{ ... }
implementation
{$R *.DFM}
uses
 Math;
procedure DrawCircle(CenterX, CenterY, Radius: Integer; Canvas: TCanvas; Color:
 TColor);
 procedure PlotCircle(x, y, x1, y1: Integer);
 begin
  Canvas.Pixels[x + x1, y + y1] := Color;
  Canvas.Pixels[x - x1, y + y1] := Color;
  Canvas.Pixels[x + x1, y - y1] := Color;
  Canvas.Pixels[x - x1, y - y1] := Color;
  Canvas.Pixels[x + y1, y + x1] := Color;
  Canvas.Pixels[x - y1, y + x1] := Color;
  Canvas.Pixels[x + y1, y - x1] := Color;
  Canvas.Pixels[x - y1, y - x1] := Color;
 end;
var
 x, y, r: Integer;
 x1, y1, p: Integer;
begin
 x := CenterX;
 y := CenterY;
 r := Radius;
 x1 := 0;
 y1 := r;
 p := 3 - 2 * r;
 while (x1 < y1) do
 begin
  plotcircle(x, y, x1, y1);
  if (p < 0) then
  p := p + 4 * x1 + 6
  else
  begin
  p := p + 4 * (x1 - y1) + 10;
  y1 := y1 - 1;
  end;
  x1 := x1 + 1;
 end;
 if (x1 = y1) then
  plotcircle(x, y, x1, y1);
end;

procedure TForm1.Button1Click(Sender: TObject);

begin

 DrawCircle(ClientWidth div 2, ClientHeight div 2, Min(ClientWidth div 2,

  ClientHeight div 2), Canvas, clBlack);

 {Add Math to the uses clause for the Min function}

end;