r - Round(sin(angle) * radius);
    arPts[I].X := x;
    arPts[I].Y := y;
  end;

  rgn := CreatePolygonRgn(arPts, pts, WINDING);
  SetWindowRgn(wnd, rgn, TRUE);
end;

The first two functions are pretty simple, just two-liners. All that''s needed to create the appropriate shapes is a handle and a TRect structure. For forms, that structure would be taken from the ClientRect property; for other controls, use the BoundsRect property.

The DrawPolygonRegion method, however, is much more complex. This is due in part to the fact that CreatePolygonRgn requires the vertices of the corners of the polygon to be passed as an array of TPoints, and partly because I wanted to draw equilateral polygons based off points rotated around a common center point. For that I had to use some trigonometry.

I wanted to not only draw polygon regions, but stars as well. Using rotational trig allowed me to do it. The way the function works if the DrawStarShape parameter is set to True is that for every even value of I in the loop, the radius of the circle is set to half its length, and to maintain the number of points of the polygon I want to draw, I double the number of points to accomodate the contraction of the radius.

At the very end of each function is a call to SetWindowRgn. This function takes as parameters a window handle, a rgn var, and a Boolean value that specifies whether the window should be re-drawn. In all cases, if you want to see the shape you''ve made, this must be always be set to True.

Below is the listing for the entire source code of my test form. On the form I''ve dropped four TButtons (one for each of the shapes: ellipse, round rectangle, polygon and star); a TPanel to demonstrate the ability to set regions for TWinControl descendants other than TForm; and a SpinEdit used in conjunction with the Polygon and Star region buttons to define the number of points that''ll be defining the shape. Here''s the code:
unit regmain;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls, StdCtrls, Spin;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    SpinEdit1: TSpinEdit;
    Button4: TButton;
    Panel1: TPanel;
    Edit1: TEdit;
    procedure DrawRndRectRegion(wnd : HWND; rect : TRect);
    procedure DrawEllipticRegion(wnd : HWND; rect : TRect);
    procedure DrawPolygonRegion(wnd : HWND; rect : TRect; NumPoints : Integer; DoStarShape : Boolean);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
&nbs