draw an arrow

{ .... }

var
BeginPoint: TPoint;

{ .... }

uses Math;

{ .... }

procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
BeginPoint.X := X;
BeginPoint.Y := Y;
end;

procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
var
B, deltaX, deltaY: Extended;
begin
Image1.Canvas.PenPos := BeginPoint;
// Beginpoint is the point from where the use drew the line
Image1.Canvas.LineTo(X, Y);

if BeginPoint.X <> X then // checks for division by zero
begin
if (BeginPoint.X > X) then
B := DegToRad(135) - ArcTan((BeginPoint.Y - Y) / (BeginPoint.X - X))
else
B := DegToRad(45) - ArcTan((BeginPoint.Y - Y) / (BeginPoint.X - X));
// the arrow will have a 45 deg corner

deltaX := 15 * Cos(B); // 15 is the length of the arrow
deltaY := 15 * Sin(B);

if (BeginPoint.X > X) then
begin
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X - Trunc(deltaX), Y + Trunc(deltaY));
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X + Trunc(deltaY), Y + Trunc(deltaX));
end
else
begin
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X - Trunc(deltaX), Y + Trunc(deltaY));
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X - Trunc(deltaY), Y - Trunc(deltaX));
end;
end
else
begin
if BeginPoint.Y > Y then
begin
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X + 10, Y + 10);
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X - 10, Y + 10);
end
else
begin
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X + 10, Y - 10);
Image1.Canvas.PenPos := Point(X, Y);
Image1.Canvas.LineTo(X - 10, Y - 10);
end;
end;
end;