[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);
}

Notebook 7 - page 1.png

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.

alt text

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 ?