unit InvalidateRectUnit;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls;

type
  TForm1 = class(TForm)
    Button2: TButton;
    Image1: TImage;
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
    procedure WMPaint(var Msg: TWMPaint); message WM_PAINT;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button2Click(Sender: TObject);
var
  InvalidRectangle: TRect; //rectangle to invalidate
begin
  {define the rectangle}
  InvalidRectangle := Rect(10, 10, 110, 110);

  {erase only the rectangular area}
  InvalidateRect(Form1.Handle, @InvalidRectangle, TRUE);
end;

procedure TForm1.WMPaint(var Msg: TWMPaint);
var
  InvalidRect: TRect;             // holds the invalid rectangular area
  PaintStruct: TPaintStruct;      // holds painting information
begin
  {retrieve the invalid rectangle}
  GetUpdateRect(Handle, InvalidRect, TRUE);

  {begin the painting process. this validates the invalid region}
  BeginPaint(Handle, PaintStruct);

  {if the entire client area is invalid...}
  if EqualRect(InvalidRect, ClientRect) then
    {...redraw the bitmap in Image1 to the canvas}
    Canvas.Draw(0, 0, Image1.Picture.Bitmap)
  else
  begin
    {...otherwise, draw a red rectangle in the entire invalid rectangular
     area, and label it as a previously invalid area}
    Canvas.Brush.Color := clRed;
    Canvas.Rectangle(InvalidRect.Left, InvalidRect.Top, InvalidRect.Right,
                     InvalidRect.Bottom);
    Canvas.TextOut(InvalidRect.Left+10, InvalidRect.Top+10, 'Invalid Rect');
  end;

  {end the painting operation}
  EndPaint(Handle, PaintStruct);
end;

end.