Bresenham's circle drawing algorithm in QT Creator C++

circle.h file:

#ifndef CIRCLE_H
#define CIRCLE_H
#include <QMainWindow>
#include "ui_circle.h"

namespace Ui {
class circle;
}

class circle : public QMainWindow
{
Q_OBJECT
public:
explicit circle(QWidget *parent = 0);
~circle();
private:
Ui::circle *ui;
private slots:
void drawcircle();
};
#endif
___________________________________________________________

circle.cpp file:

#include "circle.h"
#include "ui_circle.h"
#include"math.h"

class line //class Line
{
public:
    QImage draw(int x1,int y1,int x2,int y2, QImage img)
    {
    int dx,dy;
    int g;
    if(x2>x1)
    dx=x2-x1;
    else
    dx=x1-x2;
    if(y2>y1)
    dy=y2-y1;
    else
    dy=y1-y2;
    if(dx>=dy)
    {
    g=(2*dy)-dx;
    while(x1!=x2)
    {
    if(g>0)
    {
    if(x1<x2)
    x1++;
    else
    x1--;
    if(y1<y2)
    y1++;
    else
    y1--;
    g=g+(2*(dy-dx));
    img.setPixel(x1,y1,qRgb(255,255,255)); //set Pixel
    }
    else
    {
    if(x1<x2)
    x1++;
    else
    x1--;
    g=g+(2*dy);
    img.setPixel(x1,y1,qRgb(255,255,255));
    }
    }
    }
    if(dx<dy)
    {
    g=(2*dx)-dy;
    while(y1!=y2)
    {
    if(g>=0)
    {
    if(x1<x2)
    x1++;
    else
    x1--;
    if(y1<y2)
    y1++;
    else
    y1--;
    g=g+(2*(dx-dy));
    img.setPixel(x1,y1,qRgb(255,255,255));
    }
    else
    {
    if(y1<y2)
    y1++;
    else
    y1--;
    g=g+(2*dx);
    img.setPixel(x1,y1,qRgb(255,255,255));
    }
    }
    }
    return img;
    }
};

circle::circle(QWidget *parent) : //Class Circle
    QMainWindow(parent),
    ui(new Ui::circle)
{
    ui->setupUi(this);
    connect(this->ui->pushButton,SIGNAL(clicked()),
    this,SLOT(drawcircle()));
}

circle::~circle() //Destructor
{
    delete ui;
}

void circle::drawcircle() //Drawcircle
{
line l1;
int xc,yc;
int x1,y1,x2,y2;
int r;
int d;
QImage img(400,400,QImage::Format_RGB888);
img.fill(Qt::black);
xc = this->ui->textEdit->toPlainText().toInt();
yc = this->ui->textEdit_2->toPlainText().toInt();
r = this->ui->textEdit_3->toPlainText().toInt();
x1=0;
y1=r;
x2=x1;
y2=y1;
d=3-(2*r);
while(x1<y1)
{
x2=x1;
y2=y1;
x1++;
if(d<0)
{
d=d+(4*x1)+6;
}
else
{
y1=y1-1;
d=d+(4*(x1-y1))+10;
}
img=l1.draw(xc+x1,yc+y1,xc+x2,yc+y2,img);
img=l1.draw(xc-x1,yc+y1,xc-x2,yc+y2,img);
img=l1.draw(xc+x1,yc-y1,xc+x2,yc-y2,img);
img=l1.draw(xc-x1,yc-y1,xc-x2,yc-y2,img);
img=l1.draw(xc+y1,yc+x1,xc+y2,yc+x2,img);
img=l1.draw(xc-y1,yc+x1,xc-y2,yc+x2,img);
img=l1.draw(xc+y1,yc-x1,xc+y2,yc-x2,img);
img=l1.draw(xc-y1,yc-x1,xc-y2,yc-x2,img);
}
ui->label_4->setPixmap(QPixmap::fromImage(img));
}

Output