Изобразить вращение хорды вокруг центра окружности

unit Unit1;
 
interface
 
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, StdCtrls;
 
type
  TForm1 = class(TForm)
    Button1: TButton;
    Timer1: TTimer;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure Timer1Timer(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
 
var
  Form1: TForm1;
  var xc,yc,r,x1,y1,x2,y2,i:integer;
      f1,f2:real;
implementation
 
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
Timer1.Enabled:=false;
Timer1.Interval:=10;
color:=clWhite;
xc:=clientwidth div 2;
yc:=clientheight div 2;
r:=yc-30;
end;
 
procedure TForm1.Button1Click(Sender: TObject);
begin
with Canvas do
 begin
  pen.Color:=clBlue;
  ellipse(xc-r,yc-r,xc+r,yc+r);
 end;
f1:=0.3;f2:=pi/2-0.3;
r:=r-2;
x1:=xc+trunc(r*cos(f1));y1:=yc-trunc(r*sin(f1));
x2:=xc+trunc(r*cos(f2));y2:=yc-trunc(r*sin(f2));
Timer1.Enabled:=true;
 
end;
 
 
procedure TForm1.Timer1Timer(Sender: TObject);
begin
with Canvas do
 begin
  pen.Color:=clWhite;
  moveto(x1,y1);
  lineto(x2,y2);
 end;
f1:=f1+0.01;
f2:=f2+0.01;
x1:=xc+trunc(r*cos(f1));y1:=yc-trunc(r*sin(f1));
x2:=xc+trunc(r*cos(f2));y2:=yc-trunc(r*sin(f2));
with Canvas do
 begin
  pen.Color:=clRed;
  moveto(x1,y1);
  lineto(x2,y2);
 end;
i:=i+1; 
end;
 
end.