폼에서의 예제입니다.
반투명으로 사각을 그리는거구요.
커멘트로한건 하나씩 그릴때 습니다.

  private
    { Private declarations }
    Origin, MovePt : TPoint;
    Drawing:Boolean;

  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
     Canvas.MoveTo(X,Y);
     Origin  := Point(X,Y);
     MovePt  := Origin;
     Drawing := True;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
    if Drawing then begin
        canvas.pen.Color := clBlue;
        canvas.pen.mode := pmnotXor ; //또는 pmMerge
        canvas.Brush.Color := clBlue;
                
        //canvas.MoveTo(Origin.x, Origin.y);
        //canvas.LineTo(MovePt.x, Origin.y);
        //canvas.LineTo(MovePt.x, MovePt.y);
        //canvas.LineTo(Origin.x, MovePt.y);
        //canvas.LineTo(Origin.x, Origin.y);
        canvas.Rectangle(Rect(Origin.x, Origin.y, MovePt.x, MovePt.y));

        //canvas.MoveTo(Origin.x, Origin.y);
        //canvas.LineTo(x, Origin.y);
        //canvas.LineTo(x, y);
        //canvas.LineTo(Origin.x, y);
        //canvas.LineTo(Origin.x, origin.y);
        canvas.Rectangle(Rect(Origin.x, Origin.y,x, y));

        //Form1.Refresh;
        //canvas.FillRect(Rect(Origin.x, Origin.y,x, y));

        movept := Point(x,y);

     end;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
     {canvas.pen.Color := clBlue;
     canvas.pen.mode := pmnotXor ;
     canvas.MoveTo(Origin.x, origin.y);
     canvas.LineTo(x, origin.y);
     canvas.LineTo(x, y);
     canvas.LineTo(Origin.x,y);
     canvas.LineTo(Origin.x, origin.y);}

     Form1.Refresh;
     //canvas.FillRect(Rect(Origin.x, Origin.y,x, y));
     drawing := False;
end;
hsr///////////////////////////////////////////////////////////////////

조언 감사드립니다. 잘 이해가 됩니다.
근데..반투명이 아닌 오버라이팅(Overwriting) 시킬라면 어떻게 합니까..?

canvas.pen.mode := pmnotXor ; //또는 pmMerge
를 지우세요.
아니면 canvas.pen.mode := pmCopy ;하세요