Determine bounding rect of line in Qt

Question

I am drawing a line using QPainterPath between two points as follows:

QPainterPath line;
line.moveTo(start_p);
line.lineTo(end_p);

QPen paintpen(Qt::black);
paintpen.setWidth(1);
painter->setRenderHint(QPainter::Antialiasing);
painter->setBrush(Qt::SolidPattern);
painter->setPen(paintpen);
painter->drawPath(line);

I have defined bounding rect as:

QRectF Line::boundingRect() const
{
 return QRectF(start_p.x(), start_p.y(), end_p.x(), end_p.y());
}

I get line painted correctly when:

start_p.x() < end_p.x()

and

start_p.y() < end_p.y()

How should the bounding rect be defined so that line is drawn correctly irrespective of the relationship between the coordinates of two points(start_p and end_p)?

score 7 · Accepted Answer · answered Oct 02 '14 at 08:17

7

You might try to normalize your rectangle:

QRectF Line::boundingRect() const
{
    return QRectF(start_p.x(), start_p.y(), end_p.x(), end_p.y()).normalized();
}

answered Oct 02 '14 at 08:17

vahancho

20,808
3
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The arguments to this version of QRectF's constructor (https://doc.qt.io/qt-5/qrectf.html#QRectF-3) are x, y, width, and height. Shouldn't it be `QRectF(start_p, end_p)` (https://doc.qt.io/qt-5/qrectf.html#QRectF-2)? – Mitch Dec 04 '21 at 15:21
@Mitch, right, it might be `QRectF(start_p, end_p)` too. – vahancho Dec 06 '21 at 07:24

score 3 · Answer 2 · answered Oct 02 '14 at 08:17

3

You could either:-

Check for the conditions when the ends are greater than the start points and set the rect appropriately
Return the QPainterPath's bounding rect
Use a QGraphicsLineItem instead of reinventing the wheel.

If you just want a line, QGraphicsLineItem is likely the best way to go here.

answered Oct 02 '14 at 08:17

TheDarkKnight

27,181
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1

The third one is the best! – Gluttton May 01 '19 at 20:02

score 0 · Answer 3 · answered Oct 02 '14 at 08:13

You can use QPainterPath::boundingRect which returns the bounding rectangle of the QPainterPath. You can keep the painter path as a class member and access it in the boundingRect function :

QRectF Line::boundingRect() const
{
     return line.boundingRect();
}

Determine bounding rect of line in Qt

3 Answers3