What really is a QPolygon...

[giusepped]

Suppose the following code I supposed to do a union of polyogn

QPolygonF f,g;
 
 f<<QPointF(1,1)<<QPointF(10,29);
 g<<QPointF(100,100)<<QPointF(10,290);
 
 
  qSort(f.begin(),f.end(),lessThanPoint);
  qSort(g.begin(),g.end(),lessThanPoint);
 qDebug()<< f.united(g);

QPolygonF f,g;

 f<<QPointF(1,1)<<QPointF(10,29);
 g<<QPointF(100,100)<<QPointF(10,290);
 
 
  qSort(f.begin(),f.end(),lessThanPoint);
  qSort(g.begin(),g.end(),lessThanPoint);
 qDebug()<< f.united(g);

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Why I get the following result:

QPolygonF(QPointF(1, 1)QPointF(1, 1)QPointF(10, 29)QPointF(1, 1)QPointF(10, 290)QPointF(100, 100)QPointF(10, 290)QPointF(1, 1))



I should have simply the union of the point (a polygon of 4 points).
Maybe I am missing something?
G

[aamer4yu]

For your requirement you can use QPainter::drawPolyline
or you can see QPaintEngine::PolygonDrawMode

[giusepped]

I do not understand.
Actually, I wanna perform a merging of two curve obatained from two data set.
But: during the merging I would keep only those points who are the highest (I am supposing to draw the polygons on an xy plane).
There is a simple way in qt to merge two curves in this way?
G

[wysota]

Simply draw them both. One will cover the other and you will get what you wanted.

[giusepped]

Yes, you're right. I should study more...
Anyway, if the polygons really represent curve, I have to dela with extremes...

QPolygonF  MyPlot::combine( QPolygonF &serie1,   QPolygonF &serie2)
{
 
	 QPolygonF p1(serie1);
	 QPolygonF p2(serie2);
	 p1.prepend(QPointF(serie1.first().x(),0));
	 p1.append(QPointF(serie1.last().x(),0));
	 p2.prepend(QPointF(serie2.first().x(),0));
	 p2.append(QPointF(serie2.last().x(),0));
 
	 QPolygonF temp(p1.united(p2));
 
	 QPointF first,last;
 
 
	 qSort(temp.begin(),temp.end(),lessThanPoint);
 
	 first = temp.first();
	 last =  temp.last();
	 QPolygonF t(temp);
 
	 t.resize(t.size()-2);
 
 
	  t.append(last);
 
 
	 return t;
}

QPolygonF  MyPlot::combine( QPolygonF &serie1,   QPolygonF &serie2)
{
 
	 QPolygonF p1(serie1);
	 QPolygonF p2(serie2);
	 p1.prepend(QPointF(serie1.first().x(),0));
	 p1.append(QPointF(serie1.last().x(),0));
	 p2.prepend(QPointF(serie2.first().x(),0));
	 p2.append(QPointF(serie2.last().x(),0));

	 QPolygonF temp(p1.united(p2));
	 
	 QPointF first,last;
	
	 
	 qSort(temp.begin(),temp.end(),lessThanPoint);
	 
	 first = temp.first();
	 last =  temp.last();
	 QPolygonF t(temp);
	 
	 t.resize(t.size()-2);
	
 
	  t.append(last);

	 
	 return t;
}

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[giusepped]

I am quite stucked at this problem...the previous code does not work.
The problem is depicted in the figure.
There is a way to merge two curves or polygon and picks the envelope?
Regards

[wysota]

It's not an easy math problem so I wouldn't expect there to be a ready function for it in Qt.

[giusepped]

Actually, the union of polygons works quite well with QPOlygonF::united(), and
I expected that a similar function was available also for polylines.
The problem is that after the union, the points are not ordered in the right way....
It should be enough to have a way to open the polygon, i.e. eliminating the segment which lies on the x axis....

[wysota]

Have you tried using QPainterPath? Maybe you'll have more luck with it.

[giusepped]

I have no idea at all how to use it.
If you some suggestion, I will appreciate very much....
G

[wysota]

Add primitives to it. You can also unite it. There is a paragraph in the reference (Composing a QPainterPath) that might be helpful.

[giusepped]

I tried, but the problem is always the same.
After the union of paths, the points order is arbitary and for this reason is quite difficult to convert the path to a curve in the 2D-plane.
(For instance, the union of Polygons use the united function of QPainterPath).
G