Составление картинки из точек

TForm1 = class(TForm)
    Button1: TButton;
    Background: TImage;
    BackScreen: TImage;
    Button2: TButton;
    Screen: TPaintBox;
    procedure Button1Click(Sender: TObject);

procedure TForm1.Button1Click(Sender: TObject);
type
  Point = record
    color : TColor;
    x, y : integer;
  end;
type PointArray = array of array of Point;
var i, j       : Integer;
  oldtime      : TDateTime;
  pause        : Integer;
  pa           : PointArray;
  x0, y0       : Integer;
  step         : Integer;
  Xstep, Ystep : Integer;
  IsRunning    : Boolean;
const
  MaxStep = 100;
  MSecsPerFrame = 25;
begin
     x0:= (Screen.Width - Background.Width) div 2;
     y0:= (Screen.Height - Background.Height) div 2;
     Button1.Enabled:=False;
     Randomize;
     SetLength(pa, Background.Width, Background.Height);
 
     for i:= 0 to Background.Width - 1 do
     begin
         for j:= 0 to Background.Height - 1 do
         begin
           pa[i,j].color := Background.Canvas.Pixels[i,j];
           pa[i,j].x := Random(Screen.Width);
           pa[i,j].y := Random(Screen.Height);
         end;
     end;
     Screen.Refresh;
     BackScreen.Refresh;
     oldtime:=Now;
     BackScreen.Canvas.FillRect(Rect(0, 0, BackScreen.Width, BackScreen.Height));
     for i:= 0 to Background.Width - 1 do
         for j:= 0 to Background.Height - 1 do
         begin
           Xstep := Round(pa[i,j].x - step*(pa[i,j].x - (x0+i))/MaxStep);
           Ystep := Round(pa[i,j].y - step*(pa[i,j].y - (y0+j))/MaxStep);
           BackScreen.Canvas.Pixels[Xstep, Ystep] := pa[i,j].color;
         end;
     Screen.Canvas.CopyRect(Rect(0, 0, Screen.Width, Screen.Height),BackScreen.Canvas, Rect(0, 0, Screen.Width, Screen.Height));
     step:= step + 1;
     if step > MaxStep then Form1.Button1Click(nil);
     Application.ProcessMessages;
     pause:= Round(MSecsPerFrame-(Now-oldtime)*MSecsPerDay);
     if pause>0 then Sleep(pause);
 
end;