Get visible rectangle of QGraphicsView?

Question

I've been pulling my hair out with this one for hours. There's a thread here about it, but nothing seems to be working. QGraphicsView::rect() will return the width and height, but the left and top values aren't set properly (always 0 -- ignoring the scrolled amount). I want it in scene coordinates, but it should be easy enough to translate from any system. I have no idea what horizontalScrollBar()->value() and vert are returning...seems to be meaningless jibberish.

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@fabrizioM:

// created here
void EditorWindow::createScene() {
    m_scene = new EditorScene(this);
    m_view = new EditorView(m_scene);
    setCentralWidget(m_view);
    connect(m_scene, SIGNAL(mousePosChanged(QPointF)), this, SLOT(mousePosChanged(QPointF)));
}

/// with this constructor
EditorView::EditorView(QGraphicsScene* scene, QWidget* parent) : QGraphicsView(scene, parent) {
    setRenderHint(QPainter::Antialiasing);
    setCacheMode(QGraphicsView::CacheBackground);
    setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    setDragMode(QGraphicsView::NoDrag);
    scale(1.0, -1.0); // flip coordinate system so that y increases upwards
    fitInView(-5, -5, 10, 10, Qt::KeepAspectRatio);
    setInteractive(true);
    setBackgroundBrush(QBrush(QColor(232,232,232), Qt::DiagCrossPattern));
}

Answer 1

Nevermind. Came up with this, which seems to work.

QRectF EditorView::visibleRect() {
    QPointF tl(horizontalScrollBar()->value(), verticalScrollBar()->value());
    QPointF br = tl + viewport()->rect().bottomRight();
    QMatrix mat = matrix().inverted();
    return mat.mapRect(QRectF(tl,br));
}

Answer 2

It sounds like what you want is the scene rectangle. The ::rect() method is inherited from QWidget. See:

http://doc.qt.nokia.com/4.6-snapshot/qgraphicsview.html#sceneRect-prop

Answer 3

You can do what you've done, or use the mapToScene() functions. You can't count on the resulting scene "rectangle" being a rectangle, however, because the scene might be rotated or sheared in the view, resulting in a general polygon when mapped to the scene.

If your application never does such things, of course, you're free to assume that a rectangle is always appropriate.

Answer 4

Hi,

here is a possible solution (no clue whether this is the intended one)

QRectF XXX::getCurrrentlyVisibleRegion() const
{
        //to receive the currently visible area, map the widgets bounds to the scene

        QPointF topLeft = mapToScene (0, 0);
        QPointF bottomRight = mapToScene (this->width(), this->height());

        return QRectF (topLeft, bottomRight);
}

HTH, Bernhard