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DragStartX,DragStartY,LastX,LastY:integer;
DragAction:Boolean;
OldPenMode:TPen                    

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procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
if Button=mbLeft then begin
   DragStartX:=x;
   DragStartY:=y;
   LastX:=x;
   LastY:=y;
   DragAction:=True;
   end;
end;                    

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procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
if DragAction then begin
   OldPenMode:=Image1.Canvas.Pen;
   with Image1.Canvas do begin
        //altes Rechteck überschreiben
        Pen.Color:=clgray;
        Pen.Mode:=pmNot;
        Brush.Style:=bsClear;
        Rectangle(DragStartX,DragStartY,LastX,LastY);
        //neues Rechteck zeichnen
        if (x-DragstartX)<(y-DragstartY) then x:=(y-DragstartY+DragstartX) else y:=(x-DragstartX+DragstartY);
        Rectangle(DragStartX,DragStartY,x,y);
        LastX:=x;LastY:=y;
        end;
   end;
end;                    

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procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
if DragAction then begin
     with Image1.Canvas do begin
        //altes Rechteck überschreiben
        Pen.Color:=clgray;
        Pen.Mode:=pmNot;
        Brush.Style:=bsClear;
        Rectangle(DragStartX,DragStartY,LastX,LastY);
        DragAction:=False;
        //Einstellungen wiederherstellen
        Pen:=OldPenMode;
        end;
        if (x=DragStartX) OR (y=DragStartY) then begin
      Exit;
      end;
     end;
end;