...
public
  Row: integer;
  procedure OnIdleProc(Sender: TObject; var Done: Boolean);
...
procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.OnIdle := OnIdleProc;
end;

procedure TForm1.OnIdleProc(Sender: TObject; var Done: Boolean);
var
  i: integer;
  col: TColor;
  Gray: integer;
begin
  for i := 0 to Form1.ClientWidth - 1 do begin
  col := Form1.Canvas.Pixels[i, Row];
  Gray := GetRValue(col) + round(30 * sin(i / 30 + Row / 50));
  Form1.Canvas.Pixels[i, Row] := RGB(Gray, Gray, Gray);
  end;
  inc(Row);
  if (Row = Form1.ClientHeight) then Row := 0;
  Done := false;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
var
  i: integer;
begin
  with Form1.Canvas do begin
  Brush.Style := bsClear;
  for i := 0 to 1000 do begin
  Pen.Color := RGB(i, i, i);
  Rectangle(X - i, Y - i, X + i, Y + i);
  end;
  end;
end;