Computer Graphics:midpoint circle drawing algorithm

//---------------------------------------------------------------------------
#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::Button1Click(TObject *Sender)
{
        int pk;
        int xc,yc,x,y,r;
        xc=StrToInt(Edit1->Text);
        yc=StrToInt(Edit2->Text);
        r=StrToInt(Edit3->Text);
        pk=1-r;
 x=0;
 y=r;
 do{
 if(pk<0)
 {
  x++;
  pk=pk+2*x+3;
 }
 else
 {
  x++;
  y--;
  pk=pk+2*x-2*y+5;
 }
                Form1->Canvas->Pixels[xc+x][yc+y]=RGB(250,0,0);
                Form1->Canvas->Pixels[xc+x][yc-y]=RGB(0,250,0);
                Form1->Canvas->Pixels[xc-x][yc+y]=RGB(0,0,250);
                Form1->Canvas->Pixels[xc-x][yc-y]=RGB(250,0,0);
                Form1->Canvas->Pixels[xc+y][yc+x]=RGB(0,250,0);
                Form1->Canvas->Pixels[xc+y][yc-x]=RGB(0,0,250);
                Form1->Canvas->Pixels[xc-y][yc+x]=RGB(250,0,0);
                Form1->Canvas->Pixels[xc-y][yc-x]=RGB(0,250,0);
 }while(x<=y);



}
//---------------------------------------------------------------------------