How do I make the top left of a QGraphicsScene coincide with the top left of my QGraphicsView?

4
votes

I always have trouble with the QGraphicsScene coordinate system. My scene is created quite simply like so:

scene = new QGraphicsScene(this);
this->setScene(scene);

where this is QGraphicsView. Now I want to draw a rectangle at (0,0) This creates rectangle that is roughly in the middle of the screen. How do I make it so that (0,0) corresponds to the upper left corner of my view? This is so I know where I can position anything...

2
qgraphicsview

2 Answers

5
votes

Yo can try this method: QGraphicsView::setAlignment(Qt::Alignment alignment) (see here)

You would have to use something like:

scene->setAlignment(Qt::AlignTop|Qt::AlignLeft);
0
votes

@rsp1984

Here is an example of a header code for a class based on a QWIdget. I'm copying and pasting only the parts of the code relvant to the question, from work, so please excuse me if it does not compile because I left something out:

The Header:

#include <QWidget>
#include <QGraphicsView>
#include <QDesktopWidget>
#include <QVBoxLayout>
#include <QApplication>
#include <QGraphicsScene>

class MonitorScreen : public QWidget
{
    Q_OBJECT
public:
    explicit MonitorScreen(QWidget *parent = nullptr, const QRect &screen = QRect(), qreal SCREEN_W = 1, qreal SCREEN_H = 1);
private:
    QGraphicsView *gview;  
};

The CPP:

#include "monitorscreen.h"

MonitorScreen::MonitorScreen(QWidget *parent, const QRect &screen, qreal SCREEN_W, qreal SCREEN_H) : QWidget(parent)
{

    // Making this window frameless and making sure it stays on top.
    this->setWindowFlags(Qt::FramelessWindowHint|Qt::WindowStaysOnTopHint|Qt::X11BypassWindowManagerHint|Qt::Window);

    this->setGeometry(screen);

    // Creating a graphics widget and adding it to the layout
    gview = new QGraphicsView(this);
    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->setContentsMargins(0,0,0,0);
    layout->addWidget(gview);

    gview->setScene(new QGraphicsScene(0,0,screen.width(),screen.height(),this));
    gview->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    gview->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

    // White background
    gview->scene()->setBackgroundBrush(QBrush(Qt::gray));
}

The idea here is that the scene is the same size as the rect of the QGraphcisView widget. That way something positioned at 100,100 will be a coordinate 100, 100 of in relation to the top left corner of the QGraphicsView. I do this because my scene will NEVER be larger than widget showing it. As a matter of fact they NEED to be the same size due to my particular problem.

Keep in mind that you CAN have a larger (or smaller) scene. If your scene is larger objects that are not withing the visible view will not be drawn, but will still exist in memory.

However I've always found that doing this, greatly facilitates how you start positioning and sizing your items in any given project, as you now know 100% sure where something at any given position should appear. You can then start coding more complex behaviour.

I hope this helps!