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Frequently Asked Questions

Selection rectangle

Question:

How can I create a selection rectangle for use with selecting portions of an image for editing?

Answer:

The easiest way to do this is to use the Windows API function
DrawFocusRect. The DrawFocusRect function uses the XOR function when
drawing, so drawing the rectangle twice in the same location erases
the rectangle, and the rectangle will always be visible when drawing
over different colors.

Example:

type
  TForm1 = class(TForm)
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
  private
    { Private declarations }
    Capturing : bool;
    Captured : bool;
    StartPlace : TPoint;
    EndPlace : TPoint;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

function MakeRect(Pt1 : TPoint;
                  Pt2 : TPoint) : TRect;
begin
  if pt1.x < pt2.x then begin
    Result.Left := pt1.x;
    Result.Right := pt2.x;
  end else begin
    Result.Left := pt2.x;
    Result.Right := pt1.x;
  end;
  if pt1.y < pt2.y then begin
    Result.Top := pt1.y;
    Result.Bottom := pt2.y;
  end else begin
    Result.Top := pt2.y;
    Result.Bottom := pt1.y;
  end;
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Captured then
    DrawFocusRect(Form1.Canvas.Handle,
                  MakeRect(StartPlace,
                           EndPlace));
  StartPlace.x := X;
  StartPlace.y := Y;
  EndPlace.x := X;
  EndPlace.y := Y;
  DrawFocusRect(Form1.Canvas.Handle,
                MakeRect(StartPlace,
                         EndPlace));
  Capturing := true;
  Captured := true;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if Capturing then begin
    DrawFocusRect(Form1.Canvas.Handle,
                  MakeRect(StartPlace,
                           EndPlace));
    EndPlace.x := X;
    EndPlace.y := Y;
    DrawFocusRect(Form1.Canvas.Handle,
                  MakeRect(StartPlace,
                           EndPlace));
  end;

end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Capturing := false;
end;

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