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                        type
  TForm2 = class(TForm)
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormCreate(Sender: TObject);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
    isDown: Boolean;
    downX, downY: Integer;
  public
    { Public declarations }
    Bild: TBitMap;
  end;
implementation
{$R *.dfm}
procedure TForm2.FormCreate(Sender: TObject);
begin
  Bild := TBitMap.Create;
end;
procedure TForm2.FormDestroy(Sender: TObject);
begin
  Bild.Free;
end;
procedure TForm2.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  isDown := true;
  downX := X;
  downY := Y;
end;
procedure TForm2.FormMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  if isDown then
  begin
    Self.Repaint;
    Self.Canvas.Pen.Color := clRed;
    Self.Canvas.Pen.Width := 1;
    Self.Canvas.Pen.Style := psDot;
    Self.Canvas.Brush.Color := clGreen;
    Self.Canvas.Rectangle(downX, downY, X, Y);
    Self.Canvas.Pen.Style := psSolid;
    Self.Canvas.Brush.Color := clRed;
    Self.Canvas.Rectangle(downX - 6, downY - 6, downX + 6, downY + 6);
    Self.Canvas.Rectangle(X - 6, Y - 6, X + 6, Y + 6);
    Self.Canvas.Rectangle(X - 6, downY - 6, X + 6, downY + 6);
    Self.Canvas.Rectangle(downX - 6, Y - 6, downX + 6, Y + 6);
    Self.Canvas.Rectangle(downX - 6, (downY + Y) div 2 - 6, downX + 6,
      (downY + Y) div 2 + 6);
    Self.Canvas.Rectangle(X - 6, (downY + Y) div 2 - 6, X + 6,
      (downY + Y) div 2 + 6);
    Self.Canvas.Rectangle((downX + X) div 2 - 6, downY - 6,
      (downX + X) div 2 + 6, downY + 6);
    Self.Canvas.Rectangle((downX + X) div 2 - 6, Y - 6, (downX + X) div 2 + 6,
      Y + 6);
  end;
end;
function CaptureScreenRect(aRect: TRect): TBitMap;
var
  ScreenDC: HDC;
begin
  Result := TBitMap.Create;
  with Result, aRect do
  begin
    Width := Right - Left;
    Height := Bottom - Top;
    ScreenDC := GetDC(0);
    try
      BitBlt(Canvas.Handle, 0, 0, Width, Height, ScreenDC, Left, Top, SRCCOPY);
    finally
      ReleaseDC(0, ScreenDC);
    end;
  end;
end;
                                     
        
            Прага для "снятия экранов". Дополнительная форма растягивается по экрану и с нёё снимаеться скрин.
Без комментариев.