Mega Code Archive

Calculating an angle from two points

Question: How can I tell the given angle expressed by two points? Answer: The following example shows how to tell the angle of a given point from the coordinate 0, 0. In this example, the caption of the form shows the current point and angle from the center of the form. The y coordinates are inverted to show a more natural north orientation where north is directly up. Example: const RADTODEG = 180 / Pi; function AngleOfPt(pt : TPoint) : double; begin Result := 0; if ((Pt.x = 0) and (Pt.y < 0)) then Result := 270 else if ((Pt.x = 0) and (Pt.y > 0)) then Result := 90 else if ((Pt.x > 0) and (Pt.y >= 0)) then Result := ArcTan(Pt.y / Pt.x) * RADTODEG else if ((Pt.x < 0) And (Pt.y > 0)) then Result := 180 - (ArcTan(Pt.y / Abs(Pt.x))* RADTODEG) else if ((Pt.x < 0) And (Pt.y <= 0)) then Result := 180 + (ArcTan(Pt.y / Pt.x) * RADTODEG) else if ((Pt.x > 0) and (Pt.y < 0)) then Result := 360 - (ArcTan(Abs(Pt.y) / Pt.x) * RADTODEG) else Result:=0; end; procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer); var XYMove : TPoint; RotateAngle : Double; begin XYMove := (Point(x - (Form1.Width div 2), (Form1.Height div 2) - y)); RotateAngle := AngleOfPt(XYMove); Form1.Caption := IntToStr(XYMove.x) + #32 + IntToStr(XYMove.y) + ' = ' + FloatToStr(RotateAngle); end; procedure TForm1.FormPaint(Sender: TObject); begin Form1.Canvas.MoveTo(Form1.Width div 2, 0); Form1.Canvas.LineTo(Form1.Width div 2, Form1.Height); Form1.Canvas.MoveTo(0, Form1.Height div 2); Form1.Canvas.LineTo(Form1.Width, Form1.Height div 2); end; procedure TForm1.FormResize(Sender: TObject); begin Invalidate; end;