Как определить, лежит ли точка внутри полигона?

The main procedure is called ExploreLine. In this procedure Fst and Lst may be two consecutively points in the polyline. Srch is the point searched.

{ ... }
const {global}
  BigM = 1000000;
 
function Pend(Pi, Pf: TPoint): Real;
begin
  if (Pf.X = Pi.X) then
    Result := BigM {for a vertical line}
  else
    Result := (Pf.Y - Pi.Y) / (Pf.X - Pi.X);
end;
 
function Dist(Pi, Pf: TPoint): Real;
begin
  Result := sqrt(sqr(Pi.Y - Pf.Y) + sqr(Pi.X - Pf.X))
end;
 
function CalcPoint(Pi, Pf: TPoint; d: Word): TPoint;
var
  k, m: Real; { k=d / (1 + m2)Ѕ }
begin
  m := Pend(Pi, Pf);
  k := d / (Sqrt(1 + Sqr(m)));
  if ((Pf.X - Pi.X) < 0) then
  begin
    Result.X := Pi.X - Round(k);
    Result.Y := Pi.Y - Round(m * k);
  end
  else
  begin
    Result.X := Pi.X + Round(k);
    Result.Y := Pi.Y + Round(m * k);
  end;
end;
 
function ExploreLine(Srch, Fst, Lst: TPoint): Boolean;
var
  p: Word;
  Any: TPoint;
  lim, dis: Real;
begin
  lim := Dist(Lst, Fst);
  p := 1;
  Any := Fst;
  repeat
    Result := TestPoint(Srch, Any);
    dis := Dist(Any, Fst);
    Any := CalcPoint(Fst, Lst, Rad * p);
    Inc(p);
  until
    (Result)rr(dis >= lim);
end;

Взято с Delphi Knowledge Base: https://www.baltsoft.com/