C Implementation of Mid-Point Ellipse Drawing Algorithm

#include <graphics.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <time.h>
#include <dos.h>

int main(void)
{
int gd=DETECT,gm;
int cenx,ceny;
float Pk,a,b,x,y;
clrscr();

printf("\n\n Enter 'a' and 'b': ");
scanf("%f%f",&a,&b);

initgraph(&gd,&gm,"c:\\tc\\bgi");


cenx=getmaxx()/2;
ceny=getmaxy()/2;

Pk=b*b-b*a*a+0.25*a*a;
x=0;
y=b;
putpixel(cenx+x,ceny+y,WHITE);
putpixel(cenx+x,ceny-y,WHITE);
putpixel(cenx-x,ceny+y,WHITE);
putpixel(cenx-x,ceny-y,WHITE);

while (2*x*b*b <= 2*y*a*a)
{
if (Pk<0)
{
x=x+1;
y=y;
Pk=Pk+2*x*b*b+3*b*b;
}
else
{
x=x+1;
y=y-1;
Pk=Pk+2*x*b*b+3*b*b-2*y*a*a+2*a*a;
}
putpixel(cenx+x,ceny+y,WHITE);
putpixel(cenx+x,ceny-y,WHITE);
putpixel(cenx-x,ceny+y,WHITE);
putpixel(cenx-x,ceny-y,WHITE);
delay(40);
}


Pk=(x+0.5)*(x+0.5)*b*b+(y-1)*(y-1)*a*a-a*a*b*b;
putpixel(cenx+x,ceny+y,WHITE);
putpixel(cenx+x,ceny-y,WHITE);
putpixel(cenx-x,ceny+y,WHITE);
putpixel(cenx-x,ceny-y,WHITE);
while (y>0)
{
if (Pk>0)
{
x=x;
y=y-1;
Pk=Pk-2*y*a*a+3*a*a;
}
else
{
x=x+1;
y=y-1;
Pk=Pk-2*y*a*a+3*a*a+2*x*b*b+2*b*b;
}
putpixel(cenx+x,ceny+y,WHITE);
putpixel(cenx+x,ceny-y,WHITE);
putpixel(cenx-x,ceny+y,WHITE);
putpixel(cenx-x,ceny-y,WHITE);
delay(40);
}
gotoxy(1,25);
printf ("\npress any key to exit.");
getch();
closegraph();
return 0;
}
/*SAMPLE INPUT*/

/*Enter 'a' and 'b' 120 80*/