Question and Answer Database
FAQ1073D.txt Selection rectangle
Category :VCL
Platform :All
Product :All 32 bit
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;
7/16/98 4:31:28 PM
Last Modified: 01-SEP-99