[QT][QML][C++] Draw an interpolated polygon using C++ painter

[Shadow.01]

Hi all!

in my QT / QML application the following C++ class works perfectly and draw a simple polygon formed by six elements in a QML view.

Now, I want an interpolated polygon such that I don't see a series of lines with edges but a continous line without any edge. How can I transform my class in order to do that?

Important: I don't use a chart but a normal graph figure; due to this reason, I think that the QTSpline series I already found in some forum are not the solution. But I am obviously opened to any way.

Important: the line, as in figure, should touch all the edge. Otherwise, the result is not what I want. Due to this reason, the Bezier algorytm is not correct.

Edit: at the end of the post I explained by putting an image what result I would obtain; thanks again.

Thank you so much!

#include "mydiagram.h"
#include <QPainter>
#include <string>
#include <iostream>
#include <QtCharts>
#include <QSplineSeries>
#include <QPoint>

using namespace QtCharts;


MyDiagram::MyDiagram(QQuickItem *parent): QQuickPaintedItem(parent)
{

}

void MyDiagram::paint(QPainter *painterMyDiagram) {

    QBrush myBrush(QColor("transparent"));
    QPen myPen(QColor("grey"), 3, Qt::DashDotLine);
    painterMyDiagram->setBrush(myBrush);
    painterMyDiagram->setPen(myPen);
    painterMyDiagram->setRenderHint(QPainter::Antialiasing);
    
    static const QPointF points[6] = {
        QPointF(10.0, 80.0),
        QPointF(20.0, 10.0),
        QPointF(200.0, 30.0),
        QPointF(300.0, 160.0),
        QPointF(250.0, 250.0),
        QPointF(10.0, 80.0)
    };

    painterMyDiagram->drawPolyline(points, 6);
}

[mrjj]

Hi

• but a continous line without any edge.

You mean like a curve ish ?
Currently you are using a polyline and it has sharp edges.

You mean something like ?
https://www.toptal.com/c-plus-plus/rounded-corners-bezier-curves-qpainter

[Shadow.01]

Thanks for your reply; sorry but Bezier is not my solution because the line should touch all given points. In order to explain me better, I attach an image.

[mrjj]

@Shadow-01
Hi
super with picture.
But did you try the RoundedPolygon class from the link?

It does sort of round the edges of a polygon so might look ok and hits the points.

[Pl45m4]

@Shadow-01

Something like spline interpolation?!

This example is for QChart but maybe you can use spline interpolation for your polygon.

• https://doc.qt.io/qt-5/qsplineseries.html

Wiki:
https://en.wikipedia.org/wiki/Spline_interpolation

[Shadow.01]

@mrjj Thanks; rounded polygon doesn't exist under the item that QT creator suggested me in the list of figure I can draw; should I import something special? Or, have you an example?

Thanks again for your time!

[Shadow.01]

@Pl45m4 Thanks but not; this is not the way; as told in my question, I already tried but it works only in a chart. And I should draw a free polygon outside a chart. Any other idea?

(No, SPline in a drawPolyline give an error; only under a chart I can use it).

[mrjj]

@Shadow-01 said in [QT][QML][C++] Draw an interpolated polygon using C++ painter:

thanks; rounded polygon doesn't exist under the item that QT creator suggested me in the list of figure I can draw; should I import something

Yes, the link has a that class in the bottom. ITs not a default Qt one.

The effect sounds like what you want.

but might end being too round. I have not tried it with a shape like yours.

[Shadow.01]

@mrjj Wait, for sure the line touch the edge? It seems not. It is Bezier algorythm? From the page, it seems so..

[Shadow.01]

@mrjj Thanks but for sure is not the way. My solution should absolutely touch the edge (given points), otherwise the line is not correct. Other solutions? Thanks for your time!

[mrjj]

@Shadow-01
Hi
but look at the airplane. it does touch the edges.
Outside of such rounded polygon im not sure what to use.

[Shadow.01]

No, unfortunately don't touch. If you see the internal vertical segment of each engine, is for sure longer in the left one. Anyway, thanks. :-(

[Pl45m4]

@Shadow-01 said in [QT][QML][C++] Draw an interpolated polygon using C++ painter:

(No, SPline in a drawPolyline give an error; only under a chart I can use it).

Read my answer again.
I know that you can't use QSplineSeries outside of QChart but you should take a took at it (spline interpolation) and the Wikipedia page, I linked.

If there's nothing like this in Qt you have to draw it yourself using mathematics.

[Shadow.01]

Thanks all so much. I found a solution, by using the Bezier Algorithm with a really small curve factor number. The result is almost the same as desired.

[mrjj]

@Shadow-01
You could show an image of the final result :) 🙄
So did you use code from link or did you make some yourself ?