draw ellipse with specific points in scene

Question

i want to know how i can draw ellipse with the three points marked in the qgraphicsscene . say if the user selects three points in the scene using mousePress i will highlight the pressed points with marks . now using the the three marks i have to draw an ellipse around the three marks or making the points as a boundary for the ellipse i draw .

for implementation painterpath drawEllipse will work ..?

Answer 1

I have worked on a similar problem with PySide. You can easily adapt this code in C++:

def paintEvent( self, ev ):
    painter = QPainter( self )
    painter.setPen( QPen( Qt.red, 4 ) )
    painter.drawPoint( self.a )
    painter.drawPoint( self.b )
    painter.drawPoint( self.c )

    # 1. Find the long axis.
    maxL = max( QLineF( self.a, self.b ), QLineF( self.a, self.c ), QLineF( self.b, self.c ), key=lambda x: x.length() )
    center = maxL.pointAt( 0.5 )

    # 2. Find the small axis
    if maxL == QLineF( self.a, self.b ):
        c = self.c
    elif maxL == QLineF( self.a, self.c ):
        c = self.b
    else:
        c = self.a
    w = maxL.length()
    h = QLineF( c, center ).length() * 2.0

    # 3. Define the bounding rect for our ellipse
    rect = QRect( -w/2.0, -h/2.0, w, h )

    # 4. Rotate the painter and draw the ellipse
    painter.translate( center.x(), center.y() )
    painter.rotate( -maxL.angle() )
    painter.setPen( Qt.black )
    painter.drawEllipse( rect )

The goal is to calculate the bounding rectangle of the ellipse considering that the three points are the ends of the two axis.

So, the first step find the longer axis. The second step finds the small axis.
The rectangle is defined by the axis lengths and we just need to rotate the painter to draw our ellipse.

A better solution should exist but the mine seems to work, for the moment...

Edit: It's quick and dirty. So, it will not work for a lot of cases. A better solution would use the property: for each point on the ellipse, the sum of distance between the point and two static points is constant. So, you can calcuate this distance and determine the two other points and determine the two axis.