Midpoint Ellipse Drawing Algorithm

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <graphics.h>
#include <math.h>

#define ROUND(a) ((int)(a+0.5))

void plot_ellipse(int,int,int,int);

int xc,yc,rx,ry;

void main()
{
 int rx2,ry2,twoRx2,twoRy2;
 int i,j,x,y,p,px,py;

 int gd=DETECT,gm;
 initgraph(&gd,&gm,"");

 printf("Enter the co-ordinates of the centre:\n");
 scanf("%d%d",&xc,&yc);

 printf("Please enter the radius of the ellipse :\n");
 scanf("%d%d",&rx,&ry);
 cleardevice();

 rx2=rx*rx;
 ry2=ry*ry;
 twoRx2=2*rx2;
 twoRy2=2*ry2;
 x=0; y=ry;
 px=0; py=twoRx2*y;

 plot_ellipse(xc,yc,x,y);

 p=ROUND(ry2-(rx2*ry)+(0.25*rx2));
 while(px<py)
 {
 x++;
 px+=twoRy2;
 if(p<0)
  p+=ry2+px;
 else {
  y--;
  py-=twoRx2;
  p+=ry2+px-py;
 }
 plot_ellipse(xc,yc,x,y);
 }

 p=ROUND(ry2*(x+0.5)*(x+0.5)+rx2*(y-1)*(y-1)-rx2*ry2);
 while(y>0)
 {
 y--;
 py-=twoRx2;
 if(p>0)
  p+=rx2-py;
 else
       {
  x++;
  px+=twoRy2;
  p+=rx2-py+px;
       }
 plot_ellipse(xc,yc,x,y);
 }
 getch();
 }

 void plot_ellipse(int xc,int yc,int x,int y)
 {
 putpixel(xc+x,yc+y,4);
 putpixel(xc+x,yc-y,4);
 putpixel(xc-x,yc+y,4);
 putpixel(xc-x,yc-y,4);
 }