#include "mainwindow.h"
#include "ui_mainwindow.h"
#include<iostream>

using namespace std;

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent),
ui(new Ui::MainWindow)

{
ui->setupUi(this);

}

MainWindow::~MainWindow()
{
delete ui;
}

/this function get the value of lat/
double MainWindow::getLat(double k)
{
double value=0;

if(k<=505&&k>=0)
     { value=90-k*(18/101);}

if(k>505&&k<=1010)
      {value=k*(18/101)-90;}
return value;

}

void MainWindow:: paintEvent(QPaintEvent *event)
{

/*here i plot my image first */

QImage image;
image.load("C:/Users/hp/Documents/Qt/untitled2/earthh.jpg");
QPainter imagePainter(this);
imagePainter.begin(this);
imagePainter.drawImage(QPoint(0, 0), image);
imagePainter.end();

/then here i plot my points/

QPainterPath dotCircle;
double lat=0;

for ( double i=0;i<=1920;i+=50)
{
    for(double j=0;j<1000;j+=50)
    {

         dotCircle.addEllipse(QPoint(i,j),3,3);
         QPainter ellipsePathPainter;
         ellipsePathPainter.begin(this);
         ellipsePathPainter.setPen(QPen(Qt::transparent,1, Qt::SolidLine, Qt::SquareCap, Qt::RoundJoin));

        double k=lat;
        lat=qDegreesToRadians(getLat(j)); //the problem is here this line is only getting executed twice when j<550 and when j>505 

        double alt=0.0;
        double  champs=(U*M*sqrt(4-3*pow(cos(lat),2)))/(4*M_PI*pow(alt+Re,3))*1000000;

        if(champs>=45&&champs<60)
                 {ellipsePathPainter.setBrush(Qt::green);}
        else if(champs>=35&&champs<45)
                 { ellipsePathPainter.setBrush(Qt::yellow);}
        else if(champs>=30&&champs<35)
                 { ellipsePathPainter.setBrush(Qt::magenta);}
        else
                { ellipsePathPainter.setBrush(Qt::red);}

        ellipsePathPainter.drawPath(dotCircle);
        ellipsePathPainter.end();

        }
}

}

as the outcome the points are plotted all with the same colors because my function getlat() is only getting called twice .