ( 250, 300, 250, 200, (QPen(Qt::white, 0)));
	QGraphicsTextItem item2("250,250");
	item2.setDefaultTextColor(Qt::white);
	item2.setPos(250,250);
	item2.scale(1,-1);
	scene->addItem(&item2);
	
	view->setScene(scene);
	mw->setCentralWidget(view);
	statusbar->addPermanentWidget(view->coordinateDisplayLabel, 0);
	mw->setStatusBar(statusbar);
	
  	mw->show();
  	
  	return a.exec();
}

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Ultimately I'm trying to get the coordinate display to always match the original scene coordinates.


Hope someone can enlighten me a bit.


Thanks