Computer Graphics-coding for ellipse

//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
        : TForm(Owner)
{
}
//---------------------------------------------------------------------------
void __fastcall TForm1::Button5Click(TObject *Sender)
{
  int xc,yc,rx,ry,x,y,a,b;
 float pk;
        rx=StrToInt(Edit3->Text);
        ry=StrToInt(Edit4->Text);
        xc=StrToInt(Edit1->Text);
        yc=StrToInt(Edit2->Text);
        //For region 1
 pk=ry*ry-rx*rx*ry+((rx*rx)/4);
 x=0;
 y=ry;
        a=0;
        b=2*rx*rx*y;
 do{
  Form1->Canvas->Pixels[xc+x][yc+y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc+x][yc-y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc-x][yc+y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc-x][yc-y]=RGB(255,0,0);
                 a=a+2*ry*ry;
  if(pk<0)
  {
   pk=pk+a+ry*ry;
   x=x+1;
  }
  else
  {
   pk=pk+a-b+ry*ry;
                        b=b-2*rx*rx;
   x=x+1;
   y=y-1;
  }
 }
 while((2*ry*ry*x)<=(2*rx*rx*y));
 //For region 2

 pk=ry*ry*(x+0.5)*(x+0.5)+rx*rx*(y-1)*(y-1)-rx*rx*ry*ry;
 do{
  Form1->Canvas->Pixels[xc+x][yc+y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc+x][yc-y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc-x][yc+y]=RGB(255,0,0);
                Form1->Canvas->Pixels[xc-x][yc-y]=RGB(255,0,0);
                b=b-2*rx*rx;
  if(pk>0)
  {
   pk=pk-b+rx*rx;
   y=y-1;
  }
  else
  {
   pk=pk+a-b+rx*rx;
                        a=a+2*ry*ry;
   x=x+1;
   y=y-1;
  }
 }
 while(y>=0);
}      
//---------------------------------------------------------------------------