Author: Jonas Bilinkevicius 

I create several regions using CreatePolygonRgn function, passing an array of 
several points (ex. 4). After that, under some condition, I have to test if the 
user has clicked inside that region using PtInRegion. Now there are some problems: 
Sometimes CreatePolygonRgn returns 0 (no region created). Why is that? Under any 
circumstances I can not get any hits when passing points to PtInRegion.

Answer:

Here is a sample using a dynamic TPoint array:
1   
2   procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
3     Shift: TShiftState; X, Y: Integer);
4   type
5     PPointArray = ^TPointArray;
6     TPointArray = array[0..MaxInt div SizeOf(TPoint) - 1] of TPoint;
7   var
8     Rgn: HRGN;
9     P: PPointArray;
10  begin
11    GetMem(P, SizeOf(TPoint) * 3);
12    P[0] := Point(0, 0);
13    P[1] := Point(50, 100);
14    P[2] := Point(100, 50);
15    Rgn := CreatePolygonRgn(P[0], 3, WINDING);
16    Canvas.Brush.Color := clRed;
17    FillRgn(Canvas.Handle, Rgn, Canvas.Brush.Handle);
18    if PtInRegion(Rgn, X, Y) then
19      Beep;
20    DeleteObject(Rgn);
21    FreeMem(P);
22  end;