Identify if a point is within a polygon?

Question

I'm making a custom control in Delphi (inherited from TCustomControl) which consists of a number of polygon list items (irregular shapes). I need to implement mouse events per item, but first I need to be able to detect if the mouse position is within a given polygon (array of TPoint). I am catching the Hit Test message (WM_NCHITTEST) and this is where I will need to do this validation. I have a number of polygons, I will do a loop through each polygon item and perform this check to see if the mouse's X/Y position is within this polygon.

procedure TMyControl.WMNCHitTest(var Message: TWMNCHitTest);
var
  P: TPoint; //X/Y of Mouse
  Poly: TPoints; //array of TPoint
  X: Integer; //iterator
  I: TMyListItem; //my custom list item
begin
  P.X:= Message.XPos;
  P.Y:= Message.YPos;
  for X := 0 to Items.Count - 1 do begin
    I:= Items[X]; //acquire my custom list item by index
    Poly:= I.Points; //acquire polygon points

    //Check if Point (P) is within Polygon (Poly)...?

  end;
end;

Answer 1

You can use PtInRegion:

function PointInPolygon(Point: TPoint; const Polygon: array of TPoint): Boolean;
var
  rgn: HRGN;
begin
  rgn := CreatePolygonRgn(Polygon[0], Length(Polygon), WINDING);
  Result := PtInRegion(rgn, Point.X, Point.Y);
  DeleteObject(rgn);
end;